2 Cf. Larid, Introduction to Philosophy, pp. 330, 361.
3 For example, that hinted at by Bosanquet in his definition of the beautiful, History of aesthetic, p. 5.
4 Beauty is defined as perfection by P. Souriau, La Beaute rationnelle, 2eme partie.
5 K. Groos argues well against this violent stretching of the word beautiful, Einleitung in die Asthetik, pp. 46 seq.
6 Kant, in developing his idea of beauty as subjective, was probably influenced by Hume, who wrote: ``Beauty is no quality in things themselves; it exists merely in the mind which contemplates them'' (Essays, xxii.).
7 On the nature of these qualities see S. Witasek, Grundzuge der Allgem. Asthetik, p. 11.
8 See J. Cohn; Allgem. Esthetik, P. 96
9 Originally, as pointed out by Home and others, sight was regarded as the sense by which we received impressions of beauty. The recognition of the claims of hearing date back to Plato. (See Bosanquet, Hist. of Aesth. pp. 51-52). For recent discussions of the claims of sight and hearing see article by J. Volkelt, ``Der Aesth. Werth der niederen Sinne,'' in Zeitschrift fur Psych. u. Phys. der Sinnesorgane, vol. xxix. pp. 402 ff.; see also below, Bibliography.
10 Laws, 880 (see Bosanquet, op. cit. p. 54).
11 Plato had a glimpse of the resemblance of art to play (see Bosanquet, op. cit. p. 54). Among modern writers the idea is specially connected with the names of Schiller and Herbert Spencer. In recent works the subject is touched on by S. Wittasek, Grundzuge der allgem. Asthetik, pp. 223 fl.; Bray, Du Beau, pp. 62 ff., and by Rutgers Marshall and others referred to below in Bibliography.
12 Hence to say, as Bosanquet says (op. cit. pp. 3-4), that art is to nature as the scientific conception of the world to that of the ordinary observer, seems wide of the mark.
13 K. Lange goes very far in attributing a practical motive to features of architecture commonly supposed to have aesthetic value, e.g. a regular series of similar forms (Das Wesen der Kunst, Bd. i. pp. 277 ff.).
14 K. Lange thinks that even symmetry probably has a technical origin (op. cit. pp. 283-284).
15 The question of the place of the historical development of art in aesthetic theory is carefully considered by J. Volkelt, System der Asthetik, Bd. i. 5es Kap.
16 See, for example, a little work, The Genesis of Art-From, by G. L. Raymond.
17 Kant, stopping short of an analysis of the beauty of a concrete object, said there were no aesthetic judgments of this universal form see below). On the importance of these inductions see K. H. von Stein, Vorlesungen uber Asthetik (Einleitung).
18 Curiously enough Thomas Reid recognized a germ of aesthetic taste in animals. Essays, Of Taste, ch. v. The aesthetic importance of the observations made on animals is dealt with by L. Bray, Du Beau, pp. 233 ff.
19 See below, and Bosanquet, op. cit. pp. 382 ff.
20 The chief lines of experimental aesthetics are indicated by W. Wundt in his Physiol. Psychologie (5e Auflage), Bd. iii. pp. 142 ff. and 147 ff.
21 On the value of the judgments of experts see K. Groos, Der asth. Genuss, p. 149.
22 Examples of a forcina of the physiological method in aesthetics may be found in the Physiological Aesthetics of Grant Allen, and the Aufgabe der Kunstphysialogie, by Georg Hirsch.
23 These aesthetic prerogatives of the sensations of hearing and sight have been well brought out in the article by J. Volkelt, already referred to.
24 On the later investigations into musical consonance and harmony, harmony of colours, rhythmic and pleasing spatial forms, see Wundt, op..cit. Bd. ii. pp. 419 ff., and iii. 135 g., 140 ff., 147 ff. and 154 ff. Time-form in music is specially discussed by E. Gurney, The Power of Sound, v.
25 K. Lange, who recognizes the influence of nature and custom here denies that proportion is an aesthetic principle (Das Wesen der Kunst, 11es Kap.).
26 Alison and other English Associationists have emphasized the aesthetic importance of the principle of association. Among more recent advocates of it is G. T. Fechner. Vorschule der Asthetik, and O. Kulpe, ``Uber den associativ Factor des asthet. Eindrucks'', Fierteljahrsschrift fur wissensch. Philosophie, xxiii. pp. 145 ff.
27 This idea of imitative hand-movement in contemplating form is supported by K. Groos, Der asth. Genuss, pp. 49 ff.
28 It is commonly spoken of as ``feeling oneself into'' Einfuhlen), or as``sympathetic feeling'' (Mitempfinden.)
29 Lipps theory is developed in a number of works, the chief of which is Asthetik: Psychologie des Schonen und der Kunst, see esp. 1er Theil, 1er to 3er Abschnitt; cf. Paul Stern, Einfuhlung und Association, in which is to be found an historical sketch of the theory, and A. Hildebrand, Form in der bildenden Kunst. The play of imagination in the contemplation of form is discussed also by P. Souriau, L'Esthetique du mouvement, 3eme part., and La Suggestion dans l'art, pp. 300 ff. Cf. works of Karl Groos and K. Lange named below (Bibliography.) .
30 See P. Souriau, La Suggestion dans l'art (1ere partie).
31 Cf. K. Lange, op. cit. lfh. i. p. 208.
32 See a curious passage in Home's Elements of Criticism, chap iv., in which the emotions excited by great and elevated objects are said to express themselves externally by a special inflating inspiration, and by stretching upward and standing ``a-tiptoe'' respectively; also an article on ``Recent Aesthetics', by Vernon Lee in the Quarterly Review, 1904, part i. pp. 420-443.
33 See Hume, Essays, ``Essay of Tragedy,'' and the important discussions on the meaning of Aristotle's doctrine of the emotions of tragedy and of emotional purification or ``alleviating discharge', (kathansis) touched on by Bosanquet, op. cit. pp. 64 ff. and 234 ff.
34 That beauty implies a peculiar blending of formal and spiritual (geistige) factors is recognized by H. Riegel, Die bildende Kunste; pp. 16 ff.
35 Human Nature (first part of Tripos), ch. viii. sec. 5 (Molesworth's edition of Works, vol. iv. p. 38).
36 See among others R. Wallascheck, Primitve Music, pp. 270 ff., and Y. Hirn, The Origin of Art, pp. 9 ff.; cf. W. Jerusalem, Einleitung in die Philosophie, pp. 116, 117.
37 The idea of this social utility in aesthetic enjoyment is touched on by Kant, Criticue of Judgment (Bernard's trans.), p. 174; and is more fully worked out by Guyau, L'Art au point de vue sociologique, ch. ii. and iii.; cf. Rutgers Marshall, Aesthetic Principles, pp. 81-82.
38 On the nature of the primitive art-culture, see Rutgers Marshall, Aesthetic Principles, ch. iii.; M. Baldwin, Social and Ethical Interpretations, pp. 151 ff: Y. Hirn, The Origin of Art, ch. ii. On artistic genius and its creative process, see H. Taine, The Philosophy of Art, Part ii.; P. Souriau, L'Imagination de l'artiste; G. Seailles, Essai sur la genie dans l'art; E. Grosse, Kunstwissenschaftliche Studien iii.; Arreat, Psychologie du peintre; L. Dauriac, Essai sur l'esprit musical.
39 Only recent works are included. Important points in each are indicated by abbreviations, namely:—
Einf., Einfuhlung (expressional element in form).Evol., for bearings of evolution.Ill., for aesthetic illusion.Judg., for aesthetic judgment.Meth., for method of aesthetics.Norm., for the normative function of aestheticsPl., for theory of pleasurePlay, for Play and aesthetic enjoymentSenses, for aesthetic value of higher senses.Val., for aesthetic value.
AESTIVATION (from Lat. aestivare, to spend the aestas, or summer; the word is sometimes spelled ``estivation''), literally ``summer residence,'' a term used in zoology for the condition of torpor into which certain animals pass during the hottest season in hot and dry countries, contrasted with the similar winter condition known as hibernation (q.v..) In botany the word is used of the praefloration or folded arrangement of the petals in a flower before expansion in the summer, contrasted with ``vernation'' of leaves which unfold in the spring.
AETHELBALD, king of Mercia, succeeded Ceolred A.D. 716. According to Felix, Life of St Guthlac, he visited the saint at Crowland, when exiled by Ceolred and pursued by his emissaries before his accession, and was cheered by predictions of his future greatness. According to Bede, the whole of Britain as far north as the Humber was included within the sphere of his authority. His energy in preserving his influence is shown by several entries in the Chronicle. He made an expedition against Wessex in 733, in which year he took the royal vill of Somerton. In 740 he took advantage of the absence of Eadberht of Northumbria in a campaign against the Picts to invade his kingdom. In 743 he fought with Cuthred, king of Wessex, against the Welsh, but the alliance did not last long, as in 752 Cuthred took up arms against him. In 757 AEthelbald was slain by his guards at Seckington (Warwickshire) and buried at Repton. He seems to have been the most powerful and energetic king of Mercia between Penda and Offa. A letter of St Boniface is preserved, in which he rebukes this king for his immoralities and encroachments on church property, while recognizing his merits as a monarch. By a charter of 749 he freed ecclesiastical lands from all obligations except the trinoda necessitas.
See Bede, Hist. Ecc. (ed. Plummer), v. 23 and Continuatio s.a. 740, 750, 757; Saxon Chronicle (Earle and Plummer), s.a. 716, 733, 737, 740, 741, 743, 755; Mabillon, Acta Sanctorum, ii. pp. 264, 273, 276, 4-9, W. de G. Birch, Cartul. Saxon. 178 (1885-1893). (F. G. M. B.)
AETHELBALD, king of Wessex, was the son of AEthelwulf, with whom he led the West Saxons to victory against the Danes at Aclea, 851. According to Asser he rebelled against his father on the latter's return from Rome in 856, and deprived him of Wessex, which he ruled until his death in 860. On his father's death in 818 he married his widow, Judith.
See Asser, Life of Alfred (W. H. Stevenson, 1904), 12; Saxon Chronicle, s.a. 851, 855, 860.
AETHELBERHT, king of Kent, son of Eormenric, probably came to the throne in A.D. 560. The first recorded event of his reign was a serious reverse at the hands of Ceawlin of Wessex in the year 568 (Chronicle) at a place called Wibbandune. AEthelberht married Berhta, daughter of Charihert, king of Paris, who brought over Bishop Liudhard as her private confessor. According to Bede, AEthelberht's supremacy in 597 stretched over all the English kingdoms as far as the Humber. The nature of this supremacy has been much disputed, but it was at any rate sufficient to guarantee the safety of Augustine in his conference with the British bishops. AEthelberht exercised a stricter sway over Essex, where his nephew Saberht was king. In 597 the mission of Augustine landed in Thanet and was received at first with some hesitation by the king. He seems to have acted with prudence and moderation during the conversion of his kingdom and did not countenance compulsory proselytism. AEthelberht gave Augustine a dwelling-place in Canterbury, and Christ Church was consecrated in 603. He also made grants to found the see of Rochester, of which Justus became first bishop in 604, and his influence established Mellitus at London in the same year. A code of laws issued by him which is still extant is probably the oldest document in the English language, and contains a list of money fines for various crimes. Towards the close of his reign his pre-eminence as Bretwalda was disturbed by the increasing power of Raedwald of East Anglia. He died probably in 616, and was succeeded by his son Eadbald.
See Bede, Hist. Ecc. (Plummer) i. 25, 26, ii. 3, 5; SaxonChronicle Earle and Plummer), s.a. 568. (F. G. M. B.)
AETHELBERHT, king of the West Saxons, succeeded to the sub-kingdom of Kent during the lifetime of his father AEthelwulf, and retained it until the death of his elder brother AEthelbald in 860, when he became sole king of Wessex and Kent, the younger brothers AEthelred and Alfred renouncing their claim. He ruled these kingdoms for five years and died in 865. His reign was marked by two serious attacks on the part of the Danes, who destroyed Winchester in 860, in spite of the resistance of the ealdormen Osric and AEthelwulf with the levies of Hampshire and Berkshire, while in 865 they treacherously ravaged Kent.
Alfred's Will; W. de G. Birch, Cartul. Saxon. 553.
AETHELFLAED (ETHELFLEDA), the ``Lady of the Mercians,'' the eldest child of Alfred the Great, was educated with her brother Edward at her father's court. As soon as she was of marriageable age (probably about A.D. 886), she was married to AEthelred, earl of Mercia to whom Alfred entrusted the control of Mercia. On the accession of her brother Edward, AEthelflaed and her husband continued to hold Mercia. In 907 they fortified Chester, and in 909 and 910 either AEthelflaed or her husband must have led the Mercian host at the battles of Tettenhall and Wednesfield (or Tettenhall-Wednesfield, if these battles are one and the same). It was probably about this time that AEthelred fell ill, and the Norwegians and Danes from Ireland unsuccessfully besieged Chester. AEthelflaed won the support of the Danes against the Norwegians, and seems also to have entered into an alliance with the Scots and the Welsh against the pagans. In 911 AEthelred died and Edward took over Middlesex and Oxfordshire. Except for this AEthelflaed's authority remained unimpaired. In 912 she fortified ``Scergeat'' and Bridgenorth, Tamworth and Stafford in 913, Eddisbury and Warwick in 914, Cherbury, ``Weardbyrig'' and Runcorn in 915. In 916 she sent an expedition against the Welsh, which advanced as far as Brecknock. In 917 Derby was captured from the Danes, and in the next year Leicester and York both submitted to her. She died in the same year at Tamworth (June 12), and was buried in St Peter's church at Gloucester. This noble queen, whose career was as distinguished as that of her father and brother, left one daughter, AElfwyn. For some eighteen months AElfwyn seems to have wielded her mother's authority, and then, just before the Christmas of 919, Edward took Mercia into his own hands, and AElfwyn was ``led away'' into Wessex.
AEthelflaed and her husband wielded almost kingly authority, andthe royal title is often given them by the chroniclers. See TheSaxon Chronicle, sub ann. (especially the Mercian register inMSS. B, C and D); Florence of Worcester: Fragments of IrishAnnals (ed. O'Conor), pp. 227-237; D.N.B., s.v. (A. Mw.)
AETHELFRITH, king of Northumbria, is said to have come to the throne in A.D. 593, being the son of AEthelric (probably reigned 568-572). He married Acha, daughter of Ella (AElle), king of Deira, whom he succeeded probably in 605, expelling his son Edwin. In 603 he repelled the attack of Aidan, king of the Dalriad Scots, at Daegsastan, defeating him with great loss. The appearance of Hering, son of Hussa, AEthelfrith's predecessor, On the side of the invaders seems to indicate family quarrels in the royal house of Bernicia. Later in his reign, probably in 614, he defeated the Welsh in a great battle at Chester and massacred the monks of Bangor who were assembled to aid them by their prayers. This war may have been due partly to AEthelfrith's persecution of Edwin, but it had a strategic importance in the separation of the North Welsh from the Strathclyde Britons. In 617 AEthelfrith was defeated and slain at the river Idle by Raedwald of East Anglia, whom Edwin had persuaded to take up his cause.
See Bede, Chronica Mojora, sec. 531; Hist. Ecc. (Plummer) i. 34, ii. 2; Saxon Chronicle, s.a. 593, 603, 605, 616; Hist. Brittonum, sec. sec. 57, 63 Annales Cambriae, s.a. 613. (F. D. M. B.)
AETHELING, an Anglo-Saxon word compounded of aethele, or ethel, meaning noble, and ing, belonging to, and akin to the modern German words Adel, nobility, and adelig, noble. During the earliest years of the Anglo-Saxon rule in England the word was probably used to denote any person of noble birth. Its use was, however, soon restricted to members of a royal family, and in the Anglo-Saxon Chronicle it is used almost exclusively for members of the royal house of Wessex. It was occasionally used after the Norman Conquest to designate members of the royal family. The earlier part of the word formed part of the name of several Anglo-Saxon kings, e.g. AEthelbert, AEthelwulf, AEthelred, and was used obviously to indicate their noble birth. According to a document which probably dates from the 10th century, the wergild of an aetheling was fixed at 15,000 thrymsas, or 11,250 shillings. This wergild is equal to that of an archbishop and one-half of that of a king.
AETHELNOTH (d. 1038), archbishop of Canterbury, known also as EGELNODUS or EDNODUS, was a son of the ealdorman AEthelmaer, and a member of the royal family of Wessex. He became a monk at Glastonbury, then dean of the monastery of Christ Church, Canterbury, and chaplain to King Canute, and on the 13th of November 1020 was consecrated archbishop of Canterbury. In 1022 he went to Rome to obtain the pallium, and was received with great respect by Pope Benedict VIII. Returning from Rome he purchased at Pavia a relic said to be an arm of St Augustine of Hippo, for a hundred talents of silver and one of gold, and presented it to the abbey of Coventry. He appears to have exercised considerable influence over Canute, largely by whose aid he restored his cathedral at Canterbury. A story of doubtful authenticity tells how he refused to crown King Harold I., as he had promised Canute to crown none but a son of the king by his wife, Emma. AEthelnoth, who was called the ``Good,'' died on the 29th of October 1038, and his name appears in the lists of saints.
AETHELRED, king of Mercia, succeeded his brother Wulfhere in A.D. 675. In 676 he ravaged Kent with fire and sword, destroying the monasteries and churches and taking Rochester. AEthelred married Osthryth, the sister of Ecgfrith, king of Northumbria, but in spite of this connexion a quarrel arose between the two kings, presumably over the possession of the province of Lindsey, which Ecgfrith had won back at the close of the reign of Wulfhere. In a battle on the banks of the Trent in 679, the king of Mercia was victorious and regained the province. AElfwine, the brother of Ecgfrith, was slain on this occasion, but at the intervention of Theodore, archbishop of Canterbury, AEthelred agreed to pay a wergild for the Northumbrian prince and so prevented further hostilities. Osthryth was murdered in 697 and AEthelred abdicated in 704, choosing Coenred as his successor. He then became abbot of Bardney, and, according to Eddius, recommended Wilfrid to Coenred on his return from Rome. AEthelred died at Bardney in 716. (See WILFRID.)
SOURCES.—Eddius, Vita Wilfridi (Raine), 23, 40, 43, 45-48, 57; Bede, Hist. Ecc. (ed. Plummer), iii. 11, iv. 12, 21; Saxon Chronicle, s.a. 676, 679, 704, 716. (F. G. M. B.)
AETHELRED I., king of Wessex and Kent (866-871), was the fourth son of AEthelwulf of Wessex, and should, by his father's will, have succeeded to Wessex on the death of his eldest brother AEthelbald. He seems, however, to have stood aside in favour of his brother AEthelberht, king of Kent, to whose joint kingdoms he succeeded in 866. AEthelred's reign was one long struggle against the Danes. In the year of his succession a large Danish force landed in East Anglia, and in the year 868 AEthelred and his brother Alfred went to help Burgred, or Burhred, of Mercia, against this host, but the Mercians soon made peace with their foes. In 871 the Danes encamped at Reading, where they defeated AEthelred and his brother, but later in the year the English won a great victory at ``AEscesdun.'' A fortnight later they were defeated at Basing, but partially retrieved their fortune by a victory at ``Maeretun'' (perhaps Marden in Wiltshire), though the Danes held the field. In the Easter of this year AEthelred died, perhaps of wounds received in the wars against the Danes, and was buried at Wimborne. He left a son, AEthelwold, who gave some trouble to his cousin Edward the Elder, when the latter succeeded to the kingdom. AEthelweard the historian was also a descendant of this king.
AUTHORITIES.—The Saxon Chronicle, sub ann.; Birch, Cartul. Saxon. vol. ii. Nos. 516-526; D.N.B., s.v.; Eng. Hist. Review, i. 218-234. (A. Mw.)
AETHELRED II. (or ETHELRED) (c. 968—1016), king of the English (surnamed THE UNREADY, i.e. without rede or counsel), son of King Edgar by his second wife AElfthryth, was born in 968 or 969 and succeeded to the throne on the murder of his step-brother Edward (the Martyr) in 979. His reign was disastrous from the beginning. The year after his accession the Danish invasions, long unintermitted under Edgar the Peaceful, recommenced; though as yet their object was plunder only, not conquest, and the attacks were repeated in 981, 982 and 988. In 991 the Danes burned Ipswich, and defeated and slew the East Saxon ealdorman Brihtnoth at Maldon. After this, peace was purchased by a payment of L. 10,000-a disastrous expedient. The Danes were to desist from their ravages, but were allowed to stay in England. Next year AEthelred himself broke the peace by an attack on the Danish ships. Despite the treachery of AElfric, the English were victorious; and the Danes sailed off to ravage Lindsey and Northumbria. In 994 Olaf Tryggvason, king of Norway, and Sweyn, king of Denmark, united in a great invasion and attacked London. Foiled by the valour of the citizens, they sailed away and harried the coast from Essex to Hampshire. AEthelred now resorted to the old experiment and bought them off for L. 16,000 and a promise of supplies. Olaf also visited AEthelred at the latter's request and, receiving a most honourable welcome, was induced to promise that he would never again come to England with hostile intent, an engagement which he faithfully kept. The Danish attacks were repeated in 997, 998, 999, and in 1000 AEthelred availed himself of the temporary absence of the Danes in Normandy to invade Cumberland, at that time a Viking stronghold. Next year, however, the Northmen returned and inflicted worse evil than ever. The national defence seemed to have broken down altogether. In despair AEthelred again offered them money, which they again accepted, the sum paid on this occasion being L. 24,000. But soon afterwards the king, suspecting treachery, resolved to get rid of his enemies once and for all. Orders were issued commanding the slaughter on St Brice's day (December 2) of ``all the Danish men who were in England.'' Such a decree could obviously not be carried out literally; but we cannot doubt that the slaughter was great. This violence, however, only made matters worse. Next year Sweyn returned, his hostility fanned by the desire for revenge. For two years he ravaged and slew; in 1003 Exeter was destroyed; Norwich and Thetford in 1004. No effectual resistance was offered, despite a gallant effort here and there; the disorganization of the country was complete. In 1005 the Danes were absent in Denmark, but came back next year, and emboldened by the utter lack of resistance, they ranged far inland. In 1007 AEthelred bought them off for a larger sum than ever (L. 36,000), and for two years the land enjoyed peace. In 1009, however, in accordance with a resolution made by the witan in the preceding year, AEthelred collected such a fleet ``as never before had been in England in any king's day''; but owing to a miserable court quarrel the effort came to nothing. The king then summoned a general levy of the nation, with no better result. Just as he was about to attack, the traitor Edric prevented him from doing so, and the opportunity was lost. In 1010 the Danes returned, to find the kingdom more utterly disorganized than ever. ``There was not a chief man in the kingdom who could gather a force, but each fled as he best might; nor even at last would any there resist another.'' Incapable of offering resistance, the king again offered money, this time no less than L. 48,000. While it was being collected, the Danes sacked Canterbury and barbarously slew the archbishop Alphege. The tribute was paid soon afterwards; and about the same time the Danish leader Thurkill entered the English service. From 1013 an important change is discernible in the character of the Danish attacks, which now became definitely political in their aim. In this year Sweyn sailed up the Trent and received the submission of northern England, and then marching south, he attacked London. Failing to take it, he hastened west and at Bath received the submission of Wessex. Then he returned northwards, and after that ``all the nation considered him as full king.'' London soon acknowledged him, and AEthelred, after taking refuge for a while with Thurkill's fleet, escaped to Normandy. Sweyn died in February 1014, and AEthelred was recalled by the witan, on giving a promise to reign better in future. At once he hastened north against Canute, Sweyn's son, who claimed to succeed his father, but Canute sailed away, only to return next year, when the traitor Edric joined him and Wessex submitted. Together Canute and Edric harried Mercia, and were preparing to reduce London, when AEthelred died there on the 23rd of April 1016. Weak, self-indulgent, improvident, he had pursued a policy of opportunism to a fatal conclusion.
AEthelred's wife was Emma, or AElfgifu, daughter of Richard I. the Fearless, duke of the Nurmans, whom he married in 1002. After the king's death Emma became the wife of Canute the Great, and after his death in 1035 she struggled hard to secure England for her son, Hardicanute. In 1037, however, when Harold Harefoot became sole king, she was banished; she went to Flanders, returning to England with Hardicanute in 1040. In 1043, after Edward the Confessor had become king he seized the greater part of Emma's great wealth, and the queen lived in retirement at Winchester until her death on the 6th of March 1052. By AEthelred Emma had two sons, Edward the Confessor and the aetheling AElfred (d. 1036), and by Canute she was the mother of Hardicanute. Emma's marriage with AEthelred was an important step in the history of the relations between England and Normandy, and J. R. Green says ``it suddenly opened for its rulers a distinct policy, a distinct course of action, which led to the Norman conquest of England. From the moment of Emma's marriage Normandy became a chief factor in English politics.''
AUTHORITIES.—-The Anglo-Saxon Chronicle (edition by C. Plummer, 2 vols. Oxford, 1892-1899); Florence of Worcester (ed. B. Thorpe, London, 1848-1849); Encomium Emmae (ed. by G. H. Pertz in the Scriptores Rerum Germanicarum, Band xix., Hanover, 1866) for the latter part of the reign. See also J. M. Kemble, Codex Diplomaticus acti Saxonici (London, 1839—1848); and B. Thorpe, Ancient Laws (London, 1840). (C. S. P.*)
AETHELSTAN (c. 894-940), Saxon king, was the son (probably illegitimate) of Edward the elder. He had been the favourite of his grandfather Alfred, and was brought up in the household of his aunt AEthelflaed, the ``Lady of the Mercians.'' On the death of his father in 924, at some date after the 12th of November, AEthelstan succeeded him and was crowned at Kingston shortly after. The succession did not, however, take place without opposition. One AElfred, probably a descendant of AEthelred I., formed a plot to seize the king at Winchester; the plot was discovered and AElfred was sent to Rome to defend himself, but died shortly after. The king's own legitimate brother Edwin made no attempt on the throne, but in 933 he was drowned at sea under somewhat mysterious circumstances; the later chroniclers ascribe his death to foul play on the part of the king, but this seems more than doubtful.
One of AEthelstan's first public acts was to hold a conference at Tamworth with Sihtric, the Scandinavian king of Northumbria, and as a result Sihtric received AEthelstan's sister in marriage. In the next year Sihtric died and AEthelstan took over the Northumbrian kingdom. He now received, at Dacre in Cumberland, the submission of all the kings of the island, viz. Howel Dda, king of West Wales, Owen, king of Cumbria, Constantine, king of the Scots, and Ealdred of Bamburgh, and henceforth he calls himself ``rex totius Britanniae.'' About this time (the exact chronology is uncertain) AEthelstan expelled Sihtric's brother Guthfrith, destroyed the Danish fortress at York, received the submission of the Welsh at Hereford, fixing their boundary along the line of the Wye, and drove the Cornishmen west of the Tamar, fortifying Exeter as an English city.
In 934 he invaded Scotland by land and sea, perhaps owing to an alliance between Constantine and Anlaf Sihtricsson. The army advanced as far north as Dunottar, in Kincardineshire, while the navy sailed to Caithness. Simeon of Durham speaks of a submission of Scotland as a result; if it ever took place it was a mere form, for three years later we find a great confederacy formed in Scotland against AEthelstan. This confederacy of 937 was joined by Constantine, king of Scotland, the Welsh of Strathclyde, and the Norwegian chieftains Anlaf Sihtricsson and Anlaf Godfredsson, who, though they came from Ireland, had powerful English connexions. A great battle was fought at Brunanburh (perhaps Brunswark or Birrenswark hill in S.E. Dumfriesshire), in which AEthelstan and his brother Edmund were completely victorious. England had been freed from its greatest danger since the days of the struggle of Alfred against Cuthrum.
AEthelstan was the first Saxon king who could claim in any real sense to be lord paramount of Britain. In his charters he is continually called ``rex totius Britanniae,'' and he adopts for the first time the Greek title basileus. This was not merely an idle flourish, for some of his charters are signed by Welsh and Scottish kings as subreguli. Further, AEthelstan was the first king to bring England into close touch with continental Europe. By the marriage of his half-sisters he was brought into connexion with the chief royal and princely houses of France and Germany. His sister Eadgifu married Charles the Simple, Eadhild became the wife of Hugh the Great, duke of France, Eadgyth was married to the emperor Otto the Great, and her sister AElfgifu to a petty German prince. Embassies passed between AEthelstan and Harold Fairhair, first king of Norway, with the result that Harold's son Haakon was brought up in England and is known in Scandinavian history as Haakon Adalsteinsfostri.
AEthelstan died at Gloucester in 940, and was buried at Malmesbury, an abbey which he had munificently endowed during his lifetime. Apparently he was never married, and he certainly had no issue.
A considerable body of law has come down to us in AEthelstan's name. The chief collections are those issued at Grately in Hampshire, at Exeter, at Thunresfeld, and the Judicia civitatis Lundonie. In the last-named one personal touch is found when the king tells the archbishop how grievous it is to put to death persons of twelve winters for stealing. The king secured the raising of the age limit to fifteen.
AUTHORITIES.—-Primary: The Saxon Chronicle, sub ann.;William of Malmesbury, Gestal Regum, i. 141-157, RollsSeries, containing valuable original information (v. Stubbs'Introduction, II. lxvii.); Birch, Cartul. Saxon. vol.ii. Nos. 641-747; A.S. Laws. (ed. Liebermann), i. 146-183;AEthelweard, Florence of Worcester. Secondary: SaxonChronicle (ed. Plummer), vol. ii. pp. 132-142 D.N.B., s.v.
AETHELWEARD (ETHELWARD.) Anglo-Saxon historian, was the great-grandson of AEthelred, the brother of Alfred and ealdorman or earl of the western provinces (i.e. probably of the whole of Wessex). He first signs as dux or ealdorman in 973, and continues to sign until 998, about which time his death must have taken place. In the year 991 he was associated with archbishop Sigeric in the conclusion of a peace with the victorious Danes from Maldon, and in 994 he was sent with Bishop AElfheah (Alphege) of Winchester to make peace with Olaf at Andover. AEthelweard was the author of a Latin Chronicle extending to the year 975. Up to the year 892 he is largely dependent on the Saxon Chronicle, with a few details of his own; later he is largely independent of it. AEthelweard gave himself the bombastic title ``Patricius Consul Quaestor Ethelwerdus,'' and unfortunately this title is only too characteristic of the man. His narrative is highly rhetorical, and as he at the same time attempts more than Tacitean brevity his narrative is often very obscure. AEthelweard was the friend and patron of AElfric the grammarian.
AUTHORITIES.—-Primary: The Saxon Chronicle, 994 E; Birch,Cartularium Saxonicum; A.S. Laws (ed. Liebermann), pp. 220-224;Tabii Ethelwerdi Chron., Mon. Hist. Brit. 449-454. Secondary:Plummer, Saxon Chronicle, vol. ii. p. ci.; Napier and Stevenson,Crawford Charters, pp. 118-120; D.N.B., s.v. (A. law.)
AETHELWULF, king of the West Saxons, succeeded his father Ecgberht in A.D. 839. It is recorded in the Saxon Chronicle for 825 that he was sent with Eahlstan, bishop of Sherborne, and the ealdorman Wulfheard to drive out Baldred, king of Kent, which was successfully accomplished. On the accession of AEthelwulf, AEthelstan, his son or brother, was made sub-king of Kent, Surrey, Sussex and Essex. AEthelwulf's reign was chiefly occupied with struggles against the Danes. After the king's defeat 843-844, the Somerset and Dorset levies won a victory at the mouth of the Parret, c. 850. In 851 Ceorl, with the men of Devon, defeated the Danes at Wigganburg, and AEthelstan of Kent was victorious at Sandwich, in spite of which they wintered in England that year for the first time. In 851 also AEthelwulf and AEthelbald won their great victory at Aclea, probably the modern Ockley. In 853 AEthelwulf subdued the North Welsh, in answer to the appeal of Burgred of Mercia, and gave him his daughter AEthelswith in marriage. 855 is the year of the Donation of AEthelwulf and of his journey to Rome with Alfred. On his way home he married Judith, daughter of Charles the Bald. According to Asser he was compelled to give up Wessex to his son AEthelbald on his return, and content himself with the eastern sub-kingdom. He died in 858.
Chronicle, s.a. 823, 836, 840, 851, 853, 855. (F. G. M. B.)
AETHER, or ETHER (Gr. aither, probably from aitho, burn, though Plato in his Cratylus (41O B) derives the name from its perpetual motion— oti aei thei peri ton aera reon, aeitheer dikaios an kaloito), a material substance of a more subtle kind than visible bodies, supposed to exist in those parts of space which are apparently empty.
``The hypothesis of an aether has been maintained by different speculators for very different reasons. To those who maintained the existence of a plenum as a philosophical principle, nature's abhorrence of a vacuum was a sufficient reason for imagining an all-surrounding aether, even though every other argument should be against it. To Descartes, who made extension the sole essential property of matter, and matter a necessary condition of extension, the bare existence of bodies apparently at a distance was a proof of the existence of a continuous medium between them. But besides these high metaphysical necessities for a medium, there were more mundane uses to be fulfilled by aethers. Aethers were invented for the planets to swim in, to constitute electric atmospheres and magnetic effluvia, to convey sensations from one part of our bodies to another, and so on, till all space had been filled three or four times over with aethers. It is only when we remember the extensive and mischievous influence on science which hypotheses about aethers used formerly to exercise, that we can appreciate the horror of aethers which sober-minded men had during the 18th century, and which, probably as a sort of hereditary prejudice, descended even to John Stuart Mill. The disciples of Newton maintained that in the fact of the mutual gravitation of the heavenly bodies, according to Newton's law, they had a complete quantitative account of their motions; and they endeavoured to follow out the path which Newton had opened up by investigating and measuring the attractions and repulsions of electrified and magnetic bodies, and the cohesive forces in the interior of bodies, without attempting tdraccount for these forces. Newton himself, however, endeavoured to account for gravitation by differences of pressure in an aether; but he did not publish his theory, `because he was not able from experiment and observation to give a satisfactory account of this medium, and the manner of its operation in producing the chief phenomena of nature.' On the other hand, those who imagined aethers in order to explain phenomena could not specify the nature of the motion of these media, and could not prove that the media, as imagined by them, would produce the effects they were meant to explain. The only aether which has survived is that which was invented by Huygens to explain the propagation of light. The evidence for the existence of the luminiferous aether has accumulated as additional phenomena of light and other radiations have been discovered; and the properties of this medium, as deduced from the phenomena of light, have been found to be precisely those required to explain electromagnetic phenomena.''
This description, quoted from James Clerk Maxwell's article in the 9th edition of the Encyclopaedia Britannica, represents the historical position of the subject up till about 1860, when Maxwell began those constructive speculations in electrical theory, based on the influence of the physical views of Faraday and Lord Kelvin, which have in their subsequent development largely transformed theoretical physics into the science of the aether.
In the remainder of the article referred to, Maxwell reviews the evidence for the necessity of an aether, from the fact that light takes time to travel, while it cannot travel as a substance, for if so two interfering lights could not mask each other in the dark fringes (see INTERFERENCE OF LIGHT.) Light is therefore an influence propagated as wave-motion, and moreover by transverse undulations, for the reasons brought out by Thomas Young and Augustin Fresnel; so that the aether is a medium which possesses elasticity of a type analogous to rigidity. It must be very different from ordinary matter as we know it, for waves travelling in matter constitute sound, which is propagated hundreds of thousands of times slower than light.
If we suppose that the aether differs from ordinary matter in degree but not in kind, we can obtain some idea of its quality from a knowledge of the velocity of radiation and of its possible intensity near the sun, in a manner applied long ago by Lord Kelvin (Trans. R. S. Edin. xxi. 1854). According to modern measurements the solar radiation imparts almost 3 gramme-calories of energy per minute per square centimetre at the distance of the earth, which is about 1.3X106 ergs per sec. per cm.2 The energy in sunlight per cubic cm. just outside the earth's atmosphere is therefore about 4X10-5 ergs; applying the law of inverse squares the value near the sun's surface would be 1.8 ergs. Let E be the effective elasticity of the aether; then E=rc2, where r is its density, and c the velocity of light which is 3X1010 cm./sec. If x=A cosn (t-x/c) is the linear vibration, the stress is E dx/dx; and the total energy, which is twice the kinetic energy 1/2r(dx/dt)2dx, is 1/2rn2 A2 per cm., which is thus equal to 1.8 ergs as above. law l=2pc/n, so that if A/l=k, we have 1/2r(2pck)2= 1.8, giving r=10-22k-2 and E=10-1k-2. Lord Kelvin assumed as a superior limit of k, the ratio of amplitude to wave-length, the value 10-2, which is a very safe limit. It follows that the density of the aether must exceed 10-18, and its elastic modulus must exceed 103, which is only about 10-8 of the modulus of rigidity of glass. It thus appears that if the amplitude of vibration could be as much as 10-2 of the wave-length, the aether would be an excessively rare medium with very slight elasticity; and yet it would be capable of transmitting the supply of solar energy on which all terrestrial activity depends. But on the modern theory, which includes the play of electrical phenomena as a function of the aether, there are other considerations which show that this number 10-2 is really an enormous overestimate; and it is not impossible that the co-efficient of ultimate inertia of the aether is greater than the co-efficient of inertia (of different kind) of any existing material substance.
The question of whether the aether is carried along by the earth's motion has been considered from the early days of the undulatory theory of light. In reviving that theory at the beginning of the 19th century, Thomas Young stated his conviction that material media offered an open structure to the substance called aether, which passed through them without hindrance ``like the wind through a grove of trees.'' Any convection of that medium could be tested by the change of effective velocity of light, which would be revealed by a prism as was suggested by F. J. D. Arago. Before 1868 Maxwell conducted the experiment by sending light from the illuminated cross-wires of an observing telescope forward through the object-glass, and through a train of prisms, and then reflecting it back along the same path; any influence of convection would conspire in altering both refractions, but yet no displacement of the image depending on the earth's motion was detected. As will be seen later, modern experiments have confirmed the entire absence of any effect, such as convection would produce, to very high precision. It has further been verified by Sir Oliver Lodge that even in very narrow spaces the aether is not entrained by its surroundings when they are put into rapid motion.
A train of ideas which strongly impressed itself on Clerk Maxwell's mind, in the early stages of his theoretical views, was put forward by Lord Kelvin in 1858; he showed that the special characteristics of the rotation of the plane of polarization, discovered by Faraday in light propagated along a magnetic field, viz. that it is doubled instead of being undone when the light retraces its path, requires the operation of some directed agency of a rotational kind, which must be related to the magnetic field. Lord Kelvin was thereby induced to identify magnetic force with rotation, involving, therefore, angular momentum in the aether. Modern theory accepts the deduction, but ascribes the momentum to the revolving ions in the molecules of matter traversed by the light; for the magneto-optic effect is present only in material media. Long previously Lord Kelvin himself came nearer this view, in offering the opinion that magnetism consisted, in some way, in the angular momentum of the material molecules, of which the energy of irregular translations constitutes heat; but the essential idea of moving electric ions of both kinds, positive and negative, in the molecules had still to be introduced.
The question of the transparency of the celestial spaces presents itself in the presebt connexion. Light from stars at unfathomable distances reaches us in such quantity as to suggest that space itself is absolutely transparent, leaving open the question as to whether there is enough matter scattered through it to absorb a sensible part of the light in its journey of years from the luminous body. If the aether were itself constituted of discrete molecules, on the model of material bodies, such transparency would not be conceivable. We must be content to treat the aether as a plenum, which places it in a class by itself; and we can thus recognize that it may behave very differently from matter, though in some manner consistent with itself—-a remark which is fundamental in the modern theory.
Action across a Distance contrasted with Transmitted Action.—In the mechanical processes which we can experimentally modify at will, and which therefore we learn to apprehend with greatest fulness, whenever an effect on a body, B, is in causal connexion with a process instituted in another body, A, it is usually possible to discover a mechanical connexion between the two bodies which allows the influence of A to be traced all the way across the intervening region. The question thus arises whether, in electric attractions across apparently empty space and in gravitational attraction across the celestial regions, we are invited or required to make search for some similar method of continuous transmission of the physical effect, or whether we should rest content with an exact knowledge of the laws according to which one body affects mechanically another body at a distance. The view that our knowledge in such cases may be completely represented by means of laws of action at a distance, expressible in terms of the positions (and possibly motions) of the interacting bodies without taking any heed of the intervening space, belongs to modern times. It could hardly have been thought of before Sir Isaac Newton's discovery of the actual facts regarding universal gravitation. Although, however, gravitation has formed the most perfect instance of an influence completely expressible, up to the most extreme refinement of accuracy, in terms of laws of direct action across space, yet, as is well known, the author of this ideally simple and perfect theory held the view that it is not possible to conceive of direct mechanical action independent of means of transmission. In this belief he differed from his pupil, Roger Cotes, and from most of the great mathematical astronomers of the 18th century, who worked out in detail the task sketched by the genius of Newton. They were content with a knowledge of the truth of the principle of gravitation; instead of essaying to explain it further by the properties of a transmitting medium, they in fact modelled the whole of their natural philosophy on that principle, and tried to express all kinds of material interaction in terms of laws of direct mechanical attraction across space. If material systems are constituted of discrete atoms, separated from each other by many times the diameter of any of them, this simple plan of exhibiting their interactions in terms of direct forces between them would indeed be exact enough to apply to a wide range of questions, provided we could be certain that the laws of the forces depended only on the positions and not also on the motions of the atoms. The most important example of its successful application has been the theory of capillary action elaborated by P. S. Laplace; though even here it appeared, in the hands of Young, and in complete fulness afterwards in those of C. F. Gauss, that the definite results attainable by the hypothesis of mutual atomic attractions really reposed on much wider and less special principles—-those, namely, connected with the modern doctrine of energy.
Idea of an Aether.—-The wider view, according to which the hypothesis of direct transmission of physical influences expresses only part of the facts, is that all space is filled with physical activity, and that while an influence is passing across from a body, A, to another body, B, there is some dynamical process in action in the intervening region, though it appears to the senses to be mere empty space. The problem is whether we can represent the facts more simply by supposing the intervening space to be occupied by a medium which transmits physical actions, after the manner that a continuous material medium, solid or liquid, transmits mechanical disturbance. Various analogies of this sort are open to us to follow up: for example, the way in which a fluid medium transmits pressure from one immersed solid to another—or from one vortex ring belonging to the fluid to another, which is a much wider and more suggestive case; or the way in which an elastic fluid like the atmosphere transmits sound; or the way in which an elastic solid transmits waves of transverse as well as longitudinal displacement. It is on our familiarity with modes of transmission such as these, and with the exact analyses of them which the science of mathematical physics has been able to make, that our predilection for filling space with an aethereal transmitting medium, constituting a universal connexion between material bodies, largely depends; perhaps ultimately it depends most of all, like all our physical conceptions, on the intimate knowledge that we can ourselves exert mechanical effect on outside bodies only through the agencies of our limbs and sinews. The problem thus arises: Can we form a consistent notion of such a connecting medium? It must be a medium which can be effective for transmitting all the types of physical action known to us; it would be worse than no solution to have one medium to transmit gravitation, another to transmit electric effects, another to transmit light, and so on. Thus the attempt to find out a constitution for the aether will involve a synthesis of intimate correlation of the various types of physical agencies, which appear so different to us mainly because we perceive them through different senses. The evidence for this view, that all these agencies are at bottom connected together and parts of the same scheme, was enormously strengthened during the latter half of the 19th century by the development of a relation of simple quantitative equivalence between them; it has been found that we can define quantities relating to them, under the names of mechanical energy, electric energy, thermal energy, and so on, so that when one of them disappears, it is replaced by the others to exactly equal amount. This single principle of energy has transformed physical science by making possible the construction of a network of ramifying connexions between its various departments; it thus stimulates the belief that these constitute a single whole, and encourages the search for the complete scheme of interconnexion of which the principle of energy and the links which it suggests form only a single feature.
In carrying out this scientific procedure false steps will from time to time be made, which will have to be retraced, or rather amended; but the combination of experimental science with theory has elevated our presumption of the rationality of all natural processes, so far as we can apprehend them at all, into practical certainty; so that, though the mode of presentation of the results may vary from age to age, it is hardly conceivable that the essentials of the method are not of permanent validity.
Atomic Structure of Matter.—-The greatest obstacle to such a search for the fundamental medium is the illimitable complexity of matter, as contrasted with the theoretical simplicity and uniformity of the physical agencies which connect together its different parts. It has been maintained since the times of the early Greek philosophers, and possibly even more remote ages, that matter is constituted of independent indestructible units, which cannot ever become divided by means of any mutual actions they can exert. Since the period, a century ago, when Dalton and his contemporaries constructed from this idea a scientific basis for chemistry, the progress of that subject has been wonderful beyond any conception that could previously have been entertained; and the atomic theory in some form appears to be an indispensable part of the framework of physical science. Now this doctrine of material atoms is an almost necessary corollary to the doctrine of a universal aether. For if we held that matter is continuous, one of two alternatives would be open. We might consider that matter and aether can coexist in the same space; this would involve the co-existence and interaction of a double set of properties, introducing great complication, which would place any coherent scheme of physical action probably beyond the powers of human analysis. Or we might consider that aether exists only where matter is not, thus making it a very rare and subtle and elastic kind of matter; then we should have to assign these very properties to the matter itself where it replaces aether, in addition to its more familiar properties, and the complication would remain. The other course is to consider matter as formed of ultimate atoms, each the nucleus or core of an intrinsic modification impressed on the siurounding region of the aether; this might conceivably be of the nature of vortical motion of a liquid round a ring-core, thus giving a vortex atom, or of an intrinsic strain of some sort radiating from a core, which would give an electric atom. We recognize an atom only through its physical activities, as manifested in its interactions with other atoms at a distance from it; this field of physical activity would be identical with the surrounding field of aethereal motion or strain that is inseparably associated with the nucleus, and is carried on along with it as it moves. Here then we have the basis of a view in which there are not two media to be considered, but one medium, homogeneous in essence and differentiated as regards its parts only by the presence of nuclei of intrinsic strain or motion—-in which the physical activities of matter are identified with those arising from the atmospheres of modified aether which thus belong to its atoms. As regards laws of general physical interactions, the atom is fully represented by the constitution of this atmosphere, and its nucleus may be left out of our discussions; but in the problems of biology great tracts of invariable correlations have to be dealt with, which seem hopelessly more complex than any known or humanly possible physical scheme. To make room for these we have to remember that the atomic nucleus has remained entirely undefined and beyond our problem; so that what may occur, say when two molecules come into close relations, is outside physical science—-not, however, altogether outside, for we know that when the vital nexus in any portion of matter is dissolved, the atoms will remain, in their number, and their atmospheres, and all inorganic relations, as they were before vitality supervened.
Nature of Properties of Material Bodies.—-It thus appears that the doctrine of atomic material constitution and the doctrine of a universal aether stand to each other in a relation of mutual support; if the scheme of physical laws is to be as precise as observation and measurement appear to make it, both doctrines are required in our efforts towards synthesis. Our direct knowledge of matter can, however, never be more than a rough knowledge of the general average behaviour of its molecules; for the smallest material speck that is sensible to our coarse perceptions contains myriads of atoms. The properties of the most minute portion of matter which we can examine are thus of the nature of averages. We may gradually invent means of tracing more and more closely the average drifts of translation or orientation, or of changes of arrangement, of the atoms; but there will always remain an unaveraged residue devoid of any recognized regularity, which we can only estimate by its total amount. Thus, if we are treating of energy, we can separate out mechanical and electric and other constituents in it; and there will be a residue of which we know nothing except its quantity, and which we call thermal. This merely thermal energy—which is gradually but very slowly being restricted in amount as new subsidiary organized types become recognized in it—though transmutable in equivalent quantities with the other kinds, yet is so only to a limited extent; the tracing out of the laws of this limitation belongs to the science of thermodynamics. It is the business of that science to find out what is the greatest amount of thermal energy that can possibly be recoverable into organized kinds under given circumstances. The discovery of definite laws in this region might at first sight seem hopeless; but the argument rests on an implied postulate of stability and continuity of constitution of material substances, so that after a cycle of transformations we expect to recover them again as they were originally—-on the postulate, in fact, that we do not expect them to melt out of organized existence in our hands. The laws of thermodynamics, including the fundamental principle that a physical property, called temperature, can be defined, which tends towards uniformity, are thus relations between the properties of types of material bodies that can exist permanently in presence of each other; why they so maintain themselves remains unknown, but the fact gives the point d'appui. The fundamental character of energy in material systems here comes into view; if there were any other independent scalar entity, besides mass and energy, that pervaded them with relations of equivalence, we should expect the existence of yet another set of pualities analogous to those connected with temperature. (See ENERGETICS.)
Returning now to the aether, on our present point of view no such complications there arise; it must be regarded as a continuous uniform medium free from any complexities of atomic aggregation, whose function is confined to the transmission of the various types of physical effect between the portions of matter. The problem of its constitution is thus one which can be attacked and continually approximated to, and which may possibly be definitely resolved. It has to be competent to transmit the transverse waves of light and electricity, and the other known radiant and electric actions; the way in which this is done is now in the main known, though there are still questions as to the mode of expression and formulation of our knowledge, and also as regards points of detail. This great advance, which is the result of the gradual focussing of a century's work in the minute exploration of the exact laws of optical and electric phenomena, clearly carries with it deeper insight into the physical nature of matter itself and its modes of inanimate interaction.
If we rest on the synthesis here described, the energy of the matter, even the thermal part, appears largely as potential energy of strain in the aether which interacts with the kinetic energy associated with disturbances involving finite velocity of matter. It may, however, be maintained that an ultimate analysis would go deeper, and resolve all phenomena of elastic resilience into consequences of the kinetic stability of steady motional states, so that only motions, but not strains, would remain. On such a view the aether might conceivably be a perfect fluid, its fundamental property of elastic reaction arising (as at one time suggested by Kelvin and G. F. Fitzgerald) from a structure of tangled or interlaced vortex filaments pervading its substance, which might conceivably arrange themselves into a stable configuration and so resist deformation. This raises the further question as to whether the transmission of gravitation can be definitely recognized among the properties of an ultimate medium; if so, we know that it must be associated with some feature, perhaps very deep-seated, or on the other hand perhaps depending simply on incompressibility, which is not sensibly implicated in the electric and optical activities. With reference to all such further refinements of theory, it is to be borne in mind that the perfect fluid of hydrodynamic analysis is not a merely passive inert plenum; it is also a continuum with the property that no finite internal slip or discontinuity of motion can ever arise in it through any kind of disturbance; and this property must be postulated, as it cannot be explained.
Motion of Material Atoms through the Aether.—An important question arises whether, when a material body is moved through the aether, the nucleus of each atom carries some of the surrounding aether along with it; or whether it practically only carries on its strain-form or physical atmosphere, which is transferred from one portion of aether to another after the manner of a shadow, or rather like a loose knot which can slip along a rope without the rope being required to go with it. We can obtain a pertinent illustration from the motion of a vortex ring in a fluid; if the circular core of the ring is thin compared with its diameter, and the vorticity is not very great, it is the vortical state of motion that travels across the fluid without transporting the latter bodily with it except to a slight extent very close to the core. We might thus examine a structure formed of an aggregation of very thin vortex rings, which would move across the fluid without sensibly disturbing it; on the other hand, if formed of stronger vortices, it may transport the portion of the fluid that is within, or adjacent to, its own structure along with it as if it were a solid mass, and therefore also push aside the surrounding fluid as it passes. The motion of the well-known steady spherical vortex is an example of the latter case.
Convection of Optical Waves.—The nature of the motion, if any, that is produced in the surrounding regions of the aether by the translation of matter through it can be investigated by optical experiment. The obvious body to take in the first instance is the earth itself, which on account of its annual orbital motion is travelling through space at the rate of about 18 miles per second. If the surrounding aether is thereby disturbed, the waves of light arriving from the stars will partake of its movement; the ascertained phenomena of the astronomical aberration of light show that the rays travel to the observer, across this disturbed aether near the earth, in straight lines. Again, we may split a narrow beam of light by partial reflexion from a transparent plate, and recombine the constituent beams after they have traversed different circuits of nearly equivalent lengths, so as to obtain interference fringes. The position of these fringes will depend on the total retardation in time of the one beam with respect to the other; and thus it might be expected to vary with the direction of the earth's motion relative to the apparatus. But it is found not to vary at all, even up to the second order of the ratio of the earth's velocity to that of light. It has in fact been found, with the very great precision of which optical experiment is capable, that all terrestrial optical phenomena—reflexion, refraction, polarization linear and circular, diffraction —are entirely unaffected by the direction of the earth's motion, while the same result has recently been extended to electrostatic forces; and this is our main experimental clue.
We pass on now to the theory. We shall make the natural supposition that motion of the aether, say with velocity (u,v,w) at the point (x,y,z), is simply superposed on the velocity V of the optical undulations through that medium, the latter not being intrinsically altered. Now the direction and phase of the light are those of the ray which reaches the eye; and by Fermat's principle, established by Huygens for undulatory motion, the path of a ray is that track along which the disturbance travels in least time, in the restricted sense that any alteration of any short reach of the path will increase the time. Thus the path of the ray when the aether is at rest is the curve which makes Integralds/V least; but when it is in motion it is the curve which makes Integralds/(V+lu+my+nw) least, where (l,m,n) is the direction vector of ds. The latter integral becomes, on expanding in a series,
Integralds/V - Integral(udx + vdy + wdz)/V2 + Integral(udx + vdy + wdz)2/V3 + …,
since lds=dx. If the path is to be unaltered by the motion of the aether, as the law of astronomical aberration suggests, this must differ from Integralds/V by terms not depending on the path—that is, by terms involving only the beginning and end of it. In the case of the free aether V is constant; thus, if we neglect squares like (u/V)2, the condition is that udx + vdy + wdz be the exact differential of some function f. If this relation is true along all paths, the velocity of the aether must be of irrotational type, like that of frictionless fluid. Moreover, this is precisely the condition for the absence of interference between the component of a split beam; because, the time of passage being to the first order
Integralds/V - Integral(udx + vdy + wdz)/V2
the second term will then be independent of the path (f being a single valued function) and therefore the same for the paths of both the interfering beams. If therefore the aether can be pnt into motion, we conclude (with Stokes) that such motion, in free space, must be of strictly irrotational type.
But our experimental data are not confined to free space. if c is the velocity of radiation in free space and m the refractis'e index of a transparent body, V=C/m; thus it is the expression c-2Integralm2(u'dx + v'dy + w'dz) that is to be integrable explicitly, where now (u',v',w') is what is added to V owing to the velocity (u,v,w) of the medium. As, however, our terrestrial optical apparatus is now all in motion along with the matter, we must deal with the rays relative to the moving system, and to these also Fermat's principle clearly applies; thus V + (lu' + mv' + nw') is here the velocity of radiation in the direction of the ray, but relative to the moving material system. Now the expression above given cannot be integrable exactly, under all circumstances and whatever be the axes of co-ordinates, unless (m2u',m2v',m2w') is the gradient of a continuous function. In the simplest case, that of uniform translation, these components of the gradient will each be constant throughout the region; at a distant place in free aether where there is no motion, they must thus be equal to -u,-v, -w, as they refer to axes moving with the matter. Hence the paths and times of passage of all rays relative to the material system will not be altered by a uniform motion of the system, provided the velocity of radiation relative to the system, in material of index m, is diminished by m-2 times the velocity of the system in the direction of the radiation, that is, provided the absolute velocity of radiation is increased by 1 - m-2 times the velocity of the material system; this involves that the free aether for which m is unity shall remain at rest. This statement constitutes the famous hypothesis of Fresnel, which thus ensures that all phenomena of ray-path and refraction, and all those depending on phase, shall be unaffected by uniform convection of the material medium, in accordance with the results of experiment.
Is the Aether Stationary or mobile?—-This theory secures that the times of passage of the rays shall be independent of the motion of the system, only up to the first order of the ratio of its velocity to that of radiation. But a classical experiment of A. A. Michelson, in which the ray-path was wholly in air, showed that the independence extends to higher orders. This result is inconsistent with the aether remaining at rest, unless we assume that the dimensions of the moving system depend, though to an extent so small as to be not otherwise detectable, on its orientation with regard to the aether that is streaming through it. It is, however, in complete accordance with a view that would make the aether near the earth fully partake in its orbital motion—-a view which the null effect of convection on all terrestrial optical and electrical phenomena also strongly suggests. But the aether at a great distance must in any case be at rest; while the facts of astronomical aberration require that the motion of that medium must be irrotational. These conditions cannot be consistent with sensible convection of the aether near the earth without involving discontinuity in its motion at some intermediate distance, so that we are thrown back on the previous theory.
Another powerful reason for taking the aether to be stationary is afforded by the character of the equations of electrodynamics; they are all of linear type, and superposition of effects is possible. Now the kinetics of a medium in which the parts can have finite relative motions will lead to equations which are not linear—-as, for example, those of hydrodynamics—-and the phenomena will be far more complexly involved. It is true that the theory of vortex rings in hydrodynamics is of a simpler type; but electric currents cannot be likened to permanent vortex rings, because their circuits can be broken and the element of cyclic steadiness on which the simplicity depends is thereby destroyed.
Dynamical Theories of the Aether.—-The analytical equations which represent the propagation of light in free aether, and also in aether modified by the presence of matter, were originally developed on the analogy of the equations of propagation of elastic effects in solid media. Various types of elastic solid medium have thus been invented to represent the aether, without complete success in any case. In T. Maccullagh's hands the correct equations were derived from a single energy formula by the principle of least action; and while the validity of this dynamical method was maintained, it was frankly admitted that no mechanical analogy was forthcoming. When Clerk Maxwell pointed out the way to the common origin of optical and electrical phenomena, these equations naturally came to repose on an electric basis, the connexion having been first definitely exhibited by Fitzgerald in 1878; and according as the independent variable was one or other of the vectors which represent electric force, magnetic force or electric polarity, they took the form appropriate to one or other of the elastic theories above mentioned.
In this place it must suffice to indicate the gist of the more recent developments of the electro-optical theory, which involve the dynamical verification of Fresnel's hypothesis regarding optical convection and the other relations above described. The aether is taken to be at rest; and the strain-forms belonging to the atoms are the electric fields of the intrinsic charges, or electrones, involved in their constitution. When the atoms are in motion these strain-forms produce straining and unstraining in the aether as they pass across it, which in its motional or kinetic aspect constitutes the resulting magnetic field; as the strains are slight the coefficient of ultimate inertia here involved must be great. True electric current arises solely from convection of the atomic charges or electrons; this current is therefore not restricted as to form in any way. But when the rate of change of aethereal strain——that is, of (f,g,h) specified as Maxwell's electric displacement in free aether—-is added to it, an analytically convenient vector (u,v,w) is obtained which possesses the characteristic property of being circuital like the flow of an incompressible fluid, and has therefore been made fundamental in the theory by Maxwell under the name of the total electric current.
As already mentioned, all efforts to assimilate optical propagation to transmission of waves in an ordinary solid medium have failed; and though the idea of regions of intrinsic strain, as for example in unannealed glass, is familiar in physics, yet on account of the absence of mobility of the strain no attempt had been made to employ them to illustrate the electric fields of atomic charges. The idea of Maccullagh's aether, and its property of purely rotational elasticity which had been expounded objectively by W. J. M. Rankine, was therefore much vivified by Lord Kelvin's specification (Comptes Rendus, 1889) of a material gyrostatically constituted medium which would possess this character. More recently a way has been pointed out in which a mobile permanent field of electric force could exist in such a medium so as to travel freely in company with its nucleus or intrinsic charge—-the nature of the mobility of the latter, as well as its intimate constitution, remaining unknown.
A dielectric substance is electrically polarized by a field of electric force, the atomic poles being made up of the displaced positive and negative intrinsic charges in the atom: the polarization per unit volume (f',g',h') may be defined on the analogy of magnetism, and d/dt(f', g', h') thus constitutes truo electric current of polarization, i.e. of electric separation in the molecules, specified per unit volume. The convection of a medium thus polarized involves electric disturbance, and therefore must contribute to the true electric current; the determination of this constituent of the current is the most delicate point in the investigation. The usual definition of the component current in any direction, as the net amount of electrons which crosses, towards the positive side, an element of surface fixed in space at right angles to that direction, per unit area per unit time, here gives no definite result. The establishment and convection of a single polar atom constitutes in fact a quasi-magnetization, in addition to the polarization current as above defined, the negative poles completing the current circuits of the positive ones. But in the transition from molecular theory to the electrodynamics of extended media, all magnetism has to be replaced by a distribution of current; the latter being now specified by volume as well as by flow so that (u,v,w) dt is the current in the element of volume dt. In the present case the total dielectric contribution to this current works out to be the change per unit time in the electric separation in the molecules of the element of volume, as it moves uniformly with the matter, all other effects being compensated molecularly without affecting the propagation.1 On subtracting from this total the current of establishment of polarization d/dt/(f,g',h') as formulated above, there remains vd/dx(f',g',h') as the current of convection of polarization when the convection is taken for simplicity to be in the direction of the axis of x with velocity v. The polarization itself is determined from the electric force (P,Q,R) by the usual statical formula of linear type which becomes tor an isotropic medium
(f',g',h') = ((K-1)/4pc2)(P,Q,R),
because any change of the dielectric constant K arising from the convection of the material through the aether must be independent of the sign of v and therefore be of the second order. Now the electric force (P,Q,R) is the force acting on the electrons of the medium moving with velocity v; consequently by Faraday's electrodynamic law
(P,Q,R) = (P',Q' - vc, R'- vb)
where (P', Q', R') is the force that would act on electrons at rest, and (a,b,c) is the magnetic induction. The latter force is, by Maxwell's hypothesis or by the dynamical theory of an aether pervaded by electrons, the same as that which strains the aether, and may be called the aethereal force; it thereby produces an aethereal electric displacement, say (y,g,h), according to the relation
(f,g,h) = (4pc2) - (P', Q', R'),
in which c is a constant belonging to the aether, which turns out to be the velocity of light. The current of aethereal displacement d/dt(f,g,h) is what adds on to the true electric current to produce the total circuital current of Maxwell.