Mr. C. Christie remarks, that the centre of the arch was very near magnetic north, so that the arch would be at right angles to the magnetic meridian: that the body of light was nearly colourless, and its brightness similar to that seen on the edge of a cloud when the moon is about to rise behind it; with, however, this striking difference, that the stars were seen distinctly through the diffused light of its upper surface, and those in the tail of the Bear shone clearly in the very body of the light on the right hand. One very remarkable phenomenon recorded by Mr. C. Christie, is the darting of pencils of light through the dark cloud. The first appearance of the aurora at nine o'clock was that of a dark convex cloud cutting the luminous arch and concealing a body of light behind, the eye naturally referring the light to a more distant region, while the sharp line of division threw the clouds forward. Subsequent appearances, however, did not seem to confirm this notion, but, on the contrary, induced him to consider whether the dark cloud might not be a substratum of matter differing in nature and density from the superincumbent arch of light. Mr. C. Christie having drawn several inferences from the appearance and state of the arch, comes to the following conclusion. All the circumstances struck me as so closely resembling the disturbance of two fluids, the one superposed on the other, mutually repulsive, but compelled to mingle by forces of whose action the vividness of the pencillings seemed to indicate the intensity, and requiring intervals of repose to re-collect their scattered energies, that I cannot but conclude the luminous matter of an aurora to be a superincumbent stratum, and consequently, that its altitude is dependent on that of the dark mass beneath.'

The reader who has never witnessed the phenomenon will have an accurate conception of it from the beautiful drawings accompanying the paper from which the preceding has been extracted. Their accuracy will be borne testimony to by every one who has witnessed the pheno

menon. Sometimes two or several luminous arches are seen parallel to each other, and the colours which accompany the streamers sent up towards the zenith are most variegated and beautiful. The breaking up and restoring of the arch is a very curious fact; the luminous matter collects in large masses at different parts of the arch, and then returns again into its original state. In the present state of our knowledge we seem only to be assured of the fact, that the aurora is an electrical phenomenon; on this point all are agreed; but of the laws which govern it we are at present exceedingly ignorant.

CHAP. XII.

THEORY OF GRAVITATION-THEORY OF HEAT-THEORY OF LIGHTTHEORY OF ELECTRICITY AND MAGNETISM-CONCLUSION.

315. In the concluding chapter of this work the student's attention may with advantage be directed to some considerations of very great value, and to some features especially in the theory of the Physical Sciences, which ought not to be wholly omitted. The great object in view being to ascend from particular facts to general laws, and to form theories from which the phenomena may be reproduced, we may with advantage consider what is the actual state of our knowledge in this respect.

The theory of gravitation will first claim our consideration; on the evidence of the truth of this theory it is unnecessary to add any farther remarks to those which have been already made (Art. 3); we have traced in considerable detail in the chapters on Gravity and on Fluids the consequences which flow from this law, and the phenomena which are included in it as exhibited at the surface of the earth. The truth of the laws of gravity is so wound up with our belief, that we can scarcely conceive them otherwise than they are believed to be. But it is a most interesting and instructive question, if conducted in a proper spirit, to inquire whether there is any necessity for the law being such as it is. That any such necessity exists is quite contrary to our general belief; we are bound to believe that the Great Author of all things

could have made matter subject to any other law had it so seemed good; but very strong reasons are to be pointed out why, for the beauty and advantage of the system of the universe, this law seems immeasurably superior to every other. Reflections of this nature would furnish a most proper conclusion to the preceding work; but our limits will not admit of it, and we could scarcely hope to present any thing worth notice when compared with the beautiful remarks of Mr. Whewell on this subject.*

The universality of the law of the inverse square is most remarkable; this particular law presents itself in almost every department of Physics. Thus we are led to look for some more general principle, which may include every phenomenon in nature. Some surprising generalizations to this effect have within the last year been announced to the world by Mosotti.

It is a most important exercise for the student to contemplate the process by which a general law is arrived at. A mass of facts which previously appeared entirely unconnected, nay incoherent, stand at once in a clear relation to each other. This step, to quote the words of Mr. Whewell,† so much resembles the mode in which one intelligent being understands and apprehends the conceptions of another, that we cannot be surprised if those persons in whose minds such a step has taken place, have been most ready to acknowledge the existence and operation of a superintending Intelligence, whose ordinances it was their employment to study—when they had first read a sentence of the table of the laws of the universe, they could not doubt whether it had a Legislator.

On quitting the subject of universal gravitation we enter on a field of some uncertainty, so far as the physical theory is concerned. The theories of heat, light, electricity and magnetism, are far from settled, and we encounter much

THEORY OF HEAT.

A

485

CHAP. XII. open bed wat no vas of 19idae 1918m obam svad bic that is involved in uncertainty. We cannot, however, doubt but that here too are general laws, though we have not yet established them; indeed much has already been done in the theory of light, and some little advance has been made in the theory of the other imponderable agents; but the Newton is yet to arise who shall place these sciences on the same rank as to certainty of evidence with the theory of universal gravitation. We shall endeavour to point out the state of the theory of these sciences. 16316. Theory of Heat.-We have already (Art. 187) hinted at the hypothesis, that heat consists in the vibrations of an imponderable fluid, or ether. The phenomena of conduction and radiation, on which we have dwelt at considerable length, are so obviously consistent with the notion of the flow or transmission of some material particles, that it is not the least a matter of surprise that the hypothesis of heat, consisting in the transfer of material ja particles, should be generally received. But the two cases of the material theories of heat and light are remarkably analogous. The material theory did exceedingly well for a long time; but at last some phenomena presented themselves apparently utterly inexplicable on any such hypothesis. These phenomena are the interference of f rays of heat, so that cold is produced, and the polarization of heat; so that we may say of the theory of heat as we have already said of the undulatory theory of light (Art. 243), that a theory of undulations may be false, but we cannot conceive any theory of emission to be true,

of

Here then, as in the case of light, some other hypothesis is forced upon us, and under these circumstances there is no difficulty in entertaining the hypothesis of the vibrations of an ether; nay, of the same ether by whose motion light is supposed to be produced. We cannot suppose that the different phenomena of light and heat are produced by

* See Professor Forbes On the Refraction and Polarization of Heat,' Philosophical Magazine, March and November, 1835.