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Art. 59. An account of the Earthquake, November 1, 1755, as felt in the Lead-mines in Derbyshire.
This laft article is followed by a feries of accounts tranfmitted to the Society, of the terrible effects of this dreadful calamity, in Portugal and Barbary; alfo how far it was felt in Spain, Switzerland, Madeira, New-York, Pennsylvania, &c. But, for thefe, we are obliged, for brevity's fake, to refer to the Collection itself; as well as for the articles paffed over, without any mention, in the courfe of the foregoing Summary: See the Apology for fuch Omiffions, in our last, P. 271.
Efays and Obfervations, Phyfical and Literary, read before a Society in Edinburgh, and published by them Vol. the IId. 8vo. 6s. Printed at Edinburgh, by Hamilton and Balfour.
N our account of the firft production* of this Literary Society, we took notice of the occafion of their establishment, and the views they propofed in it to themselves; how diligently, and judiciously, they have purfued their plan, may be collected from the volume now before us, which contains thirty-fix articles, most of them curious and instructive.
The firft is an account of a new plant; by Dr. Alexander Garden, of Charles-town in Carolina. As we would not willingly pass over unnoticed, any thing that may contribute to the honour of the fair fex, we think it necefiary to mention, that the difcovery of this plant is afcribed to a young Lady †, a moft ingenious Botanift, who has named it Gardenia, in compliment to Dr. Garden.
Art. 2. Is a description, accompanied with a plate, of the Matrix, or Ovary, of the Buccinum Ampullatum; by Robert Whytt, M. D. F. R. S. &c. &c. And,
Art. 3. Gives the Drawings of fome very large Bones, by George Clerk, Efq; fuppofed to be the remains of an Elk, or fome other foreign animal, found in a fhell marlpit near the town of Dumfries.
*Review, vol. IX. p. 169.
Mifs Jenny Colden, daughter of Cadwallader Colden, Efq; a Gentleman well known, and as well refpected, in the Commonwealth of Literature.
Art. 4. Obfervations on Light and Colours. By Thomas Mel
vill, M. A.
The Editors of this volume, in a Note, inform us, that the ingenious Author of this article died in 1753, at the age of twenty-feven. Had he lived,' fay they, to have put the finishing hand to it, he would, probably, have added many things, and, perhaps, retrenched fome others, by • which it would have been rendered ftill more deferving the approbation of the public. Mr. Melvill ufed to obferve, that as, of all Sir Ifaac Newton's difcoveries, thofe relating to Light and Colours were, perhaps, the moft curious; it was • fomewhat remarkable, that few, if any, of his followers had gone one step beyond him on thefe fubjects, or attempted to complete what he had left unfinished. Our Author, therefore, proposed to have applied himfelf particularly to the ⚫ farther illuftration of the theory of Light and Colours.'
It is certainly a lofs to the learned world, that fo ingenious a Gentleman did not live to profecute this curious branch of fcience, as he was certainly very well qualified for the talk, of which the Effay before us is an undeniable proof. It is divided into eight fections, viz. 1. On the mutual Penetration of Light. 2. On the Heating of Bodies by Light, 3. On the filver-like appearance of drops of Water on the leaves of Colewort. 4. On the change which coloured bodies undergo in different Lights. 5. A Remark on Euler's Nova Theoria Lucis et Colorum. 6. Concerning the caufe of the different Refrangibility of the Rays of Light. 7. On the Imperfection of our Knowlege concerning the Inflexions of Light. 8. Queries, confifting of doubts, difficulties, and conjectures, concerning Light, Colours, and Coloured Bodies.
As a regular abftract of this Effay would too much extend the prefent article, we fhall only give a few extracts from fome particular parts of it, and refer our Readers, for further fatisfaction, to the Effay itfelf; which highly merits an attentive perufal: tho' in fome particular paffages, we apprehend our Author is miftaken; which will not appear at all furprifing, if we confider the many difficulties attending this curious fubject. What Mr. Melvil obferves, in the third fection, concerning the filver-like appearance of the drops of water on the leaves of Colewort, is very curious, and, at the fame time, fhews, the extenfive utility of Optical Principles, in leading to the knowlege of things otherwife inacceffible.
It is common,' fays Mr. Melvill, to admire the volubility and luftre of drops of rain that lie on the leaves of cole
wort, and fome other vegetables; but no philofopher, as far as I know, has put himself to the trouble of explaining this curious phænomenon. Upon infpecting them narrowly, I find, that the luftre of the drop arifes from a copious reflection of light from the flattened part of its furface contiguous to the plant: I obferve further, that when the drop rolls along a part which has been wetted, it immediately lofes all its luftre; the green plant being then feen clearly thro' it: whereas, in the other cafe, it is hardly to be dif ‹ cerned.
From these two obfervations laid together, we may certainly conclude, that the drop does not really touch the plant when it has the mercurial appearance, but hangs in the air at fome distance from it, by the force of a repulfive power; for there could not be any copious reflection of white light from its under furface, unless there were a real interval between it and the surface of the plant.
If that furface were perfectly fmooth, the under surface of the drop would be fo likewife; and would therefore fhew an image of the illuminating body by reflection, like a piece of polished filver: but, as it is confiderably rough and unequal, • the under surface becomes rough likewife; and fo by reflecting the light copiously in different directions, affumes the ⚫ refplendent white colour of unpolished filver.
After it is thus proved by an optical argument, that the ⚫ drop is really not in contact with the plant which fupports it, we eafily conceive whence its wonderful volubility arifes, and why it leaves no tract of moisture where it rolls.'
This explanation of our Author will help us to account for that common phænomenon, the fufpenfion of the drops of dew on the very fummits of the blades of grafs, &c. and their affuming that pearly, or refplendent white colour, obferved by our Author, in the drops on the leaves of colewort.
In the fixth fection our Author is inclined to think, that the differently coloured rays are projected with different velocities from the luminous body: but this, from accurate experiments made on eclipfes of Jupiter's Satellites, fince our Author wrote the Effay before us, appears to be false.
In the laft fection, Query 15, our Author endeavours to fhew, that the various colours reflected by the clouds, in the morning and evening, are not feparated from the rays by the clouds themselves, as Sir Ifaac Newton believed they were, but in paffing through the horizontal atmosphere.
Is not,' fays he, the opinion which Sir Ifaac Newton. • seems to have had, and, fince him, the generality of philofo
phers, concerning the cause of the various colours reflected by the clouds, at fun-rifing and fetting, liable to great diffi• culties? For, why fhould the particles of the clouds become, at that particular time, and never at any other, of • fuch magnitude as to feparate these colours? and why are they rarely, if ever, feen tinctured with blue and green, as ⚫ well as red, orange, and yellow? Is it not more credible, that the feparation of rays is made in paffing through the horizontal atmosphere? and that the clouds only reflect and tranfmit the fun's light, as any half transparent colourless body would do in their place? For, fince the atmosphere, as was faid in the laft query, reflects a greater quantity of • blue and violet rays than the reft, the fun's light transmitted through it, ought to draw towards yellow, orange, or red; especially when it paffes through the greatest tract of air: accordingly, every one must have remarked, that the fun's horizontal light is fometimes fo deeply tinctured, that objects directly illuminated by it, appear of a high orange, or even red; at that inftant is it any wonder that the colourless clouds reflect the fame rays in a more bright and lively manner? It is obfervable, that the clouds do not commonly affume their brighter dyes till the fun is fome minutes fet; • and that they pafs from yellow to a flaming golden colour and thence, by degrees, to red; which turns deeper and deeper, tho' fainter and fainter, till the fun leaves them altogether. Now, it is plain, that the clouds, at that time, receive the fun's light through a much longer tract of air than we do at the inftant of fetting, perhaps by the difference of a hundred miles or more; as may be computed from their height, or the duration of their colours. Is it not, therefore, natural to imagine, that, as the fun's light becomes always fomewhat yellowish, or orange, in paffing through the depth of the atmosphere horizontally, it ought to incline more and more from orange towards red, by paffing through a ftill greater length of air; fo that the clouds, according to their different altitude, may affume alf the variety of colours obferved in them at fun-rifing and fetting, by barely reflecting the fun's incident light as they receive it? I have often obferved with pleafure, when in Switzerland, that the frowy fummits of the Alps turn more and more reddish after fun-fet, in the fame manner as the clouds. What makes the fame colours much more rich ⚫ and copious in the clouds, is their femi-transparency joined with the obliquity of their fituation.
Does it not greatly confirm this explication, that these co*doured clouds immediately refume that dark leaden hue which they receive from the fky, as foon as the fun's direct rays ceafe to ftrike upon them? For, if their gaudy colours arofe, like thofe of the foap-bubble, from the particular fize • of their parts, they would preferve nearly the fame colours, tho' much fainter, when illuminated only by the atmosphere. About the time of fun-fet, or a little after, the lower part of the sky, to fome diftance on each fide from the place of ⚫his setting, feems to incline to a faint fea-green, by the mixture of his tranfmitted beams, which are then yellowish, with the ethereal blue: at greater diftances, this faint green gradually changes into a reddifh brown; because the fun's rays, by paffing through more air, begin to incline to orange: and, on the oppofite fide of the hemifphere, the colour of the horizontal sky inclines fenfibly to purple; because his tranfmitted light which mixes with the azure, by paffing through a ftill greater length of air, becomes reddith; as we have faid above.
To understand diftinctly why the fun's rays, by paffing thro' a greater and greater quantity of air, change by degrees from white to yellow, thence to orange, and laftly to red, we have only to apply to the atmosphere, what Sir Ifaac fays (book I. of his Optics, part II. prop. 10.) concerning *the colour of transparent liquors in general.'
Art. 5. An eafy method of computing the Parallaxes of the Moons
The greatest difficulty and labour attending the calculation of Solar Eclipfes, refults from the tedious methods of finding the Moon's Parallax, and therefore whoever difcovers an eafy and concife method of finding it, does great fervice to Astronomy; fuch a method we have in this paper, and which cannot fail of being kindly received by Aftronomers. Some time fince a pamphlet intitled, A new and compendious Method of investigating the parallactic Angle, without regard to the Nonagefimal Degree, was published, (See Review, vol. XI. p. 38.) in which a method nearly fimilar to that in the Ellay before us was given; but this is much easier, and rather more accu
Art. 6. A Solution of Kepler's Problem. By Matthew Stewart, Profeffor of Mathematics in the University of Edinburgh. If the fun were placed exactly in the center of the earth's orbit, and the earth defcribed equal angles in equal times, then the fun's apparent motion in the ecliptic, would be alREV. Oct. 1756.