Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

as

He adds, however, very feelingly,

• A curate is another sort of man,

Very unlike the Metropolitan,

Living, without a living, as he can.' To these new and surprising discoveries, that archbishops and eurates are not men of the same sort, and that cura have not live ings, the ingenious author has added some observations on the professional duties of the latter, which our readers will judge to be singularly appropriate, when we confess that we do not quote them flagrant instances of folly and dulness, only because they are also grossly and stupidly indecent.

We have expended more time on Mr. Colman than we had at first proposed; and yet we have not given him all the notice which he deserves. We hope, however, that we have said enough to prevent any of our readers from being misled into the purchase of his • Vagaries' as a book of amusement:- it is a book through which nothing but our duty could have enabled us to wade; and we cannot but rejoice that a work, which is so indelicate that no one ought to read it, is luckily so tiresome that nobody will read it.

Art. IX. Outlines of Natural Philosophy; being Heads of

Lectures delivered in the University of Edinburgh. By John Playfair, Professor of Natural Philosophy in the University of Edinburgh, Fellow of the Royal Society of London, and Secretary of the Royal Society of Edinburgh. Vol. I. Edinburgh, Constable and Co., London, Longman and Co.; Cadell and

Davies; and Murray. 8vo. pp. viii. 310. 1812. THERE

HERE are two ways in which lecturers on different branches of

natural philosophy have endeavoured to increase the effect resulting from their oral instructious or from their experiments. The one, by preparing very short synopses of the principal matters and putting them into the hands of students, to make them acquainted, not only with the general nature of what is to be brought before them, but with the order in which the several parts are to be presented. The other, by drawing out an entire outline of the course, with a correct enunciation of every proposition advanced; in order to put the student in complete possession of all which the lecturer thinks peculiarly striking or valuable, and to facilitate his recollection of the train of reasoning by which the several propositions were established, or of the experiments by which they were illustrated or confirmed. Of these methods, the former has been frequently

K 3

adopted;

adopted; the latter, comparatively, but seldom. In the department of natural philosophy, generally, we recollect but four books, exclusive of this before us, which give any thing like a fair abstract of the several subjects in a course; these are, Atwood's Analysis of a Course of Lectures on the Principles of Natural Philosophy, published at Cambridge in 1784; Vince's · Plan of a Course of Lectures, &c.'first published at Cambridge in 1793; Dr. Thomas Young's - Syllabus of a Course of Lectures,' &c. delivered at the Royal Institution, published in 1802; and Dr. Matthew Young's

Analysis of the Principles of Natural Philosophy,' published in 1803. Each of these is an ingenious and useful work, and the latter especially, being extended to nearly double the size of either of the others, contains much that is valuable, and not a little which a mere English student could scarcely be shewn in any other performance.

Professor Playfair, though travelling over the same regions with the authors just mentioned, and, of course, compelled to behold many things in nearly the same points of view, yet by no means confines himself to the paths of his precursors. The road which he takes is, correctly speaking, of his own striking out, and, in the main, it is direct and smooth. He examines nearly all that conies in his way with the eye of a keen observer, and, in giving us the result of his inquiries, it is not often that he can be fairly accused of either pedantry or affectation.

The subjects of which the Professor treats in the present volume, after an introduction on the properties of matter, are dynamics, mechanics, hydrostaties, hydraulics, aerostatics, and pneumatics. In discussing these, though an entire and thorough deviation from the usual route is neither to be espected nor wished, yet the plan here pursued is sufficiently original and characteristic of its author to be worth detailing. Thus, under the head dynamics, the measures of motion are first explained; the succeeding propositions relate to the first law of motion, the communication of motion by impulse, equably accelerated or retarded motion, the motion of projectiles, and notion accelerated or retarded by variable forces. The part devoted to mechanics relates to the centre of gravity, the mechanical powers, including the funicular machine, friction, mechanical agents, the motion of machines, descent of heavy bodies on plane and curved surfaces, the centre of oscillation, descents along cycloidal surfaces, and the rotation of bodies both about fixed and movable axes. An appendix to this part contains the principles of the construction of arches, and those by which the comparative strength of different beams of timber is ascertained. Under hydrostatics the author treats of the pressure of Auids, the equilibrium of solid bodies foating on fluids, and the phenomena

of

of capillary tubes. The portion appropriated to hydraulics, treats . of the motion of fuids issuing through apertures in the bottoms or sides of vessels, of conduit pipes and open canals, of the percussion and resistance of Auids, of the undulations of Auids, and of hydraulic engines, comprizing those moved by the impulse of water, those moved by the weight, and those by the reaction of water. The part on aerostatics contains propositions and observations on heat and on the equilibrium of elastic Auids. And under the head of pneumatics, the last treated in this volume, air is contemplated, first, as accelerating or retarding motion ; 2dly, as the vehicle of sound; 3dly, as the vehicle of heat and moisture. And in one or other of these sections the author treats of machines for raising water, of the steam engine, of motion produced by the explosion of gunpowder, of the resistance of the air to projectiles, of the vibrations of sonorous bodies and the propagation of those vibrations through the air, of wind and rain.

Such is the distribution of subjects: and it is but just to add, that the Professor has discussed them generally ably, and almost always perspicuously, as far as he goes. We say as far as he goes, because, in giving an abstract of a course of lectures, a writer can but seldom enter into the detail of demonstrations and experiments; and must therefore feel considerable difficulty in determining what may be safely omitted, and what it is absolutely necessary to retain. We have seen some syllabuses of lectures which were perfectly grotesque in their representations, though the lectures themselves were highly interesting and valuable. Indeed in all the works of this kind which we have seen, with the exception of the Syllabus of Dr. Thomas Young, the authors demonstrate only by fits; and the reason is obvious. Where they have a demonstration to present which is new, or striking, or more concise, or more perspicuous, than usual, it is of course inserted; while the other theorems or results are merely enunciated; the auditors of the lectures being expected to call the proofs to mind, while the general reader is left to imagine that however they may be omitted in the abstract, they were duly given in the lecture room.

This, however, is a defect which was not likely to escape the penetration of such a writer as Professor Playfair. He endeavours to supply it by referring to those places, in the works of other allthors, where the requisite information is to be found; and this is the principal novelty in his ' Outlines, though we shall have to offer a few remarks on it, presently. But before we enter upon any observations tending to the improvement of this work, we shall select two or three passages in which we think the author has been successful, either in demonstrating de novo, or in presenting the results of the inquiries of former investigators. K 4

OS

Of the former kind, is Mr. Playfair's demonstration of the second of the two subsequent properties relative to the collision of elastic bodies; the first is due to Bernoulli.

76. In perfectly elastic bodies, the sum of the products made by multiplying each of the bodies into the square of its velocity, is the same after collision that it was before it.

Let A and B be the masses of the bodies, a and b their velocities before collision, á' and 6 their velocities after it. Then, as the quantities of motion before and after collision are the same (68)

Aa + Bb = Ad + B 6',
or A (a-a') = B 6-6);
and since a + d' = 6 + b, [75]
therefore A (a? da)= B (b? + 62),
or A a2 + B 62 =

= Ad + B 6'2. "See another demonstration, Maclaurin's Account of Newton's Dis. coveries, Book 11. chap. iv. $ 12.

77. If between two unequal elastic bodies A and C, a third B be interposed; and if the leasi Å, be made to strike with any given velocity on B, the motion communicated to C will be the greatest possible, when B is a mean proportional between A and C. It is easily shewn from $74 that the velocity communicated to C is = 4 A Ba 4 AG

4 A a
(A + B)(B+C) A

AC +C
+ 1)(B+C) A + B +
B

B

AC *This fraction is a maximum, when the denominator A + B +

+C

B is a minimum, that is, since A and C are given, when B2 = AC, or when B is a mean proportional between A and C.

4 Aa

4 Aa The velocity of C is

and the A + 2 VAC +C (A + C)2'

4 A Ca motion of C =

(A + v C)2 'If the number of the bodies in geometrical progression be increased without limit, the quantity of motion communicated to the last, from a given quantity of motion in the first, however small, may also be increased without limit. Notwithstanding this, as all the bodies move backward after collision, but the last, if they form an increasing series the sum of all the motions in the direction of the first mover conti, nues = A a. Also the sum of the products of each body, into the square of its velocity, after collision, remains as it was before, equal to Aa?.'

Altogether, we think the section on the communication of motion by impulse, whence the above is quoted, very neat; there are, however, two omissions which the author will do well to supply when he has opportunity. The first is that of a proposition explaining the circumstances of the collision of bodies imperfectly elastic

in any degree. The second is of a remark tending to prevent any misinterpretation or misapplication of Bernoulli's celebrated proposition respecting the conservation of motion : they are only perfectly elastic bodies, (that is, in other words they are no bodies at all,) in which the sum of the products formed by multiplying each body by the suure of its velocity is not changed by the impact.

We should have liked to quote freely from those parts of the Professor's work in which he gives a summary of the results deduced by Du Buat and Robison, concerning the motion of water in conduit pipes, canals, and rivers, and those of Robins and Hutton, on the motion of projectiles, and to have compared the former with the calculations of a late writer in the Philosophical Transactions, who has newly modelled the whole of Du Buat's doctrine: but, as we have many remarks to offer on different parts of the book, we can only venture upon one extract of any length, which relates to a subject rather more important in practice, that is, the estimation of the mechanical agency of animals.

* 176. The strength of men, and of all animals, is most powerful when directed against a resistance that is at rest; when the resistance is overcome, and when the animal is in motion, its force is diminished; lastly, with a certain velocity, the animal can do no work, and can only keep up the motion of its own body.

"A formula having the throc properties just mentioned, wi.. afford an approximation to the law of animal force. Let P be the weight which the animal exerting itself to the utmost, or at a dead pull

, is just able to overcome; W any other weight with which it is actually loaded ; and v the velocity with which it moves when so loaded; c the velocity at which the power of drawing or carrying a load entirely ceases; then W=P(1 – ?) is an equation that has all the three conditions mentioned above.

· Not only, however, has the formula P(1—?) these conditions, but the square of it has the same, or, indeed, any function of it which vanishes when i vanishes, that is, when v = c. We are left, then, at liberty to choose any of these functions, and would assume the formula above as the simplest, if another condition did not seem necessary to be included. It is certain, that in all cases, when v approaches to c, or when the speed becomes great, a small variation in the weight is accompanied with a great variation in the velocity. The simplest formula that corresponds to this condition is, when 1 - is raised to the square.

* 177. Therefore, till experience has led to a more accurate result, we may suppose the strength of animals to follow the law expressed by the formula W = = P(1.-)

• This

с

2

« ΠροηγούμενηΣυνέχεια »