In our SUPPLEMENT No. 529 we gave an abstract of Prof. Dewars recent series of lectures on the above subject at the Royal Institution. We now present an abstract of the last and concluding lecture.
After the conclusion of his last lecture, Prof. Dewar distributed among the younger listeners small pieces of a portion of the Dhurmsala meteorite, which had been broken up for presentation to them by Mr. J.R. Gregory, whose collection of rare minerals was recently to some extent described in these pages. The lecturer stated that Sir F. Abel had given him a large piece of a large meteorite, because he thought that the speaker's piece ought to be bigger than theirs.
Professor Dewar also presented the listeners with a printed detailed account of the fall of the Dhurmsala meteorite, including the report of the occurrence sent to the Punjaub Government, and dated July 28, 1860. The following are the main facts:
"On the afternoon of Saturday, the 14th of July, 1860, between the hours of 2 and 2:30 P.M., the station of Dhurmsala was startled by a terrific bursting noise, which was supposed at first to proceed from a succession of loud blastings or from the explosion of a mine in the upper part of the station; others, imagining it to be an earthquake or very large landslip, rushed from their houses in the firm belief that they must fall upon them. It soon became apparent that this was not the case. The first report, which was far louder in its discharge than any volley of artillery, was quickly followed by another and another, to the number of fourteen or sixteen. Most of the latter reports grew gradually less and less loud. These were probably but the reverberations of the former, not among the hills, but among the clouds, just as is the case with thunder. It was difficult to say which were the reports and which the echoes. There could certainly not have been fewer than four or five actual reports. During the time that the sound lasted the ground trembled and shook convulsively. From the different accounts of three eyewitnesses there appears to have been observed a flame of fire, described as about 2 ft. in depth and 9 ft. in length, darting in an oblique direction above the station after the first explosion had taken place. The stones as they fell buried themselves from 1 ft. to 1½ ft. in the ground, sending up a cloud of dust in all directions. Most providentially, no loss of life or property has occurred. Some coolies, passing by where one fell, ran to the spot to pick up the pieces; before they had held them in their hands half a minute they had to drop them, owing to the intensity of the cold, which benumbed their fingers. This, considering the fact that they were apparently but a moment before in a state of ignition, is very remarkable. Each stone that fell bore unmistakable marks of partial fusion."
Several meteors were seen at Dhurmsala on the evening of the same day.
Dr. C.T. Jackson analyzed a portion of one meteorite weighing 4½ oz.; the piece was 2½ in. long, 1¼ in. wide, and 1 in. in average thickness. In the course of his report he stated: "Its specific gravity is 3.456 at 68 deg. Fahr., barom. 29.9. Its structure is imperfectly granular, but not crystallized, and there are small black specks of the size of a pin's head, and smaller, of malleable meteoric iron, which are readily removed from the crushed stone by the magnet. The color of the mass is ash gray. A portion of the surface is black and is scarified by fusion. Its hardness is not superior to that of olivine or massive chrysolite. Chemical analysis shows that its composition is that of a ferruginous olivine. One gramme of the stone, crushed in an agate mortar, and acted on by a magnet, yielded 0.43 gramme of meteoric iron, which was malleable. After the removal of this a qualitative analysis was made of the residual powder. Another gramme was also taken, without picking out the metallic iron, and was tested for chlorine and for phosphoric acid. The results of the qualitative analysis were that the stone contains silica, magnesia, a little alumina, oxide of iron and nickel, a little tin, an alloy of iron and nickel, phosphoric acid, and a trace of chlorine. These ingredients being determined, the plan for a quantitative analysis was laid out, and was duly executed by the usual and approved methods The following are the results of this analysis, per centum:
Silica, with traces of tin 40.000Magnesia 26.600Peroxide of iron 27.700Metallic iron 3.500Metallic nickel 0.800Alumina 0.400Chlorine 0.049Phosphoric acid not weighed —______99.049"
Messrs. Dewar and Ansdell analyzed the gases in the meteorite, of which it contained three times its volume; the gases were in the following proportions to each other:
Carbonic acid 61.29Carbonic oxide 7.52Hydrogen 30.96Nitrogen 0.23______100.00
In the twentieth volume of theAmerican Journal of Science, at page 225, I gave a preliminary account of my search, theoretic and practical, for the trans-Neptunian planet. I saythetrans-Neptunian planet, because I regard the evidence of its existence as well-founded, and further because, since the time when I was engaged upon this search, nothing has in the least weakened my entire conviction as to its existence in about that part of the sky assigned; while, as is well known, the independent researches in cometary perturbations by Prof. Forbes conducted him to a result identical with my own—a coincidence not to be lightly set aside as pure accident.
That five years have elapsed since this coincidence was remarked, and the planet is still unfound, is not sufficient assurance to me that its existence is merely fanciful. In so far as I am informed, this spot of the sky has received very little scrutiny with telescopes competent to such a search; and most observers finding nothing would, I suspect, prefer not to announce their ineffective search.
The time has now come when this search can be profitably undertaken by any observer having the rare combination of time, enthusiasm, and the necessary appliances. Strongly marked developments in astronomical photography have been effected since this optical search was conducted; and the capacity of the modern dry-plate for the registry of the light of very faint stars makes the application of this method the shortest and surest way of detecting any such object. Nor is this purely an opinion of my own. But the required apparatus would be costly; and the instrument, together with the services of an astronomer and a photographer, would, for the time being, be necessarily devoted exclusively to the work. While, however, the photographic search might have to be ended with a negative result, in so far as the trans-Neptunian planet is concerned, there would still remain the series of photographic maps of the region explored, and these would be of incalculable service in the astronomy of the future.
In the latter part of the paper alluded to above, I stated the speculative basis upon which I restricted the stellar region to be examined; also the fact that between November of 1877 and March of 1878 I was engaged in a telescopic scrutiny of this region, employing the twenty-six inch refractor of the Naval Observatory. For the purposes contemplated I had no hesitation in adopting the method of search whereby I expected to detect the planet by the contrast of its disk and light with the appearance of an average star of about the thirteenth magnitude. A power of 600 diameters was often employed, but the field of view of this eye-piece was so restricted that a power of 400 diameters had to be used most of the time. I say, too, that, "after the first few nights, I was surprised at the readiness with which my eye detected any variation from the average appearance of a star of a given faint magnitude; as a consequence whereof my observing book contains a large stock of memoranda of suspected objects. My general plan with these was to observe with a sufficient degree of accuracy the position of all suspected objects. On the succeeding night of observation they were re-observed; and, at an interval of several weeks thereafter, the observation was again verified." Subjoined to the original observations are printed these verifications in heavy-faced type.
In conducting the search, the plans were several times varied in slight detail, generally because experience with the work enabled me to make improvements in method. Usually, I prepared every few days a new zone chart of the region over which I was about to search; and these charts while containing memoranda of all the instrumental data which could be prepared beforehand, were likewise so adjusted with reference to the opposition-time of the planet as to avoid, if possible, its stationary point. The same thing, too, was kept in mind in selecting the times of subsequent observation. Notwithstanding this precaution, however, it would be well if some observer who has a large telescope should now re-examine the positions of these objects.
Researches in faint nebulæ and nebulous stars appearing likely to constitute a separate and interesting branch of the astronomy of the future, it has seemed to me that the astronomers engaged in this work may like to make a careful examination of some of the stars entered in my observing book under the category of "suspected objects." The method I adopted of insuring re-observation of these objects was by the determination, not of their absolute, but only of their relative, positions, through the agency of the larger "finder" of the great telescope. This has an aperture of five inches, a power of thirty diameters, and a field of view of seventy-eight minutes of arc. Two diagrams were usually drawn in the book for each of these objects, the one showing the relation of adjacent objects in the great telescope, and the other the configuration of the more conspicuous objects in the field of view of the finder. Adjacent to these "finder" diagrams are the settings—to the nearest minute of arc in declination, and of time in right ascension—as read from the large finding-circles, divided in black and white. The field of view of the finder is crossed by two pairs of hairlines, making a square of about twelve minutes on a side by their intersection at the center. The diagrams in all cases represent the objects as seen with an inverting eye-piece. As the adjustment of the finder was occasionally verified, as well as the readings of the large circles, there should be no trouble in identifying any of these objects, notwithstanding the fact that no estimates of absolute magnitude were recorded. The relative magnitudes, while intended to be only approximate, are still shown with sufficient accuracy for the purpose of the research, and the diagrams are, in general, faithful tracings from the original memoranda.
[Mr. Todd transcribes the observing book entire.]
[3]
By David P. Todd, M.A., from theProceedingsof the American Academy of Arts and Science.
The inestimable value of speech-reading and the practicability of its acquisition under favorable conditions is a matter of common experience and observation but justice to the deaf requires a recognition of the fact that speech-reading has its limitations. Certain English words, chiefly short ones, are practically alike to the speech-reader and the context may fail sometimes to give a clew. It is necessary, at times, in communicating with even expert speech-readers, to have recourse to writing or oral spelling to convey the names of persons, places, technical terms, etc., not in common use. Moreover, it is convenient to have accurate and rapid means of conversation under unfavorable conditions as to light and distance, or when from any cause the deaf person's voice cannot be heard.
Writing is slow, inconvenient, and often impossible. Writing upon the palm of the hand was proposed by the Abbe Deschamps in 1778, as utilizing the sense of touch, and was used in darkness by him as a substitute for speech, but it is neither accurate nor rapid. Writing in the air4with the finger is also slow and uncertain, while the action is unpleasantly conspicuous.
Finger-spelling would appear to be a far more convenient, easy, rapid, and accurate adjunct to speech or substitute for it than writing.
It is a common error to consider the ordinary manual alphabets as deaf-mute alphabets and finger-spelling as the sign-language of the deaf. Finger-spelling is to the deaf a borrowed art. It is used by many of the educated deaf and their friends as a substitute for the sign-language, and it enables them also to supply the deficiencies of the sign-language by incorporating words from written language. Scagliotti, of Turin, devised a system of initial signs5which begin with letters of the manual alphabet, and Dr. Isaac Lewis Peet, of New York, has made a similar application of manual letters to signs to suggest words of our written language to the initiated deaf. But it should not be forgotten that practice in finger-spelling is practice in our language.
The origin of finger-spelling is not known. Barrois, a distinguished orientalist, in hisDactylologie et Langage primitif,6ingeniously traces evidences of finger-spelling, from the Assyrian antiquities down to the fifteenth century upon monuments of art.
The ancient Egyptians, Greeks, and Romans were familiar with manual arithmetic and finger-numeration, as quaint John Bulwer shows by numerous citations in hisChironomia(1644). The earliest finger-alphabets extant appear to have been based upon finger-signs for numbers, as, for instance, that given by the Venerable Bede (672-735) in hisDe Loguela per Gestum Digitorum sive Indigitatione, figured in the Ratisbon edition of 1532.7Monks and others who had special reason to prize secret and silent modes of communication, beyond doubt invented and used many forms of finger alphabets as well as systems of manual signs.8The oldest plates in the library of the National Deaf Mute College are found in theThesaurus Artificiosae Memoriaeof frater Cosmas P. Rossellius of Florence, printed in 1579, which gives three forms of one-hand alphabets. Bonet's work9of 1620 gives one form of the one hand Spanish manual alphabet, which contains forms identical with certain letters in the alphabets of 1579. This was introduced into France by Pereire and taught to the Abbe de l'Epee by Saboureux de Fontenay, the gifted pupil of Pereire. The good Abbe however continued to use a French10two-hand alphabet which, he had learned when a child and which he said all school-children knew. He mentions also a Spanish alphabet in part requiring both hands, and remarks that different nations have different manual alphabets. The Abbe Deschamps, a rival of De l'Epee, made use of a finger alphabet in teaching the deaf to speak, which was not adapted to rapid use. John Bulwer, in hisChirologia, or the Naturall Language of the Hand, printed in 1644, figures five manual alphabets for secret communication.
The first alphabet which appears to have been devised expressly for use in teaching the deaf is that of George Dalgarno, of Aberdeen (1626-1687), given in his remarkable philosophical treatise,Didascalocophus, or the Deaf and Dumb Man's Tutor, Oxford, 1680. A facsimile of this alphabet is given in theAnnals, vol. ix., page 19. Words are spelled by touching with your finger the positions indicated, either upon your hand or upon the hand of your interlocutor. An alphabet of the same character, however, was not unknown at an earlier date. For Bulwer, in 1648, says: "A pregnant example of the officious nature of the Touch in supplying the defect or temporall incapacity of the other senses we have in one MasterBabingtonofBurntwoodin the County ofEssex, an ingenious gentleman, who through some sicknesse becomingdeaf, doth notwithstanding feele words, and as if he had an eye in his finger, sees signes in the darke; whose Wife discourseth very perfectly with him by a strange way of Arthrologie or Alphabet contrived on the joynts of his Fingers; who taking him by the hand in the night, can so discourse with him very exactly; for he feeling the joynts which she toucheth for letters, by them collected into words, very readily conceives what shee would suggest unto him. By which examples [referring to this case and to that of an abbot who becamedeaf, dumb, andblind, who understood writing traced upon his naked arm] you may see how ready upon any invitation of Art, theTactis, to supply the defect, and to officiate for any or all of the other senses, as being the most faithfull sense to man, being both theFounder, andVicar generallto all the rest."11
Dr. Alexander Graham Bell has modified the Dalgarno alphabet, and has made considerable use of it in its modified form as figured in theAnnals, vol. xxviii., page 133. He esteems it highly for certain purposes, especially as employing touch to assist the sight or to release the sight for other employment, as in reading speech for instance. Here a touch-alphabet may be an efficient aid to the sight, as the touch may fairly keep pace with the rapidity of oral expression in deliberate speech. An objection of Dr. Kitto to the two-hand alphabet so widely know by school-children and others in Great Britain and in this country would seem to apply with greater force to the Dalgarno alphabet: "To hit the right digit on all occasions is by far the most difficult point to learn in the use of the [two-hand] manual alphabet, and it is hard to be sure which fingers have been touched."12
It is not the purpose of the writer to attempt even a catalogue of the numerous finger alphabets, common, tactile, phonetic, "phonomimic," "phonodactylologic", and syllabic, which have been proposed for the special use of the deaf.
The one-hand alphabet used by Ponce and figured by Bonet was common in Spanish almanacs hawked by ballad-mongers upon the streets of Madrid in the days of De l'Epee, and although rejected by him, it was adopted by his pupils. This with slight modifications became the French manual alphabet which was introduced at Hartford by Dr. Thomas Hopkins Gallaudet. This alphabet is known in almost every hamlet in the land. Slight changes in the form of certain letters, or in the position of the hand, in the direction of greater perspicuity and capacity for rapid use, have taken place gradually, though there is no absolute uniformity of usage among instructors or pupils.
This "American" alphabet, as here presented, through the liberality of Dr. A. Graham Bell, has been drawn and engraved from photographs, and represents typical positions of the fingers, hand and fore-arm from a uniform point of view in front of the person spelling, or as seen in a large mirror by the user himself.13
This alphabet can be learned in less than an hour, and many have learned it by extraordinary application in ten minutes. It is recommended that the arm be held in an easy position near the body, with the fore-arm as in the plates. Each letter should be mastered before leaving it. Speed will come with use; it should not be attempted nor permitted until the forms of the letters and the appropriate positions of the hand are thoroughly familiar. The forms as given are legible from the distant parts of a public hall. In colloquial use the fingers need not be so closely held nor firmly flexed, as represented, but sprawling should be avoided. It is not necessary to move the arm, but a slight leverage at the elbow is conducive to ease and is permissible, provided the hand delivers the letters steadily within an imaginary immovable ring of, say, ten inches in diameter.
THE ONE-HAND ALPHABET IN GENERAL USE.—FRONT VIEW.
THE ONE-HAND ALPHABET IN GENERAL USE.—FRONT VIEW.
This adjunct to speech-reading is recommended for its convenience, clearness, rapidity, and ease in colloquial use, as well as for its value as an educational instrument in impressing words, phrases, and sentences in their spelled form upon the mind, in testing the comprehension of children, and in affording by easy steps a substitute for the sign-language.
In the simultaneous instruction of large classes not able to follow speech, finger-spelling "may take the place of signs to a great extent in the definition, explanation, and illustration of single words and phrases, and in questions and answers upon the lessons, and in communications of every kind to which the stock of language already acquired may be adequate."14
All who have anything to do with the school instruction of the deaf may well bear in mind the matured opinion and wise counsel of Professor Samuel Porter, of the National College, the Nestor of American instructors. In this connection, Professor Porter says:
In short, let the gestural signs come in only as a last resort, or, so far as possible, merely as supplementary to words, re-enforcing them in some instances, or employed as a test of the pupil's knowledge of words, but always, so far as possible, falling behind and taking a subordinate place. And let the pupils be required, in what they have to say to their teachers in the schoolroom or elsewhere, to employ the finger-alphabet instead of natural signs to the utmost possible extent, and this by complete sentences and not in a fragmentary way.
JOSEPH C. GORDON, M.A.,
Professor in the National College, Washington, D.C..
[4]
The brilliant but wily Sicard, whose "show" pupils were accustomed to honoring drafts at sight in appropriate responses to all sorts of questions, acting upon the motto,Mundus vult decipi, ergo decipiatur, schooled certain pupils in deciphering writing in the air, and was thus prepared, in emergencies at his public exhibitions, to convey intimations of the answers, while supposed to be using "signs" in putting questions.
[5]
Quatrieme Circulaire, Paris, 1836, p. 16. Carton'sMemoire, 1845, p. 73.
[6]
Barrois:Dactylologie et langage primitif, Paris, 1850, Firmin Didot freres.
[7]
The library of the New York Institution contains a copy of this very rare edition, bearing the titleAbacus atque velustissima Latinorum per digitos manusque numerandi (quinetiam loquendi) consuetudo, etc., Ratisbonae, 1532.
[8]
For an exhaustive account of the gesture speech in Anglo-Saxon monasteries and of the Cistercian monks, who were under rigid vows of silence, see F. Kluge:Zur Geschichte der Zeichensprache.—Angelsachsische indicia Monaslerialia,inInternational Zeitschrift fur Allgemeine Sprachwissenschaft,II. Band, I. Halfte. Leipzig, 1885.
[9]
Reduccion de lasletras y arte para ensenar a hablar los mudos, 1620. The writer is under obligations to Sr. Santos M. Robledo, of the Ministry of Public Works and Education, for advance sheets of the reprint in beautiful facsimile of this rare work ordered by the Spanish Government in 1881.
[10]
The Abbe de l'Epee did not master the Spanish alphabet, and, attaching but little importance to manual spelling, he was unsparing in his criticism ofMessieurs the dactylologists, but by "the irony of fate" this alphabet occupies a face of the pedestal of one statue to his memory, and in another statue the good Abbe is represented either as receiving this alphabet from the skies or as devoutly using it.
[11]
Philocophus: or, THE DEAFE and Dumbe Mans Friend. By I.B. [John Bulwer] sirnamed theChorosopher. London, 1648. Pp. 106,107.
[12]
Dr. Kitto remaks the following common mistakes in reading rapid two-hand spelling: the confoundingiwitheoro;dwithp;lwitht;fwithx;rwithtand with one form ofj;nwithv, and adds: "Upon the whole, the system is very defective, and is capable of great improvement."—The Lost Senses, p. 107.
[13]
See an interesting paper on figured manual alphabets by H.H. Hollister,Annals, xv., 88-93.
[14]
The Use of the Manual Alphabet, by S. Porter: Proceedings of the Eighth Convention of American Instructors, pp. 21-30. Copies of the Proceedings which contain this extremely valuable paper may be obtained of R. Mathison, Superintendent of the Ontario Institution, Belleville, Ontario.
The use of natural flowers for decorating the person is instinctive among certain peoples, and a question of fashion among others. It is in Oceanica especially that this taste seems to be nationally developed, and from the narrative of Cook we know that the Tahitian belles use in their toilet the perfumed flowers of the pua and tiare (Carissa grandisandGardenia Tahitensis), whose dazzling whiteness renders still more marked the ebony blackness of their wealth of hair.
In Europe this custom is traditional in many countries. Women of fashion scarcely ever appear at a soiree or ball without wearing a camellia or an exotic orchid on their breast or in their head-dress, and so, too, gentlemen of "high life" do not go out without a boutonniere of white violets or Cape jasmine.
But natural flowers, being ephemeral, were once replaced in the toilets of ladies by artificial ones. The artificial flower industry originated in China, and from thence passed into Italy and afterward into France. In course of time people got tired of artificial flowers for decorative purposes, and then imitation fruits made their appearance, and were worn in the toilets of dowagers and mothers of families.
Now that fashion, that tyrant born of dressmakers, milliners, and tailors of renown, obliges us to clothe ourselves according to accepted models, the kaleidoscope no longer suffices to find the most varied designs and most fantastic cuts for garbs or ornament.
In recent years pleasing objects have been borrowed from the animal kingdom, such as small birds and quadrupeds, and insects with brilliant colors and of strange forms. What formerly would have been a repulsive object (such as a great longicorn or beetle) is worn with ease by the belles of our time. The use of such objects of natural history, however, has been about confined to the decoration of head-dresses or the manufacture of jewelry.
FIG. 1.—DRESS TRIMMINGS OF FRUITS AND SEEDS. 1. Seeds of Casuarina and fruit of alder. 2. Acorn cup, involcure of beech, and pod of medick. 3. Fruit of Eucalyptus, cups of acorns, Job’s tears, and cones of cypress.
FIG. 1.—DRESS TRIMMINGS OF FRUITS AND SEEDS. 1. Seeds ofCasuarinaand fruit of alder. 2. Acorn cup, involcure of beech, and pod of medick. 3. Fruit ofEucalyptus, cups of acorns, Job’s tears, and cones of cypress.
As the need of creating new models is always making itself felt, one ingenious manufacturer, Mr. Collin, has turned toward the vegetable kingdom, and brought out an elegant and original style of dress-trimming made of certain indigenous and exotic fruits and seeds that no one would ever have thought of using for such a purpose. Instead of pendants made of wood and covered with silk or velvet, Mr. Collin uses dry fruits or seeds, which he has previously dyed, gilded, or silvered.
FIG. 2.
FIG. 2.—DRESS TRIMMINGS OF FRUITS AND SEEDS. 4 and 5. Fruit of alder. 6. Fruit ofCasuarina. 7. Fruit ofArbutus. 8. Fruit ofCasuarina.
In order that the effect may be good, it is necessary that the objects be not uniform. Their surface must be naturally carved and hollowed, and the projecting parts must detach themselves well from each other. The number of species now used is relatively large, but a selection from these will inevitably be made. Some patterns will be better liked than others, and ladies who are to wear these new trimmings this winter will be able to make their choice of them at the fashion stores. When such articles as these make their appearance, they often spread with surprising rapidity. It is now but a few days since the great dressmaker Worth adopted them, and the linen trade already has them in stock. We recently saw at Suzange's some linen aprons and collars ornamented with small groups of fruits and seeds prepared by the Collin process, and which produced a most pleasing effect. The idea has even occurred to apply these trimmings to furniture and upholstery.
In the manufacture of these articles the cones of several species ofCasuarina, the tags of alder, as well as the naturally carved fruits of certainEloeocarpiof India and Australia, were first used; then came the fruits of the umbelliferous plant,Oenanthe, the spiral pods ofMedicago, the fruit of the water-caltrops,MeliaandZizyphus, the cups of the acorn, the involucres of the beech, the seeds ofCoix lacryma, etc.
The naturalist ought to be glad to see objects that form the base of his studies taking a direction favorable to the industry of his country.
On another hand, these products themselves cannot fail to arouse the curiosity of ladies who have the instinct of observation. And, who knows? Perhaps a frock or mantle trimmed with these vegetable ornaments may prove a more certain propaganda in favor of botany than the most classic lessons on this gentle, science!—La Nature.
The first point referred to in this paper is the source of the vapor that condenses to form dew. A short historical sketch is given of the successive theories from time to time advanced on this point, showing how in early times dew was supposed to descend from the heavens, and then afterward it was suggested that it rose from the earth, while Dr. Wells, who has justly been considered the great master of this subject, thought it came neither from above nor from below, but was condensed out of the air near the surface of the earth. He combated Gersten's idea that it rose from the earth, and showed that all the phenomena observed by Gersten and others which were advanced to support this theory could be equally well explained according to the theory that it was simply formed from the vapor present at the time in the air, and which had risen from the ground during the day, and concluded that if any did rise from the ground during night, the quantity must be small, but, with great caution, he adds that "he was not acquainted with any means of determining the proportion of this part to the whole."
A few observations of the temperature of the ground near the surface, and of the air over it, first raised doubts as to the correctness of the now generally received opinion that dew is formed of vapor existing at the time in the air. These observations, made at night, showed the ground at a short distance below the surface to be always hotter than the air over it, and it was thought that so long as this excess is sufficient to keep the temperature of the surface of the ground above the dew point of the air, it will, if moist, give off vapor, and it will be this rising vapor that will condense on the grass and form dew, and not the vapor that was previously present in the air.
The first question to be determined was whether vapor does, or does not, rise from the ground on dewy nights. One method tried of testing this point was by placing over the grass, in an inverted position, shallow trays made of thin metal and painted. These trays were put over the ground to be tested after sunset and examined at night, and also next morning. It was expected that, if vapor was rising from the ground during dewy nights, it would be trapped inside the trays. The result in all the experiments was that the inside was dewed every night, and the grass inside was wetter than that outside. On some nights there was no dew outside the trays, and on all nights the inside deposit was heavier than the outside one.
An analysis of the action of these trays is given, and it is concluded that they act very much the same as if the air was quite still. Under these conditions vapor will rise from the ground so long as the vapor-tension on the surface of the ground is higher than that at the top of the grass, and much of this rising vapor is, under ordinary conditions, carried away by the passing air, and mixed with a large amount of drier air, whereas the vapor rising under the trays is not so diluted; and hence, though only cooled to the same amount as the air outside, it yields a heavier deposit of dew.
Another method of testing this point was employed, which consisted in weighing a small area of the exposed surface of the ground, as it was evident that if the soil gave off vapor during a dewy night, it must lose weight. A small turf about 6 inches (152 mm.) square was cut out of the lawn, and placed in a small shallow pan of about the same size. The pan with its turf, after being carefully weighed, was put out on the lawn in the place where the turf had been cut. It was exposed for some hours while dew was forming, and on these occasions it was always found to lose weight. It was thus evident that vapor was rising from the ground while dew was forming, and therefore the dew found on the grass was formed of part of the rising vapor, trapped or held back by coming into contact with the cold blades of grass.
The difference between these experiments, in which the exposed bodiesloseweight, and the well-known ones in which bodies are exposed to radiation, and the amount of dew formed is estimated by theincreasein their weight, is pointed out. In the former case, the bodies are in good heat-communication with the ground, whereas in the latter little or no heat is received by conduction from the earth.
Another method employed for determining whether the conditions found in nature were favorable for dew rising from the ground on dewy nights was by observations of the temperatures indicated by two thermometers, one placed on the surface of the grass and the other under the surface, among the stems, but on the top of the soil. The difference in the readings of these two thermometers on dewy nights was found to be very considerable. From 10° to 18° F. was frequently observed. A minimum thermometer placed on, and another under, the grass showed that during the whole night a considerable difference was always maintained. As a result of this difference of temperature, it is evident that vapor will rise from the hotter soil underneath into the colder air above, and some of it will be trapped by coming into contact with the cold grass.
While the experiments were being conducted on grass land, parallel observations were made on bare soil. Over soil the inverted traps collected more dew inside them than those over grass. A small area of soil was spread over a shallow pan, and after being weighed was exposed at the place where the soil had been taken out, to see if bare soil as well as grass lost weight during dewy nights. The result was that on all nights on which the tests were made the soil lost weight, and lost very nearly the same amount as the grass-land.
Another method employed of testing whether vapor is rising from bare soil, or is being condensed upon it, consisted in placing on the soil, and in good contact with it, small pieces of black mirror, or any substance having a surface that shows dewing easily. In this way a small area of the surface of the earth is converted into a hygroscope, and these test surfaces tell us whether the ground is cooled to the dew-point or not. So long as they remain clear and undewed, the surface of the soil is hotter than the dew-point, and vapor is being given off, while if they get dewed, the soil will also be condensing vapor. On all nights observed, these test-surfaces kept clear, and showed the soil to be always giving off vapor.
All these different methods of testing point to the conclusion that during dewy nights, in this climate, vapor is constantly being given off from grass land, and almost always from bare soil; that the tide of vapor almost always sets outward from the earth and but rarely ebbs, save after being condensed to cloud and rain, or on those rarer occasions on which, after the earth has got greatly cooled, a warm moist air blows over it. The results of the experiments are given, showing, from weighings, the amount of vapor lost by the soil at night, and also the heat lost by the surface soil.
It seems probable that when the radiation is strong, that soil, especially if it is loose and not in good heat-communication with the ground, will get cooled below the dew-point, and have vapor condensed upon it. On some occasions the soil certainly got wetter on the surface, but the question still remains, Whence the vapor? Came it from the air, or from the soil underneath? The latter seems the more probable source; the vapor rising from the hot soil underneath will be trapped by the cold surface-soil, in the same way as it is trapped by grass over grass-land. During frost, opportunities are afforded of studying this point in a satisfactory manner, as the trapped vapor keeps its place where it is condensed. On these occasions the under sides of the clods, at the surface of the soil, are found to be thickly covered with hoar-frost, while there is little on their upper or exposed surfaces, showing that the vapor condensed on the surface-soil has come from below.
The next division of the subject is on dew on roads. It is generally said that dew forms copiously on grass, while none is deposited on roads, because grass is a good radiator and cools quicker, and cools more, than the surface of a road. It is shown that the above statement is wrong, and that dew really does form abundantly on roads, and that the reason it has not been observed is that it has not been sought for at the correct place. We are not entitled to expect to find dew on the surface of roads as on the surface of grass. because stones are good conductors of heat, and, the vapor-tension being higher underneath than above the stones, the result is, the rising vapor gets condensed on the under sides of the stones. If a road is examined on a dewy night, and the gravel turned up, the under sides of the stones are found to be dripping wet.
Another reason why no dew forms on the surface of roads is that the stones, being fair conductors, and in heat communication with the ground, the temperature of the surface of the road is, from observations taken on several occasions, higher than that of the surface of the grass alongside. The air in contact with the stones is, therefore, not cooled so much as that in contact with the grass.
For studying the formation of dew on roads, slates were found to be useful. One slate was placed over a gravelly part of the road, and another over a hard dry part. Examined on dewy nights, the under sides of these slates were always found to be dripping wet, while their upper surfaces, and the ground all round, were quite dry.
The importance of the heat communicated from the ground is illustrated by a simple experiment with two slates or two iron weights, one of them being placed on the ground, either on grass or on bare soil, and the other elevated a few inches above the surface. The one resting on the ground, and in heat-communication with it, is found always to keep dry on dewy nights, whereas the elevated one gets dewed all over.
The effect of wind in preventing the formation of dew is referred to. It is shown that, in addition to the other ways already known, wind hinders the formation of dew by preventing an accumulation of moist air near the surface of the ground.
An examination of the different forms of vegetation was made on dewy nights. It was soon evident that something else than radiation and condensation was at work to produce the varied appearances then seen on plants. Some kinds of plants were found to be wet, while others of a different kind, and growing close to them, were dry, and even on the same plant some branches were wet, while others were dry. The examination of the leaf of a broccoli plant showed better than any other that the wetting was not what we might expect if it were dew. The surface of the leaf was not wet all over, and the amount of deposit on any part had no relation to its exposure to radiation or access to moist air; but the moisture was collected in little drops, placed at short distances apart, along the very edge of the leaf. Closer examination showed that the position of these drops had a close relation to the structure of the leaf; they were all placed at the points where the veins in the leaf came to the outer edge, at once suggesting that these veins were the channels through which the liquid had been expelled. An examination of grass revealed a similar condition of matters; the moisture was not equally distributed over the blade, but was in drops attached to the tips of some of the blades. These drops, seen on vegetation on dewy nights, are therefore not dew at all, but are an effect of the vitality of the plant.
It is pointed out that the excretion of drops of liquid by plants is no new discovery, as it has been long well known, and the experiments of Dr. Moll on this subject are referred to; but what seems strange is that the relation of it to dew does not seem to have been recognized.
Some experiments were made on this subject in its relation to dew. Leaves of plants that had been seen to be wet on dewy nights were experimented on. They were connected by means of an India-rubber tube with a head of water of about one meter, and the leaf surrounded with saturated air. All were found to exude a watery liquid after being subjected to pressure for some hours, and a broccoli leaf got studded all along its edge with drops, and presented exactly the same appearance it did on dewy nights. A stem of grass was also found to exude at the tips of one or two blades when pressure was applied.
The question as to whether these drops are really exuded by the plant, or are produced in some other way, is considered. The tip of a blade of grass was put under conditions in which it could not extract moisture from the surrounding air, and, as the drop grew as rapidly under these conditions as did those on the unprotected blades, it is concluded that these drops are really exuded by the plant. Grass was found to get "dewed" in air not quite saturated.
On many nights no true dew is formed, and nothing but these exuded drops appear on the grass; and on all nights when vegetation is active, these drops appear before the true dew; and if the radiation is strong enough and the supply of vapor sufficient, true dew makes its appearance, and now the plants get equally wet all over, in the same manner as dead matter. The difference between true dew on grass and these exuded drops can be detected at a glance. The drops are always exuded at a point near the tip of the blade, and form a drop of some size, while true dew is distributed all over the blade. The exuded liquid forms a large diamond-like drop, while the dew coats the blade with a pearly luster.
Toward the end of the paper the radiating powers of different surfaces at night is considered, and after a reference to some early experiments on this subject, the paper proceeds to describe some experiments made with the radiation thermometer described by the author in a previous paper. When working with this instrument, it is placed in a situation having a clear view of the sky all round, and is fixed at the same height as the ordinary thermometer screen, which is worked along with it, the difference between the thermometer in the screen and the radiation thermometer being observed. This difference in clear nights amounts to from 7° to 10°. By means of the radiation thermometer the radiating powers of different surfaces were observed. Black and white cloths were found to radiate equally well; soil and grass were also almost exactly equal to each other. Lampblack was equal to whitening. Sulphur was about two-thirds of black paint, and polished tin about one-seventh of black paint. Snow in the shade on a bright day was at midday 7° colder than the air, while a black surface at the same time was only 4° colder. This difference diminished as the sun got lower, and at night both radiated almost equally well. In the concluding pages of the paper some less important subjects are considered.