Chapter 17

Very elaborate observations have been made during several Arctic expeditions of the azimuths of the summits of auroral arcs. At Cape Thorsden (7) in 1882-1883 the mean azimuth derived from 371 arcs was 24° 12′ W., or 11° 27′ to the W. of the magnetic meridian. As to the azimuths in individual cases, 130 differed from the mean by less than 10°, 118 by from 10° to 20°, 82 by from 20° to 30°, 21 by from 30° to 40°, 14 by from 40° to 50°; in six cases the departure exceeded 50°, and in one case it exceeded 70°. Also, whilst the mean azimuths deduced from the observations between 6A.M.and noon, between noon and 6P.M., and between 6P.M.and midnight, were closely alike, their united mean being 22.4° W. of N. (or E. of S.), the mean derived from the 113 arcs observed between midnight and 6A.M.was 47.8° W. At Jan Mayen (8) in 1882-1883 the mean azimuth of the summit of the arcs was 28.8° W. of N., thus approaching much more closely to the magnetic meridian 29.9° W. As to individual azimuths, 113 lay within 10° of the mean, 37 differed by from 10° to 20°, 18 by from 20° to 30°, 6 by from 30° to 40°, whilst 6 differed by over 40°. Azimuths were also measured at Jan Mayen for 338 auroral bands, the mean being 22.0° W., or 7.9° to the east of the magnetic meridian. Combining the results from arcs and bands, Carlheim-Gyllensköld gives the “anomaly” of the auroral meridian at Jan Mayen as 5.7° E. At the British Polar station of 1882, Fort Rae (62° 23′ N. lat., 115° 44′ W. long.), he makes it 15.7° W. At Godthaab in 1882-1883 the auroral anomaly was, according to Paulsen, 15.5° E., the magnetic meridian lying 57.6° W. of the astronomical.

14.Auroral Zenith.—Another auroral direction having apparently a close relation to terrestrial magnetism is the imaginary line drawn to the eye of an observer from the centre of the corona—i.e.the point to which the auroral rays converge. This seems in general to be nearly coincident with the direction of the dipping needle.

Thus at Cape Thorsden (7) in 1882-1883 the mean of a considerable number of observations made the angle between the two directions only 1° 7′, the magnetic inclination being 80° 35′, whilst the coronal centre had an altitude of 79° 55′ and lay somewhat to the west of the magnetic meridian. Even smaller mean values have been found for the angle between the auroral and magnetic “zeniths”—as the two directions have been called—e.g.0° 50′ at Bossekop (16) in 1838-1839, and 0° 7′ at Treurenberg (17) (79° 55′ N. lat., 16° 51′ E. long.) in 1899-1900.

15.Relations to Magnetic Storms.—That there is an intimate connexion between aurora when visible in temperate latitudes and terrestrial magnetism is hardly open to doubt. A bright aurora visible over a large part of Europe seems always accompanied by a magnetic storm and earth currents, and the largest magnetic storms and the most conspicuous auroral displays have occurred simultaneously. Noteworthy examples are afforded by the auroras and magnetic storms of August 28-29 and September 1-2, 1859; February 4, 1872; February 13-14 and August 12, 1892; September 9, 1898; and October 31, 1903. On some of these occasions aurora was brilliant in both the northern and southern hemispheres, whilst magnetic disturbances were experienced the whole world over. In high latitudes, however, where both auroras and magnetic storms are most numerous, the connexion between them is much less uniform. Arctic observers, both Danish and British, have repeatedly reported displays of aurora unaccompanied by any special magnetic disturbance. This has been more especially the case when the auroral light has been of a diffused character, showing only minor variability. When there has been much apparent movement, and brilliant changes of colour in the aurora, magnetic disturbance has nearly always accompanied it. In the Arctic, auroral displays seem sometimes to be very local, and this may be the explanation. On the other hand, Arctic observers have reported an apparent connexion of a particularly definite character. According to Paulsen (18), during the Ryder expedition in 1891-1892, the following phenomenon was seen at least twenty times by Lieut. Vedel at Scoresby Sound (70° 27′ N. lat., 26° 10′ W. long.). An auroral curtain travelling with considerable velocity would approach from the south, pass right overhead and retire to the north. As the curtain approached, the compass needle always deviated to the west, oscillated as the curtain passed the zenith, and then deviated to the east. The behaviour of the needle, as Paulsen points out, is exactly what it should be if the space occupied by the auroral curtain were traversed by electric currents directed upwards from the ground. The Danish observers at Tasiusak (10) in 1898-1899 observed this phenomenon occasionally in a slightly altered form. At Tasiusak the auroral curtain after reaching the zenith usually retired in the direction from which it had come. The direction in which the compass needle deviated was west or east, according as the curtain approached from the south or the north; as the curtain retired the deviation eventually diminished.

Kr. Birkeland (19). who has made a special study of magnetic disturbances in the Arctic, proceeding on the hypothesis that they arise from electric currents in the atmosphere, and who has thence attempted to deduce the position and intensity of these currents, asserts that whilst in the case of many storms the data were insufficient, when it was possible to fix the position of the mean line of flow of the hypothetical current relatively to an auroral arc, he invariably found the directions coincident or nearly so.

16. In the northern hemisphere to the south of the zone of greatest frequency, the part of the sky in which aurora most generally appears is the magnetic north. In higher latitudes auroras are most often seen in the south. The relative frequency in the two positions seems to vary with the hour, the type of aurora, probably with the season of the year, and possibly with the position of the year in the sun-spot cycle.

At Jan Mayen (8) in 1882-1883, out of 177 arcs whose position was accurately determined, 44 were seen in the north, their summits averaging 38.5° above the northern horizon; 88 were seen in the south, their average altitude above the southern horizon being 33.5°; while 45 were in the zenith. At Tasiusak (10) in 1898-1899 the magnetic directions of the principal types were noted separately. The results are given in Table VI.Table VI.Direction.Absolute Number for each Type.Percentagefrom allTypes.Arcs.Bands.Curtains.Rays.Patches.N.916515410N.E.91322049E.31122639S.E.5611076S.45431161524S.W.99212139W.31122269N.W.282855

Table VI.

Table VI. accounts for only 81% of the total displays; of the remainder 15% appeared in the zenith, while 4% covered the whole sky. Auroral displays generally cover a considerable area, and are constantly changing, so the figures are necessarily somewhat rough. But clearly, whilst the arcs and bands, and to a lesser extent the patches, showed a marked preference for the magnetic meridian, the rays showed no such preference.

At Cape Thorsden (7) in 1882-1883 auroras as a whole were divided into those seen in the north and those seen in the south. The variation throughout the twenty-four hours in the percentage seen in the south was as follows:—Hour.0-3.3-6.6-9.9-12.A.M.69554435P.M.55706565The mean from the whole twenty-four hours is sixty-three. Between 3A.M.and 3P.M.the percentage of auroras seen in the south thus appears decidedly below the mean.17. The following data for the apparent angular width of arcs were obtained at Cape Thorsden, the arcs being grouped according to the height of the lower edge above the horizon. Group I. contained thirty arcs whose altitudes did not exceed 11° 45′; Group II. thirty arcs whose altitudes lay between 12° and 35°; and Group III, thirty arcs whose altitudes lay between 36° and 80°.Group.I.II.III.Greatest width.11.5°12.0°21.0°Least width.1.0°0.75°2.0°Mean width.3.45°4.6°6.9°There is here a distinct tendency for the width to increase with the altitude. At the same time, arcs near the horizon often appeared wider than others near the zenith. Furthermore, Gyllensköld says that when arcs mounted, as they not infrequently did, from the horizon, their apparent width might go on increasing right up to thezenith, or it might increase until an altitude of about 45° was reached and then diminish, appearing much reduced when the zenith was reached. Of course the phenomenon might be due to actual change in the arc, but it is at least consistent with the view that arcs are of two kinds, one form constituting a layer of no great vertical depth but considerable real horizontal width, the other form having little horizontal width but considerable vertical depth, and resembling to some extent an auroral curtain.18. According to numerous observations made at Cape Thorsden, the apparent angular velocity of arcs increases on the average with their altitude. Dividing the whole number of arcs, 156, whose angular velocities were measured into three numerically equal groups, according to their altitude, the following were the results in minutes of arc per second of time (or degrees per minute of time):—Group.I.II.III.All.Mean altitude10.5°34.6°72.3°..Greatest velocity4.8115.12109.09..Mean velocity0.482.428.673.86Each group contained auroras which appeared stationary. The intervals to which the velocities referred were usually from five to ten minutes, but varied widely. The velocity 109.09 was much the largest observed, the next being 52.38; both were from observations lasting under half a minute.19. In 1882-1883 the direction of motion of arcs was from north to south in 62% of the cases at Jan Mayen, and in 58% of the cases at Cape Thorsden. This seems the more common direction in the northern hemisphere, at least for stations to the south of the zone of maximum frequency, but a considerable preponderance of movements towards the north was observed in Franz Joseph Land by the Austrian Expedition of 1872-1874. The apparent motion of arcs is sometimes of a complicated character. One end only, for example, may appear to move, as if rotating round the other; or the two ends may move in opposite directions, as if the arc were rotating about a vertical axis through its summit.

The mean from the whole twenty-four hours is sixty-three. Between 3A.M.and 3P.M.the percentage of auroras seen in the south thus appears decidedly below the mean.

17. The following data for the apparent angular width of arcs were obtained at Cape Thorsden, the arcs being grouped according to the height of the lower edge above the horizon. Group I. contained thirty arcs whose altitudes did not exceed 11° 45′; Group II. thirty arcs whose altitudes lay between 12° and 35°; and Group III, thirty arcs whose altitudes lay between 36° and 80°.

There is here a distinct tendency for the width to increase with the altitude. At the same time, arcs near the horizon often appeared wider than others near the zenith. Furthermore, Gyllensköld says that when arcs mounted, as they not infrequently did, from the horizon, their apparent width might go on increasing right up to thezenith, or it might increase until an altitude of about 45° was reached and then diminish, appearing much reduced when the zenith was reached. Of course the phenomenon might be due to actual change in the arc, but it is at least consistent with the view that arcs are of two kinds, one form constituting a layer of no great vertical depth but considerable real horizontal width, the other form having little horizontal width but considerable vertical depth, and resembling to some extent an auroral curtain.

18. According to numerous observations made at Cape Thorsden, the apparent angular velocity of arcs increases on the average with their altitude. Dividing the whole number of arcs, 156, whose angular velocities were measured into three numerically equal groups, according to their altitude, the following were the results in minutes of arc per second of time (or degrees per minute of time):—

Each group contained auroras which appeared stationary. The intervals to which the velocities referred were usually from five to ten minutes, but varied widely. The velocity 109.09 was much the largest observed, the next being 52.38; both were from observations lasting under half a minute.

19. In 1882-1883 the direction of motion of arcs was from north to south in 62% of the cases at Jan Mayen, and in 58% of the cases at Cape Thorsden. This seems the more common direction in the northern hemisphere, at least for stations to the south of the zone of maximum frequency, but a considerable preponderance of movements towards the north was observed in Franz Joseph Land by the Austrian Expedition of 1872-1874. The apparent motion of arcs is sometimes of a complicated character. One end only, for example, may appear to move, as if rotating round the other; or the two ends may move in opposite directions, as if the arc were rotating about a vertical axis through its summit.

20.Height.—If an auroral arc represented a definite self-luminous portion of space of small transverse dimensions at a uniform height above the ground, its height could be accurately determined by observations made with theodolites at the two ends of a measured base, provided the base were not too short compared to the height. If a very long base is taken, it becomes increasingly open to doubt whether the portions of space emitting auroral light to the observers at the two ends are the same. There is also difficulty in ensuring that the observations shall be simultaneous, an important matter especially when the apparent velocity is considerable. If the base is short, definite results can hardly be hoped for unless the height is very moderate. Amongst the best-known theodolite determinations of height are those made at Bossekop in Norway by the French Expedition of 1838-1839 (16) and the Norwegian Expedition of 1882-1883, and those made in the latter year by the Swedes at Cape Thorsden and the Danes at Godthaab. At Bossekop and Cape Thorsden there were a considerable proportion of negative or impossible parallaxes. Much the most consistent results were those obtained at Godthaab by Paulsen (15). The base was 5.8 km. (about 3½ miles) long, the ends being in the same magnetic meridian, on opposite sides of a fiord, and observations were confined to this meridian, strict simultaneity being secured by signals. Heights were calculated only when the observed parallax exceeded 1°, but this happened in three-fourths of the cases. The calculated heights—all referring to the lowest border of the aurora—varied from 0.6 to 67.8 km. (about 0.4 to 42 m.), the average being about 20 km. (12 m.). Regular arcs were selected in most cases, but the lowest height obtained was for a collection of rays forming a curtain which was actually situated between the two stations.

In 1885 Messrs Garde and Eherlin made similar observations at Nanortalik near Cape Farewell in Greenland, but using a base of only 1250 metres (about ¾ m.). Their results were very similar to Paulsen’s. On one occasion twelve observations, extending over half an hour, were made on a single arc, the calculated heights varying in a fairly regular fashion from 1.6 to 12.9 km. (about 1 to 8 m.). The calculated horizontal distances of this arc varied between 5 and 24 km. (about 3 and 15 m.), the motion being sometimes towards, sometimes away from the observers, but not apparently exceeding 3 km. (nearly 2 m.) per minute. Heights of arcs have often been calculated from the apparent altitudes at stations widely apart in Europe or America. The heights calculated in this way for the under surface of the arc, have usually exceeded 100 m.; some have been much in excess of this figure. None of the results so obtained can be accepted without reserve, but there are several reasons for believing that the average height in Greenland is much below that in lower latitudes. Heights have been calculated in various less direct ways, by observing for instance the angular altitude of the summit of an arc and the angular interval between its extremities, and then making some assumption such as that the portion visible to an observer may be treated as a circle whose centre lies over the so-called auroral pole. The mean height calculated at Arctic stations, where careful observations have been made, in this or analogous ways, has varied from 58 km. (about 36 m.) at Cape Thorsden (Gyllensköld) to 227 km. (about 141 m.) at Bossekop (Bravais). The height has also been calculated on the hypothesis that auroral light has its source where the atmospheric pressure is similar to that at which most brilliancy is observed when electric discharges pass in vacuum tubes. Estimates on this basis have suggested heights of the order of 50 km. (about 31 m.). There are, of course, many uncertainties, as the conditions of discharge in the free atmosphere may differ widely from those in glass vessels. If the Godthaab observations can be trusted, auroral discharges must often occur within a few miles of the earth’s surface in Arctic regions. In confirmation of this view reference may be made to a number of instances where observers—e.g.General Sabine, Sir John Franklin, Prof. Selim Lemström, Dr David Walker (at Fort Kennedy in 1858-1859), Captain Parry (Fort Bowen, 1825) and others—have seen aurora below the clouds or between themselves and mountains. One or two instances of this kind have even been described in Scotland. Prof. Cleveland Abbe (20) has given a full historical account of the subject to which reference may be made for further details.21.Brightness.—In auroral displays the brightness often varies greatly over the illuminated area and changes rapidly. Estimates of the intensity of the light have been based on various arbitrary scales, such for instance as the size of type which the observer can read at a given distance. The estimate depends in the case of reading type on the general illumination. In other cases scales have been employed which make the result mainly depend on the brightest part of the display. At Jan Mayen (8) in 1882-1883 a scale was employed running from 1, taken as corresponding to the brightness of the milky way, to 4, corresponding to full moonlight. The following is an analysis of the results obtained, showing the number of times the different grades were reached:—Scale ofIntensity.1.2.3.4.MeanIntensity.Arcs27531311.87Bands468349222.24Rays30116138282.21Corona31412122.81On one or two occasions at Jan Mayen auroral light is described as making the full moon look like an ordinary gas jet in presence of electric light, whilst rays could be seen crossing and brighter than the moon’s disk. Such extremely bright auroras seem very rare, however, even in the Arctic. There is a general tendency for both bands and rays to appear brightest at their lowest parts; arcs seldom appear as bright at their summits as nearer the horizon. It is not unusual for arcs and bands to look as if pulses or waves of light were travelling along them; also the direction in which these pulses travel does not seem to be wholly arbitrary. Movements to the east were twice as numerous at Jan Mayen and thrice as numerous at Traurenberg as movements to the west. In some cases changes of intensity take place round the auroral zenith, simulating the effect that would be produced by a cyclonic rotation of luminous matter. In the case of isolated patches the intensity often waxes and wanes as if a search-light were being thrown on and turned off.

21.Brightness.—In auroral displays the brightness often varies greatly over the illuminated area and changes rapidly. Estimates of the intensity of the light have been based on various arbitrary scales, such for instance as the size of type which the observer can read at a given distance. The estimate depends in the case of reading type on the general illumination. In other cases scales have been employed which make the result mainly depend on the brightest part of the display. At Jan Mayen (8) in 1882-1883 a scale was employed running from 1, taken as corresponding to the brightness of the milky way, to 4, corresponding to full moonlight. The following is an analysis of the results obtained, showing the number of times the different grades were reached:—

On one or two occasions at Jan Mayen auroral light is described as making the full moon look like an ordinary gas jet in presence of electric light, whilst rays could be seen crossing and brighter than the moon’s disk. Such extremely bright auroras seem very rare, however, even in the Arctic. There is a general tendency for both bands and rays to appear brightest at their lowest parts; arcs seldom appear as bright at their summits as nearer the horizon. It is not unusual for arcs and bands to look as if pulses or waves of light were travelling along them; also the direction in which these pulses travel does not seem to be wholly arbitrary. Movements to the east were twice as numerous at Jan Mayen and thrice as numerous at Traurenberg as movements to the west. In some cases changes of intensity take place round the auroral zenith, simulating the effect that would be produced by a cyclonic rotation of luminous matter. In the case of isolated patches the intensity often waxes and wanes as if a search-light were being thrown on and turned off.

22.Colour.—The ordinary colour of aurora is white, usually with a distinct yellow tint in the brighter forms, but silvery white when the light is faint. When the light is intense and changing rapidly, red is not infrequently present, especially towards the lower edge. Under these circumstances, green is also sometimes visible, especially towards the zenith. Thus a bright auroral ray may seem red towards the foot and green at its summit, with yellow intervening. In some cases the green may be only a contrast effect. Other colours,e.g.violet, have occasionally been noticed but are unusual.

23.Spectrum.—The spectrum of aurora consists of a number of lines. Numerous measurements have been made of the wave-lengths of the brightest. One line, in the yellow green, is so dominant optically as often to be described as the auroral line. Its wave-length is probably very near 5571 tenth-metres, and it is very close to, if not absolutely coincident with, a prominent line in the spectrum of krypton. This line is so characteristic that its presence or absence is the usual criterion for decidingwhether an atmospheric light is aurora. The Swedish Expedition (17) of 1899-1902, engaged in measuring an arc of the meridian in Spitsbergen, were unusually well provided spectrographically, and succeeded in taking photographs of aurora in conjunction with artificial lines—chiefly of hydrogen—which led to results claiming exceptional accuracy. In the spectrograms three auroral rays—including the principal one mentioned above—were pre-eminent. For the two shorter wave-lengths, for whose measurement he claims the highest precision, the observer, J. Westman, gives the values 4276.4 and 3913.5. In addition, he assigns wave-lengths for 156 other auroral lines between wave-lengths 5205 and 3513. The following table gives the wave-lengths of the photographically brightest of these, retaining four significant figures in place of Westman’s five.

Table VII.

There are a number of optically bright lines of longer wave-length. For the principal of these Angot (1) gives the following wave-lengths (unit 1 µµ or 1 × 10−9metre):—630, 578, 566, 535, 523, 500.

Out of a total of 146 auroral lines, with wave-lengths longer than 3684 tenth-metres, Westman identifies 82 with oxygen or nitrogen lines at the negative pole in vacuum discharges. Amongst the lines thus identified are the two principal auroral lines having wave-lengths 4276.4 and 3913.5. The interval considered by Westman contains at least 300 oxygen and nitrogen lines, so that approximate coincidence with a number of auroral lines was almost inevitable, and an appreciable number of the coincidences may be accidental. E.C.C. Baly (21), making use of the observations of the Russian expedition in Spitsbergen in 1899, accepts as the wave-lengths of the three principal auroral lines 5570, 4276 and 3912; and he identifies all three and ten other auroral lines ranging between 5570 and 3707 with krypton lines measured by himself. In addition to these, he mentions other auroral lines as very probably krypton lines, but in their case the wave-lengths which he quotes from Paulsen (22) are given to only three significant figures, so that the identification is more uncertain. The majority of the krypton lines which Baly identifies with auroral lines require for their production a Leyden jar and spark gap.

If, as is now generally believed, aurora represents some form of electrical discharge, it is only reasonable to suppose that the auroral lines arise from atmospheric gases. The conditions, however, as regards pressure and temperature under which the hypothetical discharges take place must vary greatly in different auroras, or even sometimes in different parts of the same aurora. Further, auroras are often possessed of rapid motion, so that conceivably spectral lines may receive small displacements in accordance with Doppler’s principle. Thus the differences in the wave-lengths of presumably the same lines as measured by different Arctic observers may be only partly due to unfavourable observational conditions. Many of the auroral lines seen in any single aurora are exceedingly faint, so that even their relative positions are difficult to settle with high precision.24. Whether or not auroral displays are ever accompanied by a characteristic sound is a disputed question. If sound waves originate at the seat of auroral displays they seem hardly likely to be audible on the earth, unless the aurora comes very low and great stillness prevails. It is thus to the Arctic one looks for evidence. According to Captain H.P. Dawson (26), in charge of the British Polar Station at Fort Rae in 1882-1883, “The Indians andvoyageursof the Hudson Bay Company, who often pass their nights in the open, say that it [sound] is not uncommon ... there can be no doubt that distinct sound does occasionally accompany certain displays of aurora.” On the one occasion when Captain Dawson says he heard it himself, “the sound was like the swishing of a whip or the noise produced by a sharp squall of wind in the upper rigging of a ship, and as the aurora brightened and faded so did the sound which accompanied it.” If under these conditions the sound was really due to the aurora, the latter, as Captain Dawson himself remarks, must have been pretty close.25. Usually the electric potential near the ground is positive compared to the earth and increases with the height (seeAtmospheric Electricity). Several Arctic observers, however, especially Paulsen (18) have observed a diminution of positive potential, or even a change to negative, for which they could suggest no explanation except the presence of a bright aurora. Other Arctic observers have failed to find any trace of this phenomenon. If it exists, it is presumably confined to cases when the auroral discharge comes unusually low.26.Artificial Phenomena resembling Aurora.—At Sodankylä, the station occupied by the Finnish Arctic Expedition of 1882-1883, Selim Lemström and Biese (23) described and gave drawings of optical phenomena which they believed to be artificially produced aurora. A number of metallic points, supported on insulators, were connected by wires enclosing several hundred square metres on the top of a hill. Sometimes a Holtz machine was employed, but even without it illumination resembling aurora was seen on several occasions, extending apparently to a considerable height. In the laboratory, Kr. Birkeland (19) has produced phenomena bearing a striking resemblance to several forms of aurora. His apparatus consists of a vacuum vessel containing a magnetic sphere—intended to represent the earth—and the phenomena are produced by sending electric discharges through the vessel.27.Theories.—A great variety of theories have been advanced to account for aurora. All or nearly all the most recent regard it as some form of electrical discharge. Birkeland (19) supposes the ultimate cause to be cathode rays emanating from the sun; C. Nordmann (24) replaces the cathode rays by Hertzian waves; while Svante Arrhenius (25) believes that negatively charged particles are driven through the sun’s atmosphere by the Maxwell-Bartoli repulsion of light and reach the earth’s atmosphere. For the size and density of particles which he considers most likely, Arrhenius calculates the time required to travel from the sun as forty-six hours. By modifying the hypothesis as to the size and density, times appreciably longer or shorter than the above would be obtained. Cathode rays usually have a velocity about a tenth that of light, but in exceptional cases it may approach a third of that of light. Hertzian waves have the velocity of light itself. On either Birkeland’s or Nordmann’s theory, the electric impulse from the sun acts indirectly by creating secondary cathode rays in the earth’s atmosphere, or ionizing it so that discharges due to natural differences of potential are immensely facilitated. The ionized condition must be supposed to last to a greater or less extent for a good many hours to account for aurora being seen throughout the whole night. The fact that at most places the morning shows a marked decay of auroral frequency and intensity as compared to the evening, the maximum preceding midnight by several hours, is certainly favourable to theories which postulate ionization of the atmosphere by some cause or other emanating from the sun.Authorities.—The following works are numbered according to the references in the text:—(1) A. Angot,Les Aurores polaires(Paris, 1895); (2) H. Fritz,Das Polarlicht(Leipzig, 1881); (3) Svante August Arrhenius,Lehrbuch der kosmischen Physik; (4) Joseph Lovering, “On the Periodicity of the Aurora Borealis,”Mem. American Acad.vol. x. (1868); (5) Sophus Tromholt,Catalog der in Norwegen bis Juni 1878 beobachteten Nordlichter; (6)Observations internationales polaires(1882-1883),Expédition Danoise, tome i. “Aurores boréales”; (7) Carlheim-Gyllensköld, “Aurores boréales” inObservations faites au Cap Thorsden Spitzberg par l’expédition suédoise, tome ii. 1; (8) “Die Österreichische Polar Station Jan Mayen” inDie Internationale Polarforschung, 1882-1883, Bd. ii. Abth. 1; (9) Henryk Arctowski, “Aurores australes” inExpédition antarctique belge ... Voyage du S. Y. “Belgica”; (10) G.C. Amdrup,Observations ... faites par l’expédition danoise; H. Ravn,Observations de l’aurore boréale de Tasiusak; (11)K. Sven. Vet.-Akad. Hand. Bd. 31, Nos. 2, 3, &c.; (12)Sitz. d. k. Akad. d. Wiss.(Vienna), Math. Naturw. Classe, Bd. xcvii. Abth. iia, 1888; (13)Proc. Roy. Soc., 1906, lxxvii. A, 141; (14)Kongl. Sven. Vet.-Akad. Hand.Bd. 15, No. 5, Bd. 18, No. 1; (15)Bull. Acad. Roy. Danoise, 1889, p. 67; (16)Voyages ... pendant les années 1838, 1839 et 1840 sur ... la Recherche, “Aurores boréales,” by MM. Lottin, Bravais, &c.; (17)Missions scientifiques ... au Spitzberg ... en 1899-1902, Mission suédoise, tome ii. VIIIeSection, C. “Aurores boréales”; (18)Bull. Acad. R. des Sciences de Danemark, 1894, p. 148; (19) Kr. Birkeland,Expédition norvégienne 1899-1900 pour l’étude des aurores boréales(Christiania, 1901); (20)Terrestrial Magnetism, vol. iii. (1898), pp. 5, 53, 149; (21)Astrophysical Journal, 1904, xix. p. 187; (22)Rapports présentés au Congrès International de Physique réuni à Paris, 1900, iii. 438; (23)Expédition polaire finlandaise(1882-1884), tome iii.; (24) Charles Nordmann,Thèses présentées à la Faculté des Sciences de Paris(1903); (25)Terrestrial Magnetism, vol. 10, 1905, p. 1; (26)Observations of the International Polar Expeditions 1882-1883 Fort Rae... by Capt. H.P. Dawson, R.A.

24. Whether or not auroral displays are ever accompanied by a characteristic sound is a disputed question. If sound waves originate at the seat of auroral displays they seem hardly likely to be audible on the earth, unless the aurora comes very low and great stillness prevails. It is thus to the Arctic one looks for evidence. According to Captain H.P. Dawson (26), in charge of the British Polar Station at Fort Rae in 1882-1883, “The Indians andvoyageursof the Hudson Bay Company, who often pass their nights in the open, say that it [sound] is not uncommon ... there can be no doubt that distinct sound does occasionally accompany certain displays of aurora.” On the one occasion when Captain Dawson says he heard it himself, “the sound was like the swishing of a whip or the noise produced by a sharp squall of wind in the upper rigging of a ship, and as the aurora brightened and faded so did the sound which accompanied it.” If under these conditions the sound was really due to the aurora, the latter, as Captain Dawson himself remarks, must have been pretty close.

25. Usually the electric potential near the ground is positive compared to the earth and increases with the height (seeAtmospheric Electricity). Several Arctic observers, however, especially Paulsen (18) have observed a diminution of positive potential, or even a change to negative, for which they could suggest no explanation except the presence of a bright aurora. Other Arctic observers have failed to find any trace of this phenomenon. If it exists, it is presumably confined to cases when the auroral discharge comes unusually low.

26.Artificial Phenomena resembling Aurora.—At Sodankylä, the station occupied by the Finnish Arctic Expedition of 1882-1883, Selim Lemström and Biese (23) described and gave drawings of optical phenomena which they believed to be artificially produced aurora. A number of metallic points, supported on insulators, were connected by wires enclosing several hundred square metres on the top of a hill. Sometimes a Holtz machine was employed, but even without it illumination resembling aurora was seen on several occasions, extending apparently to a considerable height. In the laboratory, Kr. Birkeland (19) has produced phenomena bearing a striking resemblance to several forms of aurora. His apparatus consists of a vacuum vessel containing a magnetic sphere—intended to represent the earth—and the phenomena are produced by sending electric discharges through the vessel.

27.Theories.—A great variety of theories have been advanced to account for aurora. All or nearly all the most recent regard it as some form of electrical discharge. Birkeland (19) supposes the ultimate cause to be cathode rays emanating from the sun; C. Nordmann (24) replaces the cathode rays by Hertzian waves; while Svante Arrhenius (25) believes that negatively charged particles are driven through the sun’s atmosphere by the Maxwell-Bartoli repulsion of light and reach the earth’s atmosphere. For the size and density of particles which he considers most likely, Arrhenius calculates the time required to travel from the sun as forty-six hours. By modifying the hypothesis as to the size and density, times appreciably longer or shorter than the above would be obtained. Cathode rays usually have a velocity about a tenth that of light, but in exceptional cases it may approach a third of that of light. Hertzian waves have the velocity of light itself. On either Birkeland’s or Nordmann’s theory, the electric impulse from the sun acts indirectly by creating secondary cathode rays in the earth’s atmosphere, or ionizing it so that discharges due to natural differences of potential are immensely facilitated. The ionized condition must be supposed to last to a greater or less extent for a good many hours to account for aurora being seen throughout the whole night. The fact that at most places the morning shows a marked decay of auroral frequency and intensity as compared to the evening, the maximum preceding midnight by several hours, is certainly favourable to theories which postulate ionization of the atmosphere by some cause or other emanating from the sun.

Authorities.—The following works are numbered according to the references in the text:—(1) A. Angot,Les Aurores polaires(Paris, 1895); (2) H. Fritz,Das Polarlicht(Leipzig, 1881); (3) Svante August Arrhenius,Lehrbuch der kosmischen Physik; (4) Joseph Lovering, “On the Periodicity of the Aurora Borealis,”Mem. American Acad.vol. x. (1868); (5) Sophus Tromholt,Catalog der in Norwegen bis Juni 1878 beobachteten Nordlichter; (6)Observations internationales polaires(1882-1883),Expédition Danoise, tome i. “Aurores boréales”; (7) Carlheim-Gyllensköld, “Aurores boréales” inObservations faites au Cap Thorsden Spitzberg par l’expédition suédoise, tome ii. 1; (8) “Die Österreichische Polar Station Jan Mayen” inDie Internationale Polarforschung, 1882-1883, Bd. ii. Abth. 1; (9) Henryk Arctowski, “Aurores australes” inExpédition antarctique belge ... Voyage du S. Y. “Belgica”; (10) G.C. Amdrup,Observations ... faites par l’expédition danoise; H. Ravn,Observations de l’aurore boréale de Tasiusak; (11)K. Sven. Vet.-Akad. Hand. Bd. 31, Nos. 2, 3, &c.; (12)Sitz. d. k. Akad. d. Wiss.(Vienna), Math. Naturw. Classe, Bd. xcvii. Abth. iia, 1888; (13)Proc. Roy. Soc., 1906, lxxvii. A, 141; (14)Kongl. Sven. Vet.-Akad. Hand.Bd. 15, No. 5, Bd. 18, No. 1; (15)Bull. Acad. Roy. Danoise, 1889, p. 67; (16)Voyages ... pendant les années 1838, 1839 et 1840 sur ... la Recherche, “Aurores boréales,” by MM. Lottin, Bravais, &c.; (17)Missions scientifiques ... au Spitzberg ... en 1899-1902, Mission suédoise, tome ii. VIIIeSection, C. “Aurores boréales”; (18)Bull. Acad. R. des Sciences de Danemark, 1894, p. 148; (19) Kr. Birkeland,Expédition norvégienne 1899-1900 pour l’étude des aurores boréales(Christiania, 1901); (20)Terrestrial Magnetism, vol. iii. (1898), pp. 5, 53, 149; (21)Astrophysical Journal, 1904, xix. p. 187; (22)Rapports présentés au Congrès International de Physique réuni à Paris, 1900, iii. 438; (23)Expédition polaire finlandaise(1882-1884), tome iii.; (24) Charles Nordmann,Thèses présentées à la Faculté des Sciences de Paris(1903); (25)Terrestrial Magnetism, vol. 10, 1905, p. 1; (26)Observations of the International Polar Expeditions 1882-1883 Fort Rae... by Capt. H.P. Dawson, R.A.

(C. Ch.)

AURUNCI,the name given by the Romans to a tribe which in historical times occupied only a strip of coast on either side of the Mons Massicus between the Volturnus and the Liris, although it must at an earlier period have extended over a considerably wider area. Their own name for themselves inthe 4th centuryB.C.wasAusŏnes, and in Greek writers we find the nameAusŏniaapplied to Latium and Campania (see Strabo v. p. 247; Aristotle,Pol.iv. (vii.) 10; Dion. Hal. i. 72), while in the Augustan poets (e.g.Virgil,Aen.vii. 795) it is used as one of many synonyms for Italy. In history the tribe appears only for a brief space, from 340 to 295B.C.(Mommsen,C.I.L.x. pp. 451, 463, 465), and their struggle with the Romans ended in complete extermination; their territory was parcelled out between the Latin colonies of Cales (Livy viii. 16) and Suessa Aurunca (id.ix. 28) which took the place of an older town calledAusona(id.ix. 25; viii. 15), and the maritime colonies Sinuessa (the olderVescia) and Minturnae (both in 295B.C., Livy x. 21). The coin formerly attributed to Suessa Aurunca on the strength of its supposed legendAurunkudhas now been certainly referred to Naples (see R.S. Conway,Italic Dialects, 145, andVerner’s law in Italy, p. 78, where the change ofstoris explained as probably due to the Latin conquest). Seeing that the tribe was blotted out at the beginning of the 3rd centuryB.C., we can scarcely wonder that no record of its speech survives; but its geographical situation and the frequency of theco-suffix in that strip of coast (besidesAurunciitself we have the namesVescia,Mons Massicus,Marica,GlanicaandCaedicii; seeItalic Dialects, pp. 283 f.) rank them beyond doubt with their neighbours the Volsci (q.v.).

(R. S. C.)

AUSCULTATION(from Lat.auscultare, to listen), a term in medicine, applied to the method employed by physicians for determining, by the sense of hearing, the condition of certain internal organs. The ancient physicians appear to have practised a kind of auscultation, by which they were able to detect the presence of air or fluids in the cavities of the chest and abdomen. Still no general application of this method of investigation was resorted to, or was indeed possible, till the advance of the study of anatomy led to correct ideas regarding the locality, structure and uses of the various organs of the body, and the alterations produced in them by disease. In 1761 Leopold Auenbrugger (1722-1809), a Viennese physician, published hisInventum Novum, describing the art of percussion in reference more especially to diseases of the chest. This consisted in tapping with the fingers the surface of the body, so as to elicit sounds by which the comparative resonance of the subjacent parts or organs might be estimated. Auenbrugger’s method attracted but little attention till the French physician J.N. Corvisart (1755-1828) in 1808 demonstrated its great practical importance, and then its employment in the diagnosis of affections of the chest soon became general. Percussion was originally practised in the manner above mentioned (immediate percussion), but subsequently the method ofmediate percussionwas introduced by P.A. Piorry (1794-1879). It is accomplished by placing upon the spot to be examined some solid substance, upon which the percussion strokes are made with the fingers. For this purpose a thin oval piece of ivory (called apleximeter, or stroke-measurer) may be used, with a small hammer; but one or more fingers of the left hand applied flat upon the part answer equally well, and this is the method which most physicians adopt. Percussion must be regarded as a necessary part of auscultation, particularly in relation to the examination of the chest; for the physician who has made himself acquainted with the normal condition of that part of the body in reference to percussion is thus able to recognize by the ear alterations of resonance produced by disease. But percussion alone, however important in diagnosis, could manifestly convey only limited and imperfect information, for it could never indicate the nature or extent of functional disturbance.

In 1819 the distinguished French physician R.T.H. Laënnec (1781-1826) published hisTraité de L’auscultation médiate, embodying the present methods of auscultatory examination, and venturing definite conclusions based on years of his own study. He also invented the stethoscope (στῆθος, the breast, andσκοπεῖν, to examine). Since then many men have widened the scope of auscultation, notably Skoda, Wintrich, A. Geigel, Th. Weber and Gerhardt. According to Laënnec the essential of a good stethoscope was its capability of intensifying the tone vibrations. But since his time the opinion of experts on this matter has somewhat changed, and there are now two definite schools. The first and older condemns the resonating stethoscope, maintaining that the tones are bound to be altered; the second and younger school warmly advocates its use. In America, more than elsewhere, there is a type of phonendoscope much used by the younger men, which has the advantage that it can be used when the older type of instrument fails, viz. when the patient is recumbent and too ill to be moved. By slipping it beneath the patient’s back a fairly accurate idea of the breathing over the bases of the lungs behind can often be obtained.

Stethoscopes have been made of many forms and materials. They usually consist of a hollow stem of wood, hard rubber or metal, with an enlarged tip slightly funnel-shaped at one end, and an ear-plate with a hole in the middle, fastened perpendicularly to the other end. To enable the instrument to be more conveniently carried, the ear-plate can be unscrewed from the tube. The length of the stem of the instrument is of minor importance, but its bore should be as nearly as possible that of the entrance of the external ear. A flexible stethoscope in general use both in England and America transmits the sound from a funnel through tubes to the ears of the observer. This is the common form of a binaural resonating stethoscope. It is convenient and gives a loud tone, but is condemned by the older school, who say that the resonance is confusing, and that the slightest movement in handling gives rise to perplexing murmurs. Nevertheless, it is this form of instrument which has by far the greatest vogue. It is probable, however, that the most skilled physicians of all find a special use in each form, the monaural non-resonating type being more sensitive to high-pitched sounds, and of greater assistance in differentiating the sounds and murmurs of the heart, the ordinary binaural form being more useful in examining the lungs and other organs. In using the stethoscope, it must be applied very carefully, so that the edge of the funnel makes an air-tight connexion with the skin, and in the monaural form the ear must be but lightly applied to the ear-plate, not pressing heavily on the patient.

The numerous diseases affecting the lungs can now be recognized and discriminated from each other with a precision which, but for auscultation and the stethoscope, would have been altogether unattainable. The same holds good in the case of the heart, whose varied and often complex forms of disease can, by auscultation, be identified with striking accuracy. But in addition to these its main uses, auscultation is found to render great assistance in the investigation of many obscure internal affections, such as aneurysms and certain diseases of the oesophagus and stomach. To the accoucheur the stethoscope yields valuable aid in the detection of some forms of uterine tumours, and especially in the diagnosis of pregnancy—the only evidence now accepted as absolutely diagnostic of that condition being the hearing of the foetal heart sounds.

AUSONIUS, DECIMUS MAGNUS(c. 310-395), Roman poet and rhetorician, was born at Burdigala [Bordeaux]. He received an excellent education, especially in grammar and rhetoric, but confesses that his progress in Greek was unsatisfactory. Having completed his studies, he practised for some time as an advocate, but his inclination lay in the direction of teaching. He set up (in 334) a school of rhetoric in his native place, which was largely attended, his most famous pupil being Paulinus, afterwards bishop of Nola. After thirty years of this work, he was summoned by Valentinian to the imperial court, to undertake the education of Gratian, the heir-apparent. The prince always entertained the greatest regard for his tutor, and after his accession bestowed upon him the highest titles and honours, culminating in the consulship (379). After the murder of Gratian (383), Ausonius retired to his estates near Burdigala. He appears to have been a (not very enthusiastic) convert to Christianity. He died about 395.

His most important extant works are: in prose,Gratiarum Actio, an address of thanks to Gratian for his elevation to the consulship;Periochae, summaries of the books of theIliadandOdyssey; and one or twoepistolae; in verse,Epigrammata, including several free translations from the Greek Anthology;Ephemeris, the occupations of a day;ParentaliaandCommemoratio Professorum Burdigalensium, on deceased relatives and literary friends;Epitaphia, chiefly on the Trojan heroes;Caesares, memorial verses on the Roman emperors from Julius Caesar to Elagabalus;Ordo Nobilium Urbium, short poems on famous cities;Ludus Septem Sapientum, speeches delivered by the Seven Sages of Greece;Idyllia, of which the best-known are theMosella, a descriptive poem on the Moselle, and the infamousCento Nuptialis. We may also mentionCupido Cruciatus, Cupid on the cross;Technopaegion, a literary trifle consisting of a collection of verses ending in monosyllables;Eclogarum Liber, on astronomical and astrological subjects;Epistolae, including letters to Paulinus and Symmachus; lastly,Praefatiunculae, three poetical epistles, one to the emperor Theodosius. Ausonius was rather a man of letters than a poet; his wide reading supplied him with material for a great variety of subjects, but his works exhibit no traces of a true poetic spirit; even his versification, though ingenious, is frequently defective.

There are no MSS. containing the whole of Ausonius’s works. Editio princeps, 1472; editions by Scaliger 1575, Souchay 1730, Schenkl 1883, Peiper 1886; cf.Mosella, Böcking 1845, de la Ville de Mirmont (critical edition with translation) 1889, andDe Ausonii Mosella, 1892, Hosius 1894. See Deydou,Un Poète bordelais(1868); Everat,De Ausonii Operibus(1885); Jullian,Ausone et Bordeaux(1893); C. Verrier and R. de Courmont,Les Épigrammes d’Ausone(translation with bibliography, 1905); R. Pichon,Les Derviers Écrivains profanes(1907).

AUSSIG(CzechOustí nad Labem), a town of Bohemia, Austria, 68 m. N. of Prague by rail. Pop. (1900) 37,255, mostly German. It is situated in a mountainous district, at the confluence of the Biela and the Elbe, and, besides being an active river port, is an important junction of the northern Bohemian railways. Aussig has important industries in chemicals, textiles, glass and boat-building, and carries on an active trade in coal from the neighbouring mines, stone and stoneware, corn, fruit and wood. It was the birthplace of the painter, Raphael Mengs (1728-1779). Aussig is mentioned as a trading centre as early as 993. It was made a city by Ottokar II. in the latter part of the 13th century. In 1423 it was pledged by King Sigismund to the elector Frederick of Meissen, who occupied it with a Saxon garrison. In 1426 it was besieged by the Hussites, who on the 16th of June, though only 25,000 strong, defeated a German army of 70,000, which had been sent to its relief, with great slaughter. The town was stormed and sacked next day. After lying waste for three years, it was rebuilt in 1429. It suffered much during the Thirty Years’ and Seven Years’ Wars, and in 1830 it had only 1400 inhabitants. Not far from Aussig is the village of Kulm, where, on the 29th and 30th of August 1813, a battle took place between the French under Vandamme and an allied army of Austrians, Prussians and Russians. The French were defeated, and Vandamme surrendered with his army of 10,000 men.

AUSTEN, JANE(1775-1817), English novelist, was born on the 16th of December 1775 at the parsonage of Steventon, in Hampshire, a village of which her father, the Rev. George Austen, was rector. She was the youngest of seven children. Her mother was Cassandra Leigh, niece of Theophilus Leigh, a dry humorist, and for fifty years master of Balliol, Oxford. The life of no woman of genius could have been more uneventful than Miss Austen’s. She did not marry, and she never left home except on short visits, chiefly to Bath. Her first sixteen years were spent in the rectory at Steventon, where she began early to trifle with her pen, always jestingly, for family entertainment. In 1801 the Austens moved to Bath, where Mr Austen died in 1805, leaving only Mrs Austen, Jane and her sister Cassandra, to whom she was always deeply attached, to keep up the home; his sons were out in the world, the two in the navy, Francis William and Charles, subsequently rising to admiral’s rank. In 1805 the Austen ladies moved to Southampton, and in 1809 to Chawton, near Alton, in Hampshire, and there Jane Austen remained till 1817, the year of her death, which occurred at Winchester, on July 18th, as a memorial window in the cathedral testifies.

During her placid life Miss Austen never allowed her literary work to interfere with her domestic duties: sewing much and admirably, keeping house, writing many letters and reading aloud. Though, however, her days were quiet and her area circumscribed, she saw enough of middle-class provincial society to find a basis on which her dramatic and humorous faculties might build, and such was her power of searching observation and her sympathetic imagination that there are not in English fiction more faithful representations of the life she knew than we possess in her novels. She had no predecessors in this genre. Miss Austen’s “little bit (two inches wide) of ivory” on which she worked “with so fine a brush”—her own phrases—was her own invention.

Her best-known, if not her best work,Pride and Prejudice, was also her first. It was written between October 1796 and August 1797, although, such was the blindness of publishers, not issued until 1813, two years afterSense and Sensibility, which was written, on an old scenario called “Eleanor and Marianne,” in 1797 and 1798. Miss Austen’s inability to find a publisher for these stories, and forNorthanger Abbey, written in 1798 (although it is true that she sold that MS. in 1803 for £10 to a Bath bookseller, only, however, to see it locked away in a safe for some years, to be gladly resold to her later), seems to have damped her ardour; for there is no evidence that between 1798 and 1809 she wrote anything but the fragment called “The Watsons,” after which year she began to revise her early work for the press. Her other three books belong to a later date—Mansfield Park,EmmaandPersuasionbeing written between 1811 and 1816. The years of publication wereSense and Sensibility, 1811;Pride and Prejudice, 1813;Mansfield Park, 1814; andEmma, 1816—all in their author’s lifetime.PersuasionandNorthanger Abbeywere published posthumously in 1818. All were anonymous, agreeably to their author’s retiring disposition.

AlthoughPride and Prejudiceis the novel which in the mind of the public is most intimately associated with Miss Austen’s name, bothMansfield ParkandEmmaare finer achievements—at once riper and richer and more elaborate. But the fact thatPride and Prejudiceis more single-minded, that the love story of Elizabeth Bennet and D’Arcy is not onlyofthe book butisthe book (whereas the love story of Emma and Mr Knightley and Fanny Price and Edmund Bertram have parallel streams), has givenPride and Prejudiceits popularity above the others among readers who are more interested by the course of romance than by the exposition of character. Entirely satisfactory as isPride and Prejudiceso far as it goes, it is, however, thin beside the niceness of analysis of motives inEmmaand the wonderful management of two housefuls of young lovers that is exhibited inMansfield Park.

It has been generally agreed by the best critics that Miss Austen has never been approached in her own domain. No one indeed has attempted any close rivalry. No other novelist has so concerned herself or himself with the trivial daily comedy of small provincial family life, disdaining equally the assistance offered by passion, crime and religion. Whatever Miss Austen may have thought privately of these favourite ingredients of fiction, she disregarded all alike when she took her pen in hand. Her interest was in life’s little perplexities of emotion and conduct; her gaze was steadily ironical. The most untoward event in any of her books is Louisa’s fall from the Cobb at Lyme Regis, inPersuasion; the most abandoned, Maria’s elopement with Crawford, inMansfield Park. In pure ironical humour Miss Austen’s only peer among novelists is George Meredith, and indeedEmmamay be said to be herEgoist, or theEgoisthisEmma. But irony and fidelity to the fact alone would not have carried her down the ages. To these gifts she allied a perfect sense of dramatic progression and an admirably lucid and flowing prose style which makes her stories the easiest reading.

Recognition came to Miss Austen slowly. It was not until quite recent times that to read her became a necessity of culture. But she is now firmly established as an English classic, standing far above Miss Burney (Madame d’Arblay) and Miss Edgeworth, who in her day were the popular women novelists of real life,while Mrs Radcliffe and “Monk” Lewis, whose supernatural fancies’Northanger Abbeywas written in part to ridicule, are no longer anything but names. Although, however, she has become only lately a household word, Miss Austen had always her panegyrists among the best intellects—such as Coleridge, Tennyson, Macaulay, Scott, Sydney Smith, Disraeli and Archbishop Whately, the last of whom may be said to have been her discoverer. Macaulay, whose adoration of Miss Austen’s genius was almost idolatrous, consideredMansfield Parkher greatest feat; but many critics give the palm toEmma. Disraeli readPride and Prejudiceseventeen times. Scott’s testimony is often quoted: “That young lady had a talent for describing the involvements, feelings and characters of ordinary life which is to me the most wonderful I have ever met with. The big bow-wow I can do myself like any one going; but the exquisite touch which renders commonplace things and characters interesting from the truth of the description and the sentiment is denied to me.”

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