ARAGO'S METEOROLOGICAL ESSAYS. *

THIS volume forms the commencement of a series which is to con- tain a translation of Arago's principal works by or under the au- spices of some leading men of science. The Popular Astronomy, for instance, is in the hands of Admiral Smyth and Mr. Robert Grant. Other portions will be translated by Professor Baden Powell. The translation before us appears under the superintend- ence of Colonel Sabine ; who has added a number of explanatory or corrective foot-notes, and thoroughly edited the work.

In point of greatness and general interest of subject, the Me-

teorological _Essays cannot compete with the Astronomy, or perhaps with some other works of the author. They have this further dis- advantage, that the conclusions arrived at are not so certain or so complete as in some branches of natural philosophy, where, though much is yet to be done, and there is much that never can be done by human faculties, all is •" teres atque rotundus " as far as it goes. Such is not the case with the subjects of this volume. The first topic of the book is Thunder and Lightning. The cause is

well enough known to be electribity ; but how little do we really know about electricity—how much do we not imagine or expect. How many phenomena are connected with thunder and lightning and the state of the atmosphere at the time of thunder-storms, which we cannot explain. " Eleotro-Magnetism" is newer, if not more recondite or mysterious than thunder and lightning. The facts in the volume connected with "Animal Electricity" are few, and the treatment of the subject curt. By this term, however, Arago does not precisely mean the animal magnetism of the con- jurors, but the electric .power present in the torpedo and the gymnotus. The essay includes the exposure of a case of "a so-called electric girl,' and a few remarks on table-turning. Ter- restrial Magnetism chiefly treats of the variations of the compass. Though as wonderful, and probably as obscure as other branches of meteorology, men are more familiar with it from the daily use of the compass;. and the subject in a certain sense seems less recondite, owing to the extensive observations that have been made upon the magnetic variations in different parts of the world. The question has this popular disadvantage, that however practically important in navigation, it is of a minute and technical character. The Au- rora Borealis lands us again in doubt as to its origin and influence. From the corrective remarks of Colonel Sabine, Arago would seem to."have ascribed a closer connexion between the aurora borealis and an influence on the magnetic needle than can as yet be logic- ally supported. But if the subjects of the volume are inferior to some other branches of natural philosophy in breadth and certainty, they ex- hibit in a very remarkable degree the philosophic character of the author. "Thunder and Lightning" and "Aurora Borealis" more especially display his extensive labour in collecting facts, his power of patient research, and that philosophic apprehension which slights no fact that may bear upon the conclusion sought, however small in itself or humble the recorder. The practised experimenter is visible in the rejection of all that is needless; the trained philoso- pher and logician are evident in the cogent statement of all that is retained. The characteristic which, according to Humboldt in his introduction to this edition of Arago's works prefixed to the pre- sent 'volume, first drew the attention of Lagrange to the rising philosopher, is seen throughout—" that faculty of penetration which in complex problems discerns and rapidly and clearly lays hold of the decisive point." According to the same authority, amid the various subjects that claimed his attention, Arago had "a • Meteorological Essays, by Francis Arago, Member of the Institute. With an Introduction, by Alexander von Humboldt. Translated under the superintendence of Colonel Sabine, B.A., Trees. and V.P.R.S. Published by Longman and Co.

marked preference for all the phenomena of meteorological optics ; he was especially fond of investigating the laws which regulate the perpetual variations of the colour of the sea, the intensity of re- fleeted light on the surface of the clouds, and the play of atmo- spheric refractions." This native bias probably induced Arago to undertake the " essays" which form the present volume. With the first subject, " Thunder and Lightning," he was indeed practically connected by various applications that were made to him both on public and private account in reference to conductors ; and with the practical management of these electric safety-rods he concludes his theme. The whole volume, however, is strictly essays, rather than tree- tises,—a collection of facts to assist future observers in coming to a conclusion, rather than a treatise on the sciences. Conclusions there are, but they are particular or isolated. Indeed, the whole wears more the appearance of a vast collection of facts and thoughts, systematically arranged under their proper heads, to serve as a storehouse for future use, than as a regular didactic exposition. In a readable sense, the Essays may vie with the most interesting collection of the wonders of natural phenomena, With the advantage of frequently noting the conclusion the facts contain, and by their classification always indicating the law they tend to establish.

The most elaborate of the essays is the one on Thunder and Lightning ; indeed it occupies half the volume. From the nature of the case this was to be expected. These phenomena are frequent and universal ; from their character and their occasionally destruc- tive effects they not only command attention but excite terror; in ignorant ages, mid even in ages far from ignorant, they have been associated with religion, as directly indicating the will of the gods. These.three circumstances of inherent grandeur, fear not altogether unreasonable, and superstition, have produced more extensive re- cords in connexion with thunder-storms than any other meteorolo- gical phenomena. The wide range over which Arago has pursued his researches is something marvellous. From the classical writers to forgotten travels or obscure journals, all is laid under contribu- tion. Whether he does not sometimes ascribe more accuracy to the observer than the observer probably possessed may be a question. The critical acumen so early observed by Lagrange, which perceives what truth the fact will really illustrate, is ever visible. Of course the more extraordinary facts are made by modern observers, with more or less tincture of science; for it is only knowledge which really knows how to observe. The following curious facts are an ex- ample of this learned kind of observation ; they are taken from the chapter headed "Lightning sometimes fuses, and instantly vitrifies certain earthy substances."

"M. Esmond, who saw the same phenomena on several summits of the Pyrenees, did me the kindneses of setting down at my request the following interesting notice. The Pic du Midi is a mountain which stands in a great measure isolated, and rises high above surrounding points. Its summit is of very small extant; it consists of a mica schist of extreme hardness, dividing into rather thick tablets, which adhere strongly to each other; it does not into to lamina), but into parallelipirdons with oblique angles, like trap. Its colour is a grey black, rendered a little silvery by the presence of mica. The lightning acts upon it superficially only, causing a coating of yellowish enamel, or glaze, on which there are blisters or bubbles ; these latter are sometimes spherical, sometimes, from the convexity having burst, concave ; they are usually opaque, but sometimes semi-transparent. There are rocks whose entire face is varnished or glazed by this kind of enamel, and covered with bubbles often of the size of a pea ; but the interior of the rock remains perfectly unchanged, the fused part not being more than four-hundredths of an inch thick.

"'The summit of the Mont Perdu, which I reached twenty years ago, pre- sented to me the same phenomena. It is almost entirely covered with snow, and does not, like the Pie du Midi, offer continuous surfaces of rocks' but only small fragments heaped together without order. These consist of a calcareous stone bituminous and fetid, but containing a large intermixed portion of exceedingly fine quartzose sand. Many of these fragments bear evident marks of lightning. Their surface is loaded with bubbles of yellow enamel ; and, as on the Put du Midi, the fusion is only superficial, and does not penetrate to the interior of the stones notwithstanding their small size; and, what is not less remarkable, the heat, which has been sufficient to vitrify the surface has not taken from the stone that cadaverous odour, front which we can so easily free it either by dissolving it in an acid or by heating it a little strongly. "'Lastly, at the Roche Sanadoire, a mountain in the Departement du Puy- de-Deane, composed of clinkstone porphyry, and which I believe to be of volcanic origin, I saw, twelve years ago, the surface of the rocks vitrified and covered with bubbles from the action of lightning. Here also the fusion is superficial, and shows itself by bubbles and blisters in a glaze of very small thickness.'" The " Impartation of Magnetism by the action of Lightning" has some curious facts connected with important practical consi- derations.

"I have read, somewhere, a story of a stroke of lightning in a shoemaker's shop in Swabia, which had the effect of so magnetizing all the tools, that the poor artisan could no longer make use of them. Ile had to be constantly freeing his hammer, pincers, and knife from the nails, needles, and awls, which were constantly getting caught by them as they lay together on the bench.

"When the New York arrived at Liverpool, in May 1827, after having been twice struck by lightning Dr. Scoresby found that the nails of the par- titions and pannele which babeen broken, the iron fastenings of the masts which had fallen on the deck, the knives and forks which at the instant of the discharge were in the biscuit-room, and lastly, that the steel points of the mathematical instruments had become very decidedly magnetic.

"The effects of lightning on the needles of sea-compasses have often led to very serious consequences. We have already stated a case, in which, .after a stroke of lightning, the crew, deceived by the false indications of their com- passes, wrecked their ship on the very dangers from which they thought thee were receding as fast as their sails could carry them. Lightning, by instantaneously magnetizing the multitude of pieces of hard iron or steel throughout a ship, may moreover create powerful centres of attraction ; and hence, even without the compasses being themselves affected, local devia- tions may result, the more hurtful because on the high seas the navigator has but few means at his command for ascertaining their existenoe, and es- pecially for determining their value. Nor are these two kinds of perturba- tions the only ones to be guarded against. When lightning magnetizes the different pieces of steel which enter into the composition of a chronometer, and especially its balance-wheel, a new force, that of the earth's magnetism, becomes superadded to those which originally regulated the march of these admirable but very delicate piece of mechanism. This new force sometimes causes sensible accelerations or retardations • and from these result, after a certain number of days' navigation, very dangerous errors in the supposed geographies! longitude. For example, the chronometers of the New York were on their arrival at Liverpool 33m 55' in advance of what they would have given if the ship had not been struck by lightning."

The cause of thunder and lightning, or more truly the way in

which the cause operates, is a point on which Arago could not make up his mind. "Before we proceed to discuss the sh-fferent methods which have been proposed for the most efficient protection against lightning, let us look back on the long course which we have been passing over ; not, assuredly, with the view of deriving from what has been said a theory wherein all the phe- nomena described shall fall at once into due place, but with the far more modest hope of arriving, by means of interoonmarison, and the light which the phenomena may be made to throw on each other, at the discovery of some truths, which the examination of each fact taken singly has not yet disclosed to us.

"From the most ancient times it has been known that sound is not a ma- terial substance. Aristotle, for example, had perfectly recognized that sound results from simple undulations of common air. At the present day the same may be unhesitatingly said of light, with a single modification. Light is also a consequence of undulatory movement, not of the air, but of a certain extremely rare and highly elastic medium, filling the whole universe, and which it has been agreed to call ether.

"Is the phenomenon of which we have been treating, (that of thunder and lightning,) which manifests itself almost always simultaneously by light and by sound, to be clamed in the same category ? Although a declared partisan of the theory of luminous undulations, I own that on the above question I remain in a state of complete indecision.

"When I view Mr. Wheatatone's experiments as perfectly assured, and when my attention rests on the incomparable rapidity with which lightning traverses the iSerial regions, and the solid bodies through which it is pro- pagated at the surface of the earth, I feel little inclined to imagine it com- posed of material molecules, or of a mass of very small projectiles : undula- tions seem far more accordant with such velocities. But then again my mind reverts to those great mechanical effects—to the actual removal of material masses of considerable weight—effected by strokes of lightning. If I then recall to my recollection that in experiments in which by the methods of proceeding the test applied has been of exceeding delicacy, without the slightest deviation being obtained,—when operating, for example, with levers suspended in a vocuum to spiders' threada2 and with light concentrated in the focus of .the. largest mirrors or the -largest -lenges —all my doubts are renewed, and the,idea of,fulminating undulations Fen& itself to me encum- bered with ten thousand difficulties." -