'But a simpler method of reading off divisions with accuracy in common instruments, is the application of a vernier, an apparatus so called from its inventor. The space occupied by eleven divisions of the scale being divided into ten parts in the index, the concidence of any of the divisions of the index with those of the scale shows by its distance from the end, the number of tenths to be added to the entire divisions. (Plate vii. fig. 92.)'

The reference to its place is in these words simply: A vernier, indicating SS3 of the divisions of its scale. P. 105.' Of the figure itself, we must complain that its execution is such, that to an eye of moderate powers, no less than three of the divisions of the index appear to coincide with the divis ons of the scale. But this en passant. In the description of the instrument itself, we arrive by a single leap from the premises at the conclusion. Had the doctor condescended to give two or three of the intermediate steps, we think he would have saved inost of his readers a toil, which many of them will think a greater evil than remaining ignorant of the use of a vernier. To those who can read Newton's Principia, or who perchance are versed in Cocker's Arithmetic, more words were not needful. But does Dr. Young write for such persons only? We hope that the purchasers of his book will be infiuitely more numerous. beg our readers to observe that we have cited this example as one of ill-placed brevity, the consequence, probably, of the writer's having comprehended in his design too great a multiplicity of objects. We might, if we thought right, produce other examples to illustrate our other objections. But we wish to avoid the appearance of captious criti

cism.

We

That Dr. Young is profoundly skilled in the methods of mathematical analysis, and the sciences depending upon them, no one can doubt, who is acquainted with the many ingenious speculations by which he has distinguished himself. But he does not appear to us to have paid due attention to the metaphysics of philosophy, by reason of which he has sometimes fallen into the use of language, which we dee:n obscure and unphilosophical. Force is a species of power; it is power applied to the generation of motion. Force denotes always a species of relation, and we doubt whether it is possible for the mind to conceive it as possessing an absolute and independent existence. Whether forces therefore can, strictly speaking, have that sort of existence which is susceptible of proportion, whether they can be properly represented by magnitudes, and thus be a subject of mathematical demonstration we extremely doubt. word, we doubt whether force can legitimately be called a

In a

quantity, and therefore whether the expression of double, treble, quadruple force, &c. has any intelligible signification. Let us examine those with which we are acquainted; for some of which we talk most familiarly, gravitation for example, is entirely hidden from us, except by its effects. Volition is a true and proper force, which considered as a cause and in its effects is present to us every moment. if we were to talk of a double or triple volition, should we not be using unintelligible jargon? Heat considered as a cause of expansion is also a force; the expansion may be double or treble; but a double or a treble heat is what no one can understand.

But

When, therefore, we undertake to measure forces, and to express them by arbitrary signs, be they algebraical characters or mathematical figures, it is under some secret hypothe. sis that causes are proportional to their effects; a posi tion very commonly laid down, as a self-evident truth; but to which we cannot assent, as we see that effects are often susceptible of proportion, whilst their causes are wholly incapable of it. We have thrown out these observations as they prove to our own minds, that the laws of motion, the fundamental properties of the lever, the laws of the descent of heavy bodies, in truth all the fundamental principles of dynamics, are reallynot mathematical but experimental truths, and that all attempts to prove them to be necessary truths, either from metaphysical or from mathematical considerations, must ever fail. Had D. Young justly considered the proper boundaries between mathematical and experimental truths, we think he never would have written the following sentence :

'The law discovered by Galileo, that the space described is as the square of the time of descent, and that it is also equal to half the space, which would be described in the same time with the final velocity, is one of the most useful and interesting propositions in the whole science of mechanics. Its truth is easily shown, from mathematical considerations, by comparing the time with the base and the velocity with the perpendicular of a triangle, gradually increasing, of which the area will represent the space.'

That such is the law of an uniform force, requires no triangle to make evident; it may easily be shown from equal movements of velocity being produced, which is no more than the definition of an uniform force. Experiment proves this to be the law of falling bodies on the surface of the earth, and, independent of experiment, we think that no mathematical consideration could prove it. Still farther removed from legitimate reasoning, is the sort of attempt at

demonstration, which he has taken from Maclaurin on the fundamental property of the lever.

Supposing two equal weights, of an ounce each, to be fixed at the ends of the equal arms of a lever of the first kind; in this case it is obvious there will be an equilibrium, since the re is no reason why either weight should preponderate.'

We say it is not obvious at all. It might have happened that the end nearest the north pole, for example, should have always preponderated; or it might have followed any other imaginable law; or we might not have been able to discover that the result was regulated by any law whatever. Such a state of things would doubtless have been very in consistent with the economy of human life; but it is no more repulsive to reason, than the phænomena of the magnetic needle. That we can see no reason why the event should be otherwise than it is, is an argument that we did not expect to be brought forward in the present day, when it seems universally agreed, that, there exists no necessary relation between cause and effect in any of the phanomena of the physical world.

Hydrodynamics, or the properties of fluid matter, is the second division of Dr. Young's lectures. Under this general head are comprehended hydrostatics, acoustics, and optics. The latter science has commonly fallen under a different arrangement, but Dr. Young has chosen to consider optics as a branch of hydrodynamics, preferring the Huygenian theory of the undulations of an elastic medium to the Newtonian of the emission of particles of light from luminous bodies. Under various heads, we find explained the principles of balloons, barometers, locks and syphons, whirlpools, waves, motions of rivers, weres, form of a ship, hydrometer, embankments, dikes, reservoirs, floodgates, canals, piers, harbours, water-pipes, stop cocks and valves, overshot-wheel, undershot-wheel, breast-wheel, windmills, smoke-jack, kite, pumps,fire-engine, air-pump, condensers, corn-fan, chiminies, steam-engine, gunpowder, air-gun, speaking-trumpet, whispering-gallery, invisible-girl, harp, lyre, harpsichord, spinet, pianoforte, dulcimer, clarichord, guitar, vielle, trumpet, marigni, Eolian-harp, human voice, drum, stacada, bell, harmonica, vox humana, pipe, photometers, magnifiers, simple microscope, burning-glasses, camera obscura, solar microscope, Incernal microscope, phantasmagoria, double microscopes, telescopes common, Herschel's, Newton's, Gregory's, and Cassegrain's double magnifier, achromatic glasses, micrometers, divided speculum, aerial perspective, panorama, ●cular spectre.

We shall select as a specimen of the execution of this work, his observations on vision, a subject to which he has paid more than common attention. After describing the formation of the image on the surface of the retina, and attempting to account for an inverted image causing the sensation of an erect object, Dr. Young thus gives his opinion on another subject, which has caused much disputation among philosophers.

'The mode in which the accommodation of the eye to different distances is effected, has long been a subject of investigation and dispute among opticians and physiologists, but I apprehend that at present there is little farther room for doubting that the change is produced by an increase of the convexity of the crystalline lens, arising from an internal cause. The arguments in favour of this conclusion are of two kinds. Some of them are negative, derived from the impossibility of imagining any other mode, without exceeding the limits of the actual dimensions of the eye, and from the examination of the eye in its different states by several tests, capable of detecting any other changes if they had existed: for example, by the application of water to the cornea, which completely removes the effect of its convexity, without impairing the power of altering the focus, and by holding the whole eye, when turned inwards, in such a manner as to render any material alteration of its length utterly impossible. Other arguments are deduced from positive evidence of the change of form of the crystalline, furnished by the particular effects of refraction and aberration, which are observable in the different states of the eye, effects which furnish direct proof that the figure of the lens must vary its surfaces, which are nearly spherical in the quiescent form of the lens, assuming a different determinable curvature when it is called into exertion. The objections whieh have been made to this conclusion are founded only on the appearance of a slight alteration of focal length in an eye, from which the crystalline had been extracted; but the fact is neither sufficiently ascertained, nor was the apparent change at all considerable and even if it were proved that an eye without a lens is capable of a certain small alteration, it would by no means follow that it could undergo a change five times or ten times as great.?

a

On the power of judging of distances we have the following observations:

When the images of the object fall on certain corresponding points of the retina in each eye, they appear to the sense only as one; but if they fall on parts not corresponding, the object appears double; and in general all objects at the same distance, in any one position of the eyes, appear alike, either double or single. The optical axes, or the directions of the rays falling on the points

of most perfect vision, naturally meet at a great distance, that is, they are nearly parallel to each other, and in looking at a nearer object we make them converge towards it, wherever it may be situated, by means of the external muscles of the eye; while in perfect eyes the refractive powers are altered, at the same time, by an involuntary sympathy, so as to form a distinct image of an object at a given distance. This correspondence of the situation of the axis with the focal length is in most cases unalterable; but some have perhaps a power of deranging it in a slight degree, and in others, the adjustment is imperfect; but the eyes seem to be in most persons inseparably connected together with respect to the changes that their refractive powers undergo, although it sometimes happens that those powers are originally very different in the opposite eyes.

These motions enable us to judge pretty accurately, within certain limits, of the distance of an object; and beyond these limits, the degree of distinctness or confusion of the image still continues to assist the judgment. We estimate distances much less accurately with one eye than with both, since we are deprived of the assistance usually afforded by the relative assistance of the optical axes; thus we seldom succeed at once in attempting to pass a finger or a hooked rod sideways through a ring, with one eye shut. Our idea of distance is also usually regulated by the knowledge of the real magnitude of an object, while we observe its angular magnitude: and on the other hand a knowledge of the real or imaginary distance of the object often directs our judgment of its actual magnitude. The quantity of light intercepted by the air interposed, and the intensity of the blue tint, which it occasions, are also elements of our involuntary calculation: hence, in a mist, the obscurity increases the apparent distance, and consequently the supposed magnitude of an unknown object. We naturally observe, in estimating a distance, the number and extent of the intervening objects; so that a distant church in a woody and hilly country appears more remote than if it were situated in a plain; and for a similar reason the apparent distance of an object at sea is smaller than its true distance. The city of London is unquestionably larger than Paris; but the difference appears at first sight much greater than it really is; and the smoke produced by the coal fires of London, is proba bly the principal cause of the deception.

The sun, moon and stars, are much less luminous, when they are near the horizon, than when they are more elevated, on account of the greater quantity of their light, that is intercepted, in its longer passage through the atmosphere: we also observe a much greater variety of nearer objects almost in the same direction ; we cannot, therefore, help imagining them to be more distant, when they rise or set, than at other times; and since they subtend the same angle they appear to be actually larger. For similar reasons the apparent figure of the starry heavens, even when free from clouds, is that of a flattened vault, its summit appearing to be much nearer to us than its horizontal parts, and any of the, constellations seems to be