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Art. 59. An account of the Earthquake, November 1, 1755,
as felt in the Lead-mines in. Derbyshire. This last article is followed by a series of accounts transmitted to the Society, of the terrible effects of this dreadful calamity, in Portugal and Barbary, also how far it was felt in Spain, Switzerland, Madeira, New-York, Pennsylvania, &c. But, for these, we are obliged, for brevity's fake, to refer to the Collection itself; as well as for the articles pafled over, without any mention, in the course of the foregoing Summary : See the Apology for such Omiffions, in our last, p. 271.
Elays and Obfervations, Physical 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 first production * of this Literary
Society, we took notice of the occasion of their establishment, and the views they proposed in it to themselves; how diligently, and judiciously, they have pursued their plan, may be collected from the volume now before us, which contains thirty-six articles, most of them curious and instructive.
The first 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 sex, we think it neceliary to mention, that the discovery of this plant is ascribed to a young Ladyt, a most ingenious Botanist, 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 some very large Bones, by George Clerk, Esq; fupposed to be the remains of an Elk, or fome other foreign animal, found in a shell marlpit near the town of Dumfries.
* Review, vol. IX. p. 169.
+ Miss Jenny Colden, daughter of Cadwallader Colden, Efq; a Gentleman well known, and as well respected, 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-fevent. Had he lived,' say they, to have put the finishing hand to it, he would, 'probably, have add
ed many things, and, perhaps, retrenched some others, by & which it would have been rendered still more deserving the • approbation of the public. Mr. Melvill ufed to observe, that
as, of all Sir Ifaac Newton's discoveries, those relating to • Light and Colours were, perhaps, the most curious; it was
fomewhat remarkable, that few, if any, of his followers had gone one step beyond him on these subjects, or attempted to
complete what he had left unfinished.' Our Author, there• fore, proposed to bave applied himself particularly to the • farther illustration of the thcory of Light and Colours.'
It is certainly a lofs to the learned world, that so ingenious a Gentleman did not live to profecute this curious branch of frience, as he was certainly very well qualified for the talk, of which the Efray 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 silver-like appearance of drops of Water on the leaves of Colewort. 4. On the change wbich coloured bodies undergo in different Lights. 5. A Remark on Euler's Nova Theoria Lucis et Colorun 6. Concerning the cause of the different Refrangibility of the Rays of Light. 7. On the Imperfection of our Knowlege concerning the Inflexions of Light. 8. Queries, consisting of doubts, difficulties, and conjectures, concerning Light, Colours, and Coloured Bodies.
As a regular abstract of this Effay would too much extend the present article, we shall only give a few extracts from fome particular parts of it, and refer our Readers, for further fatisfaction, to the Efray itfelf; which highly merits an attentive perasal : tho' in some particular paflages, we apprehend our Author is mistaken; which will not appear at all.surprising, if we consider the many difficulties attending this curious fubject. What Mr. Melvil observes, in the third section, concerning the silver-like appearance of the drops of water on the leaves of Colewort, is very curious, and, at the same time, thews, the extensive utility of Optical Principles, in leading to the knowlege of things otherwise inaccessible.
" It is common,' says Mr. Melvill, to admire the volubility and lustre of drops of rain that lie on the leaves of cole
wort, and some other vegetables ; but no philosopher, as far as I know, has put himself to the trouble of explaining
this curious phænomenon. Upon inspecting them narrow<ly, I find, that the lustre of the drop arises from a copious
reflection of light from the Aattened part of its furface conĆ
tiguous to the plant: I observe further, that when the drop • rolls along a part which has been wetted, it immediately
loses, all its lustre; the green plant being then seen clearly "thro' it: whereas, in the other case, it is hardly to be discerned. <From these two observations 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 repulsive power; for • there could not be any copious reflection of white light from cits under surface, unless there were a real interval between
it and the surface of the plant. • If that surface were perfectly smooth, the under surface of the drop would be so likewise; and would therefore shew an image of the illuminating body by reflection, like a piece of polished silver : but, as it is considerably rough and unequal,
the under surface becomes rough likewise; and so by reflect' ing the light copiously in different directions, assumes the
resplendent white colour of unpolished silver. • After it is thus proved by an optical argument, that the drop is really not in contact with the plant which supports it, we easily conceive whence its wonderful volubility arises, 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 suspension of the drops of dew on the very summits of the blades of grass, &c. and their assuming that pearly, or resplendent white colour, observed by our Author, in the drops on the leaves of colewort.
In the fixth section 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 eclipses of Jupiter's Satellites, fince our Author wrote the Eflay before us, appears to be false.
In the laft section, Query 15, our Author endeavours to fhew, that the various colours reflected by the clouds, in the morning and evening, are not separated from the rays by the clouds themselves, as Sir Isaac Newton believed they were, but in passing through the horizontal atmosphere.
• Is not,' says he, ''the opinion which Sir Ifaac Newton. ! seems to have had, and, since him, the generality of philoso
phers, concerning the cause of the various colours reflected
by the clouds, at sun-rising and setting, liable to great diffi• culties? For, why should the particles of the clouds be
come, at that particular time, and never at any other, of • such magnitude as to separate these colours ? and why are
they rarely, if ever, seen tinctured with blue and green, as well as red, orange, and yellow? Is it not more credible, that the separation of rays is made in passing through the hori, zontal atmosphere and that the clouds only reflect and
transmit the sun's light, as any half transparent colourless • body would do in their place? For, since the atmosphere,
as was said in the lait query, reflects a greater quantity of • blue and violet rays than the rest, the sun's light transmitted
through it, ought to draw towards yellow, orange, or red ; especially when it passes through the greatest tract of air : accordingly, every one must have remarked, that the sun's
horizontal light is sometimes so deeply tinctured, that ob«jects directly illuminated by it, appear of a high orange, or
even red; at that instant is it any wonder that the colourless $ clouds reflect the same rays in a more bright and lively man
ner? It is observable, that the clouds do not commonly - assume their brighter dyes till the sun is some minutes set; s and that they pass 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 al
together. 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 instant of setting, perhaps by the differ cence 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 sun's light « becomes always somewhat yellowish, or orange, in passing
through the depth of the atmosphere horizontally, it ought « to incline more and more from orange towards red; by
passing through a ftill greater length of air; so that the clouds, according to their different altitude, may assume all the variety of colours observed in them at fun-rising and feta
ting, by barely reflecting the sun's incident light as they re(ceive it? I have often observed with pleasure, when in
Switzerland, that the snowy summits of the Alps turn more ( and more reddith after sun-fet, in the same manner as < the clouds. What makes the same colours much more rich • and copious in the clouds, is their semi-transparency joined (with the obliquity of their situation.
• Does it not greatly confirm this explication, that these co*loured clouds immediately resume that dark leaden hue which they receive from the sky, as soon as the sun's direct rays cease to strike upon them? For, if their gaudy colours
arofe, like those of the foap-bubble, from the particular fize 6 of their parts, they would preserve nearly the same 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 some distance on each side from the place of
his setting, feems to incline to a faint fea-green, by the mix5
ture of his transmitted beams, which are then yellowish, with the ethereal blue : at greater distances, this faint green gradually changes into a reddish brown; because the sun's
rays, by paffing through more air, begin to incline to orange: * and, on the oppofite fide of the hemdphere, the colour of
the horizontal sky inclines fenfibly to purple; because his transmitted light which mixes with the azure, by palling * through a ftill greater length of air, becomes reddith; as
we have faid above. 1. To undertand 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 lastly to red, we have only to apply to the atmosphere, what Sir Isaac
fays (book I. of his Optics, part II. prop. 10.) concerning * the colour of transparent liquors in general.' Art. 5. An easy method of computing the Parallaxes of the Moona
By The greateft dificulty and labour attending the calculation of Solar
Eclipses, results from the tedious methods of finding the Moon's Parallax, and therefore whoever discovers an ealy and concise method of finding it, does great service to Astronomy,
such a method we have in this paper, and which cannot fail of being kindly received by Astronomers. Some time since a pamphlet intitled, A new and compendious Method of inveftigating the parallactic Angle,
without regard to the Nonagesiinal Degree, was published, (See Review, vol. XI. p. 38.) in which a method nearly fimilar to that in the Eliay before us was given ; but this is much easier, and rather more accu.fate. Art. 6. A Solution of Kepler's Problem. By Matthew Stewart,
Professor of Mathematics in the University of Edinburgh.
If the fun were placed exactly in the center of the earth's orbit, and the earth described equal angles in equal times; then the sun's apparent motion in the ecliptic, would be alREV.Qát. 1756,