where he proves that, as the earth would never be nearer the comet than 24800000 British leagues, there could be no danger of collision." -pp. 369-70.

We may observe that the alarm of which Mrs. Somerville here speaks, affords an example of the confusion of ideas, which popular views of scientific matters often involve; and thus shows us how valuable a boon it is to the mass of readers, when persons of real science, like Mrs. Somerville, condescend to write for the wider public, as in this work she does. The apprehensions with regard to Biela's (or, as it ought rather to be called, Gambart's) comet, which were entertained by our worthy neighbours, tout le monde of Paris, were of a kind somewhat peculiar. The expected arrival of this visiter, with his fiery train, produced a commotion scarcely inferior to that which was excited among the good people of Strasburg by the stranger in the red-plush inexpressibles. That his head or his tail would do us irreparable harm-that he would burn us with his nucleus—or drown or poison us with his atmosphere were slight terrors compared with those excited by the combination of terms perturbations' and 'orbite de la terre.' It appeared that the comet would cross the earth's orbit; what mischief might not come of this? It was true that the earth would not be near the crossing at that time; but then, might not the orbit itself be seriously injured? Instead of an imaginary line in the trackless ocean of space, the fears of our friends appear to have represented to them the earth's orbit as a sort of railroad, which might be so damaged by what Mr. Campbell calls the ' bickering wheels and adamantine car' of the fiery giant,' that the earth must stick or run off, the next time the revolving seasons brought her to the fatal place. In M. Arago's agreeable and instructive article in the Annuaire du Bureau des Longitudes,' written in order to calm the panic arising from these horrible imaginings,' he

says,

6

'Shall I be so fortunate as to do this? I hope so; yet without being very confident. Have I not seen persons who, while they acknowledged that the earth would not receive, in 1832, any direct blow from the comet, still believed that this body could not go through our orbit without altering its form; as if this orbit was a material thing; as if the parabolic path which a bomb is just going to describe, could be affected by passing through the space which other bombs had traversed before!!'

But we must not dwell too long on one part of Mrs. Somerville's work; we must recollect that her professed object is to illustrate The Connexion of the Physical Sciences.' This is a noble object; and to succeed in it would be to render a most important service to science. The tendency of the sciences has

long

6

long been an increasing proclivity to separation and dismemberment. Formerly, the learned' embraced in their wide grasp all the branches of the tree of knowledge; the Scaligers and Vossiuses of former days were mathematicians as well as philologers, physical as well as antiquarian speculators. But these days are past; the students of books and of things are estranged from each other in habit and feeling. If a moralist, like Hobbes, ventures into the domain of mathematics, or a poet, like Goethe, wanders into the fields of experimental science, he is received with contradiction and contempt; and, in truth, he generally makes his incursions with small advantage, for the separation of sympathies and intellectual habits has ended in a destruction, on each side, of that mental discipline which leads to success in the other province. But the disintegration goes on, like that of a great empire falling to pieces; physical science itself is endlessly subdivided, and the subdivisions insulated. We adopt the maxim oue science only can one genius fit.' The mathematician turns away from the chemist; the chemist from the naturalist; the mathematician, left to himself, divides himself into a pure mathematician and a mixed mathematician, who soon part company; the chemist is perhaps a chemist of electro-chemistry; if so, he leaves common chemical analysis to others; between the mathematician and the chemist is to be interpolated a 'physicien' (we have no English name for him), who studies heat, moisture, and the like. And thus science, even mere physical science, loses all traces of unity. A curious illustration of this result may be observed in the want of any name by which we can designate the students of the knowledge of the material world collectively. We are informed that this difficulty was felt very oppressively by the members of the British Association for the Advancement of Science, at their meetings at York, Oxford, and Cambridge, in the last three summers. There was no general term by which these gentlemen could describe themselves with reference to their pursuits. Philosophers was felt to be too wide and too lofty a term, and was very properly forbidden them by Mr. Coleridge, both in his capacity of philologer and metaphysician; savans was rather assuming, besides being French instead of English; some ingenious gentleman proposed that, by analogy with artist, they might form scientist, and added that there could be no scruple in making free with this termination when we have such words as sciolist, economist, and atheist-but this was not generally palatable; others attempted to translate the term by which the members of similar associations in Germany have described themselves, but it was not found easy to discover an English equivalent for natur-forscher. The process of examination which it implies might suggest such undignified compounds

as

as nature-poker, or nature-peeper, for these naturæ curiosi; but these were indignantly rejected.

The inconveniences of this division of the soil of science into infinitely small allotments have been often felt and complained of. It was one object, we believe, of the British Association, to remedy these inconveniences by bringing together the cultivators of different departments. To remove the evil in another way is one object of Mrs. Somerville's book. If we apprehend her purpose rightly, this is to be done by showing how detached branches have, in the history of science, united by the discovery of general principles.

' In some cases identity has been proved where there appeared to be nothing in common, as in the electric and magnetic influences; in others, as that of light and heat, such analogies have been pointed out as to justify the expectation that they will ultimately be referred to the same agent; and in all there exists such a bond of union, that proficiency cannot be attained in any one without a knowledge of others.-Preface.

We may add, that in the same way in which a kindred language proves the common stock and relationship of nations, the connexion of all the sciences which are treated of in the work now before us is indicated by the community of that mathematical language which they all employ. Our space does not allow us to dwell on the illustration of this point, but we may select a passage or two. We cannot even refer to the curious sections on the properties of light; on the fringes of shadows, the colours of thin plates, the results of polarization, and of the analysis of polarized light after passing through crystals; on the evidence and proofs of the undulatory theory; which last great question our author, rightly, as we conceive, judges to be now nearly settled in favour of the undulationists. But we may quote what she says on one of the analogies which we have already noticed :

It has been observed that heat, like light and sound, probably consists in the undulations of an elastic medium. All the principal phenomena of heat may actually be illustrated by a comparison with those of sound. The excitation of heat and sound are not only similar, but often identical, as in friction and percussion; they are both communicated by contact and radiation; and Dr. Young observes, that the effect of radiant heat in raising the temperature of a body upon which it falls resembles the sympathetic agitation of a string, when the sound of another string, which is in unison with it, is transmitted

*When the German association met at Berlin, a caricature was circulated there, representing the 'collective wisdom' employed in the discussion of their mid-day meal with extraordinary zeal of mastication, and dexterity in the use of the requisite implements, to which was affixed the legend- Wie die natur-forscher natur-forschen,' which we venture to translate the poking of the nature-pokers.'

to

to it through the air. Light, heat, sound, and the waves of fluids, are all subject to the same laws of reflection, and, indeed, their undulatory theories are perfectly similar. If, therefore, we may judge from analogy, the undulations of some of the heat-producing rays must be less frequent than those of the extreme red of the solar spectrum; but if the analogy were perfect, the interference of two hot rays ought to produce cold, since darkness results from the interference of two undulations of light-silence ensues from the interference of two undulations of sound-and still water, or no tide, is the consequence of the interference of two tides. The propagation of sound, however, requires a much denser medium than that either of light or heat; its intensity diminishes as the rarity of the air increases; so that at a very small height above the surface of the earth the noise of the tempest ceases, and the thunder is heard no more in those boundless regions where the heavenly bodies accomplish their periods in eternal and sublime silence.'-pp. 250, 251.

We refer to the following on account of the novelty of the subject:

After Mr. Faraday had proved the identity of the magnetic and electric fluids by producing the spark, heating metallic wires, and accomplishing chemical decomposition, it was easy to increase these effects by more powerful magnets and other arrangements. The following apparatus is now in use, which is in effect a battery, where the agent is the magnetic instead of the voltaic fluid, or, in other words, electricity.

A very powerful horse-shoe magnet, formed of twelve steel plates in close approximation, is placed in a horizontal position. An armature consisting of a bar of the purest soft iron has each of its ends bent at right angles, so that the faces of those ends may be brought directly opposite and close to the poles of the magnet when required. Two series of copper wires-covered with silk, in order to insulate them—are wound round the bar of soft iron as compound helices. The extremities of these wires, having the same direction, are in metallic connexion with a circular disc, which dips into a cup of mercury, while the ends of the wires in the opposite direction are soldered to a projecting screw-piece, which carries a slip of copper with two opposite points. The steel magnet is stationary; but when the ar mature, together with its appendages, is made to rotate horizontally, the edge of the disc always remains immersed in the mercury, while the points of the copper slip alternately dip in it and rise above it. By the ordinary laws of induction, the armature becomes a temporary magnet while its bent ends are opposite the poles of the steel magnet, and ceases to be magnetic when they are at right angles to them, It imparts its temporary magnetism to the helices which concentrate it; and while one set conveys a current to the disc, the other set conducts the opposite current to the copper slip. But as the edge of the revolving disc is always immersed in the mercury, one set of wires is

constantly

constantly maintained in contact with it, and the circuit is only completed when a point of the copper slip dips in the mercury also; but the circuit is broken the moment that point rises above it. Thus, by the rotation of the armature, the circuit is alternately broken and renewed; and as it is only at these moments that electric action is manifested, a brilliant spark takes place every time the copper point touches the surface of the mercury. Platina wire is ignited, shocks smart enough to be disagreeable are given, and water is decomposed with astonishing rapidity, by the same means, which proves beyond a doubt the identity of the magnetic and electric agencies, and places Mr. Faraday, whose experiments established the principle, in the first rank of experimental philosophers.'-pp. 339, 340.

The following speculations are somewhat insecure, but they are proposed as conjectures rather than assertions, and are well worth notice :

From the experiments of Mr. Faraday, and also from theory, it is possible that the rotation of the earth may produce electric currents in its own mass. In that case, they would flow superficially in the meridians, and if collectors could be applied at the equator and poles, as in the revolving plate, negative electricity would be collected at the equator, and positive at the poles; but without something equivalent to conductors to complete the circuit, these currents could not exist.

'Since the motion, not only of metals but even of fluids, when under the influence of powerful magnets, evolves electricity, it is probable that the gulf stream may exert a sensible influence upon the forms of the lines of magnetic variation, in consequence of electric currents moving across it, by the electro-magnetic induction of the earth. Even a ship passing over the surface of the water, in northern or southern latitudes, ought to have electric currents running directly across the line of her motion. Mr. Faraday observes, that such is the facility with which electricity is evolved by the earth's magnetism, that scarcely any piece of metal can be moved in contact with others without a development of it, and that consequently, among the arrangements of steam engines and metallic machinery, curious electro-magnetic combinations probably exist, which have never yet been noticed.

'What magnetic properties the sun and planets may have it is impossible to conjecture, although their rotation might lead us to infer that they are similar to the earth in this respect. According to the observations of MM. Biot and Gay-Lussac, during their aerostatic expedition, the magnetic action is not confined to the surface of the earth, but extends into space. A decrease in its intensity was perceptible; and as it most likely follows the ratio of the inverse square of the distance, it must extend indefinitely. It is probable that the moon has become highly magnetic by induction, in consequence of her proximity to the earth, and because her greatest diameter always points towards it. Should the magnetic, like the gravitating force,