Abbildungen der Seite

has not perhaps attracted all the attention which it deseryes. · Mr. Davies, in a very interesting paper, has fully confirmed the accuracy of Mr. Sym's observations.'-pp. 309, 310.

The second part of this work, which the author devotes to electricity, is much more limited than we could bave wished. We have no notion whatever of that fastidious sense of delicacy which would prevent a professor of chemistry in Glasgow University, from entertaining the whole subject of electricity, because, forsooth, by taking such a range he would infringe on the province of the professor of natural philosophy. In this metropolis of ours, the very pandemonium of jobbing, interchanges of accommodation such as this excite no surprise. But that the freedom of instruction should be controlled, or made subservient to the monopolizing regulations of a few professors in the simple and patriarchal regions of Scotland, is more than we were prepared to expect. Dr. Thomson, in yielding to this abuse, has ventured to explain only. those branches of electricity which are most intimately connected with chemistry. The whole of the accurate knowledge which we possess of this interesting science (electricity) is of modern achievement. Dr. Thomson traces minutely and with rigorous impartiality as to men and countries, every step of the progress which electricity has made to its present improved condition. To the work itself we must refer the student for a thorough acquaintance with some of the laws of this wonderful principle, and the best practicable modes of employing it as an agent in decomposition,

and other processes, in which its power is so signally displayed. But we cannot omit some passages on the electricity of the atmosphere, which, in its manifestations at certain periods of the year, but too formidably calls forth the attention of mankind. The phenomena of thunder and lightning are thus explained by our author:

Air, and all gases, are non-conductors ; but vapour and clouds, which are composed of it, are conductors. Clouds consist of small hollow bladders of vapour, charged each with the same kind of electricity. It is this electric charge which prevents the vesicles from uniting together, and falling down in the form of rain. Even the vesicular form which the vapour assumes, is probably owing to the particles being charged with electricity. The mutual repulsion of the electric particles may be considered as sufficient (since they are prevented from leaving the vesicle by the action of the surrounding air, and of the surrounding vesicles,) to give the vapour the vesicular form.

• In what way these clouds come to be charged with electricity, it is not easy to say. But as electricity is evolved during the act of evaporation, the probability is, that clouds are always charged with electricity, and that they owe their existence, or at least their form, to that fluid. It is very probable that when two currents of dry air are moving different ways, the friction of the two surfaces may evolve electricity. Should these currents be of different temperatures, a portion of the vapour which they always contain, will be deposited; the electricity evolved will be taken up by that vapour, and will cause it to assume the vesicular state constituting

[ocr errors]


[ocr errors]

a cloud, Thus we can see in general how clouds come to be formed, and how they contain electricity. This electricity may be either vitreous or resinous according to circumstances. And it is conceivable, that by long continued opposite currents of air, the charge accumulated in a cloud may be considerable. Now, when two clouds charged, the one with positive, and the other with negative, electricity, happen to approach within a certain distance, the thickness of the coating of electricity increases on the two sides of the clouds which are nearest each other. This accumulation of thickness soon becomes so great as to overcome the pressure of the atmosphere, and a discharge takes place, which occasions the flash of lightning. The noise accompanying the discharge constitutes the thunder clap, the long continuance of which partly depends upon the reverberations from neighbouring objects. It is therefore loudest and largest, and most tremendous, in hilly countries. * A thunder storm in this country commonly commences in the following

A low dense cloud begins to form in a part of the atmosphere that was previously clear. This cloud increases fast, but only from its upper part, and spreads into an arched form, appearing like a large heap of cotton wool. Its under surface is level, as if it rested on a smooth plane. The wind is hushed, and every thing appears preternaturally calm and still.

Numberless small ragged clouds, like teazled flakes of cotton, soon begin to make their appearance, moving about in various directions and perpetually changing their irregular surface, appearing to increase by gradual accumulation. As they move about they approach each other, and appear to stretch out their ragged arms towards each other. They do not often come in contact; but after approaching very near each other, they evidently recede either in whole, or by bending away their ragged arms.

During this confused motion, the whole mass of small clouds approaches the great one above it: and when near it, the clouds of the lower mass frequently coalesce with each other, before they coalesce with the upper cloud. But as frequently the upper cloud coalesces without them. Its lower surface, from being level and smooth, now becomes ragged, and its tatters stretch down towards the others, and long arms are extended towards the ground. The heavens now darken apace, the whole mass sinks down; wind rises and frequently shifts in squalls; small clouds move swiftly in various directions ; lightning darts from cloud to cloud. A spark is sometimes seen coexistent through a vast horizontal extent, of a zigzag shape, and of different brilliancy in different parts. Lightning strikes between the clouds and the earth-frequently in two places at once. A very heavy rain falls—the cloud is dissipated, or it rises high and becomes light and thin.

* These electrical discharges obviously dissipate the electricity, the cloud condenses into water, and occasions the sudden and heavy rain which always terminates a thunder storm. The previous motions of the clouds, which act like electrometers, indicate the electrical state of different parts of the atmosphere.

"Thunder then only takes place when the different strata of air are in different electrical states. The clouds interposed between these strata, are also electrical, and owe their vesicular nature to that electricity. They

[ocr errors]

are also conductors. Hence they interpose themselves between strata in different states, and arrange themselves in such a manner as to occasion the mutual discharge of the strata in opposite states. The equilibrium is restored, the clouds deprived of their electricity collapse into rain, and the thunder terminates.

· These electrical discharges sometimes take place without any noise. In that case the flashes are very bright, but they are single flashes passing visibly from one cloud to another, and confined usually to a single quarter of the heavens. When they are accompanied by the noise which we call thunder, a number of simultaneous flashes, of different colours, and constituting an interrupted zigzag line, may generally be observed stretching to an extent of several miles. These seem to be occasioned by a number of successive or almost simultaneous discharges from one cloud to another; these intermediate clouds serving as intermediate conductors, or stepping stones for the electrical Auid. It is these simultaneous discharges, which occasions the rattling noise, which we call thunder. Though they are all made at the same time, yet as their distances are different, they only reach our ear in succession, and thus occasion the lengthened rumbling noise so different from the snap, which accompanies the discharge of a Leyden jar.

. If the electricity were confined to the clouds, a single discharge (or a single flash of lightning) would restore the equilibrium. The cloud would collapse and discharge itself in rain, and the serenity of the heavens would be restored. But this is seldom the case. I have witnessed the most vivid discharges of lightning, from one cloud to another, which enlightened the whole horizon, continue for several hours, and amounting to a very considerable number, not fewer certainly than fifty, and terminating at last in a violent thunder storm. We see that these discharges, though the quantity of electricity must have been immense, did not restore the equilibrium. It is obvious from this, that not only the clouds but the strata of air themselves, must have been strongly charged with electricity. The clouds being conductors served the purpose of discharging the electricity with which they were loaded, when they came within the striking distance. But the electric stratum of air with which the cloud was in contact, being a non-conductor, would not lose its electricity by the discharge of the cloud. It would immediately supply the cloud with which it was in contact with a new charge. And this repeated charging and discharging process would continue to go on till the different strata of excited air were brought to their natural state.'—pp. 442 -- 446.

We should observe, that the explanations of Dr. Thomson are facilitated by a great number of well executed plates, so that we do not hesitate to award to this volume the distinction of being the best elementary work extant, on the fundamental sciences of heat and chemical electricity.

Art. VII.— The Cabinet Cyclopædia— History-England. By the

Right Hon. Sir James Mackintosh, M.P. Vol. i. 12mo. pp. 382.

London : Longman and Co. and Taylor. 1830. Either from a weak invention in the art of puffing, or from a weaker and baser effort of sheer malignity, a charge was recently

brought forward against Dr. Lardner of having, among other similar practices, presented this volume to the public under the authority of a name to which it had in reality no pretensions. The charge has been laboriously and rather ostentatiously refuted; for with the simple and unequivocal answers of Sir James Mackintosh and Sir Walter Scott, the matter should at once have been terminated, whereas the learned Doctor has thought necessary not only to print a long and pompous vindication of his own, but also two or three notes from other celebrated authors, whose promised works have not yet made their appearance. Whether the latter have deposed by way of compurgators, or witnesses to the characters of the accused, or as pledges for their own good behaviour, we are at a loss to conjecture. In either case their testimony was more than superfluous. The over-zealous advice of Dr. Southey to cause the inventor of the puff, or the accusation (we really cannot say which it was), to be prosecuted in a court of law, savours little of the temperament of the man who erewhile sang the glories of Wat Tyler.

One need but open the first page of this volume in order to be convinced, that the writer of it could have been no other than the able and distinguished person whose name it bears. It is impossible to mistake Sir James Mackintosh's style. Whether in writing or in speaking, his manner of composition is the same, He is always ambitious of originality of logical arrangement, and shining diction. But his chief peculiarity is his love for generalizing, for reducing facts to theories, or rather for spinning out of ascertained data general reflections. In gratifying this mental propensity, he very frequently bewilders the memory of his readers, thought we doubt whether, from the great subtlety of his own intellect, he is ever conscious that he wanders out of the way

which he has proposed to follow. He seems often to imagine that he is expounding clear principles, when his readers feel that he is in truth dealing in difficult and abstruse problems. Hence the little work before us, whatever be its merits in other respects, is a collection of criticisms rather than a narrative.

It is an essay upon the history of England, so far as it goes, containing a review of leading events and of their consequences; the author now sketching prominent characters, next discussing a point of constitutional law, refuting the assertions of preceding historians, examining the authorities by which they were guided, shewing the better evidence which he prefers, and all this in the body of his text, after the fashion of a commentator.

That the learned author's observations are often sound, and, when not too much refined, highly instructive, we have no disposition to deny. We admire the singular acuteness and felicity with which he has traced the early history of those institutions which are the grand bulwarks of our liberties. His notices of the Saxon, Roman, and feudal laws, his reflections on the great charter, his

views of the origin of juries, and of the infancy and wondrous growth of our parliamentary constitution, are replete with learning and ability, and often adorned by eloquence of a superior order. But the narrative portions of his work, which should properly comprehend the results of his criticisms, and be not easily separable from his reflections, fade into the exercise of a schoolboy when compared with the splendid composition of Hume, or the beautiful and transparent style of Dr. Lingard. In this respect; so far as one volume enables us to judge, Sir James Mackintosh has but added one to the histories of England, which are unreadable to the general mass of eđucated persons. The student, the lawyer, the learned foreigner, anxious to be acquainted with the rise and development of our institutions, will repair to this work as a source of abundant knowledge; but unless the glaring defects of the narrative, its long and intricate, often obscure, sentences, its rough and gravelly course, be amended in the succeeding volumes, it never can be popular.

We suspect that the learned author had originally commenced his historic labours on a more comprehensive scale : indeed it has been understood for years that his leisure from parliamentary duties was devoted to this dignified occupation; and hence, perhaps, have arisen most of the imperfections of which we have reason to complain. His mind having been imbued with legal studies, he would naturally direct his chief attention to the constitutional part of our history, and his pen having been long practised in criticism, he would be contented with mere notes of facts, giving the larger space to his favourite train of observation. The decided and brilliant success which has borne Lingard's production beyond every attempt at competition, would have rendered a new work of similar proportions an unprofitable speculation. Dr. Lardner's Cyclopædia offered the opportunity for an epitome; but the necessary limitation of a history within the compass of an abridgment, by no means excuses inattention to fluency and clearness of narration: On the contrary, if we may judge from the labours of Goldsmith and Bossuet, and from the little historical Resumés which, since the restoration, have emanated from the French press, we should say that the necessity of presenting a series of facts within a confined space, is rather favourable than injurious to energy, rapidity, and beauty in this department of literature. Besides, it must be admitted that eight volumes of the size now before us, closely printed in small letter, present a range for the display of talent, which can hardly be said to be restricted. It would be ludicrous to give the name of a compend to a publication exceeding three thousand pages, and which might, if the author chooses, amount to double that number.

The reader will, perhaps, be surprised to see from the pen of Şir James Mackintosh, such a sentence as the following. Speaking of the Picts, he says :- It will not be wondered that every thing

« ZurückWeiter »