for example, which will become heated to redness by the action of the pile.

Another remark which I have made on the light which escapes between the charcoal points, is that it may be employed as we employ the solar light, to illuminate any object we wish to daguerréotype. We obtained, by the daguerréotype, an imprint, it is true, though a slight one, but sufficiently decisive not to leave any doubt, from a plaster bust which we exposed for ten minutes to the light in question. This experiment is not easily made, because the luminous arc is intermittent, likewise that it is not always directed in the same manner, giving place consequently to shadows whose position is variable. But still, I have seen sufficient to prove that after having been reflected on a white body, the light which escapes between the two points of charcoal fixed to the two poles of the pile has still the property to act on a daguerréotype plate.

I cannot conclude this short notice without observing that the greater part of the experiments contained in it were made with the co-operation of M. M. Colloden, Melly, Suskind, and Bonigol, to whom I am happy here to express my gratitude for their able assistance.

NOTE.

"The experiments of M. Ersted appear to me to be worthy of repetition in a circumstance which will still add to the interest which they ought to inspire, in bringing us a step nearer towards the explication of the phenomenon, hitherto so incomprehensible-the aurora borealis."

There is at the Institution of the Royal Society of London, a voltaic battery composed of 2000 double plates of four inches square. By making use of this powerful piece of apparatus, Sir Humphry Davy has observed that there was an electrical discharge produced between the two points of charcoal adapted to the extremities of the positive and negative conductors, when even these points were distant from each other or of an inch. The first effect of the discharge is to redden the charcoal. Now, immediately on the incandescence being established, the points may be gradually withdrawn from each other four inches, without breaking the intermediate light. This light is extremely bright, and larger in the middle than at its extremities; it forms, in fact, an arch.

"The experiment succeeded much better in rarified air. Under a pressure of a quarter of an inch, the discharge from one charcoal point to the other commenced at the distance of half an inch; afterwards, the charcoal points being gradually withdrawn from each other, Sir H. Davy obtained a continuous purple flame, and which measured seven inches in length.

"It is very natural to suppose that such an electric current will act on the magnetic needle just as if it was moved along a conjunctive metallic wire; nevertheless the experiment seems to me to merit being recommended to philosophers who have voltaic

great power at their command, especially with regard to the views it gives rise to relative to the aurora borealis. Will it not also be, independently of any immediate application, a phenomenon worthy of remark, that the production in vacuo, or in air highly rarified, of a flame which, acting on the magnetic needle, may be in its turn attracted or repulsed by the poles of the magnet."

ATMOSPHERIC ELECTRICITY.

Read by Mr. SPENCER, at the Polytechnic Institution, Liverpool.

SECTION III.

At the close of the preceding section it was stated that rain is accompanied with electricity, not only during the shower, but in much greater quantity immediately before it occurs. When the electricity is attracted by the earth's surface, it will arrive there much sooner than the globules of water that compose the rain; consequently, we have several indications of rain being about to fall, independent of those furnished us by the barometer.

These are often exhibited by various affections of the human frame; and little doubt can exist that they are pretty general throughout the animal and vegetable kingdoms, had we the same means of observing them. For the present I shall leave this portion of the subject untouched, as my personal experience does not furnish me with an example; but we frequently hear complaints made of the aching of old wounds, corns, increased pain from rheumatic affections, &c., all in connexion with rain being about to fall.

There is, however, one indication of rain that has recently come under my observation, of which I can speak more positively. Having had, within the present winter months, a good deal of experience in the new art of photography, I have been surprised in dull weather, even when the sky was beginning to lour, to find that a picture was obtained in one-third or one-fourth of the time it would have otherwise taken, and that, when such took place, rain followed soon after; but, on the other hand, after the rain had continued for some time, this rapidity of chemical action departed, and a picture would then take rather longer than it would on a day having the same amount of light, when no rain had fallen. If, as already stated, we associate electricity and chemical action together, or as being identical, the solution of this enigma is easy. To render this, however, still clearer, I shall enter somewhat more into detail, and, while doing so, will explain, perhaps, more fully the immediate cause of rain as it occurs, under ordinary circumstances, in our climate.

When the condition of the atmosphere admits masses of cloud to float near the surface of the land or ocean, the electricity contained in them becomes attracted by either of these bodies, and, in consequence, continued streams of electricity are given out of the cloud

towards the earth. This electricity, under ordinary circumstances' is unseen, the distance between the attracting body and the attracted being so considerable, and the attraction being equal over such an immense area as that in which rain occurs, renders the electricity given out of the most feeble possible tension, but not so much so that it cannot be detected by our most delicate instruments: indeed, sensitive electrometers betray evident marks of electrical disturbance prior to the fall of rain. When a sufficient quantity of electricity is thus discharged from the cloud, that the small globules composing it cannot hold each other in electrical repulsion, they condense into larger drops, and, from their specific gravity, fall as rain, but still surrounded by a feeble atmosphere of the subtle fluid, which descends with them to the earth. This latter portion of electricity, so retained, may be familiarly likened to the residuum that is left in the Leyden phial after its first discharge, a feebler quantity being always detected. It sometimes happens that masses of cloud, not in the highest possible state of electric tension, are discharged without noise, by a shower of rain falling through them from a mass of cloud in an upper stratum of atmosphere. In such cases, the weather is what is termed thick and close, and the small dropped rain is attended with flashes of harmless sheet lightning unseen during the day.

In tropical climates, where evaporation takes place in much greater degree than our own, immense masses of thunder cloud are sometimes discharged by the attraction of a mass of negative cloud in a higher or lower stratum of the atmosphere, often floating immediately above or below it, as the case may be. In such instances the lightning is discharged in almost vertical streams, and spouts of water, caused by the sudden condensation of the globules forming the cloud, are discharged with fearful rapidity to the earth these having been known to cut circular basons, in a gravelly soil, to the depth of six or ten feet, with as much precision as by an artificial process. They have also been observed to take a progressive motion, and, in those instances, a deep trench has been cut for many hundred yards; yet, so sharp have been the edges of such cuttings, that loose leaves have been left undisturbed on their margin. These land spouts are always observed accompanied with lightning, streaming more or less vertically, and, reasoning from the foregoing, it must be as necessary a result as the developement of the latent caloric itself.

The phenomena of water-spouts next demand our attention.

From all the accounts of observers I have been able to collect, there seems no doubt, in my own mind, that water-spouts are merely an extraordinary effect arising as corrollary out of the principles already laid down, and may be, on the same ground, very satisfactorily explained. Most observers state they are attended with sharp claps of thunder. There are many other coexistent circumstances that point out their electrical origin.

Suppose we take by no means an unusual meteorological state of things, and reason from it. Let us suppose an immense storm cloud filled to repletion with free electricity, as we have already seen all clouds are, in greater or less degree. Such a cloud will be positive as respects the surface of the ocean, evaporation being always going on to render it so, coupled with the known fact, that aqueous surfaces are the best possible conductors. In consequence of this state of things, an immense attractive power will exist between the cloud and the surface of the ocean, until the cloud is gradually drawn from its altitude and begins to approach the surface of the water. This approach would seem, by the current testimony of close observers, to be accelerated as the distance decreases, and there can be little doubt that it obeys the well known law, the air being still, of an increased velocity of approach as the squares of the distance decrease. Now, by paying attention to the preceding statement with respect to clouds in general, it is sufficiently obvious what, under the circumstances, must take place. Although the under side of the clouds towards the earth or sea are comparatively level, yet there are irregularities enough to render some portions of them nearer the sea than others. A few yards would disturb the attracting equilibrium sufficient to cause an ultimate discharge at the point where the irregularity took place. Let us suppose that a rag of cloud hung down, and, by the way, this is in strict accordance with the observed facts, as also are all the other phenomenon I shall predicate. On the other hand, any wave that lifted up its head on the ocean beneath the point where the shred of cloud impended, would accelerate or induce the spout at that place.

Whenever these two points would come within discharging distance, the electricity contained in the cloud would rush towards the sea the water contained in it would necessarily discharge itself at the same point. We have here the whole phenomenon of the waterspout. Sometimes three or four have been observed at one time, and all proceeding from one cloud: at other times, a boiling up of the ocean has been observed immediately under the spot where the spout was about to take place, as if there was a desire in the ocean to meet the spout at that particular point. This is exactly what should take place, and, had the particulars of a water-spout never reached me farther than the bare fact, I could, with the greatest ease, have predieated that such would take place that it would be a necessary result of what has been already laid down. It is, however, this fact alone that has rendered the phenomena of waterspouts apparently inexplicable, as it induced some observers to pronounce that the rush of water was from the sea upwards, and not from the cloud and downwards. The cause, however, will, I trust, be obvious. Bodies dissimilarly electrified attract each other. The cloud being highly positive on the one hand, and the ocean being in an opposite state, a mutual attraction takes place between them, and were it not for the high specific gravity of the water, as

compared with the opake vapour of the cloud, it would ascend still higher. Similar attractions are often observed on the land even on a much smaller scale. While making these observations on waterspouts, I wish it to be understood I do it with great deference, because I differ with a high meteorological authority, Mr. Espy, on this point; and that water occasionally rushes upwards from the sea it would be difficult to deny, or how else are we to account for showers of fishes and other marine productions that are sometimes precipitated on the land? This phenomenon is accounted for, by this gentleman, by supposing a vacuum has been created immediately above the spot where the spout takes place, in consequence of the discharge of latent caloric during the process of cloud forming.

In the present state of my knowledge, I cannot, by possibility, conceive that this is adequate to produce the observed effect, more especially when we find water-spouts taking place when all is comparatively calm for many leagues around. Were there a rush of air, in and upwards, observed within any reasonable distance from where the phenomenon was occurring, then we might assimilate it with the tornado; but, on the contrary, water-spouts have been observed at the mouths of some of the narrowest African rivers, while the vegetion at each side remained comparatively quiescent; even vessels have entered between the spout and the land without any observable difference in the breeze which bore them onward.

There are two other great meteorological phenomena which I must briefly explain to enter into each of them fully would require much more space than is at present allotted to me. The aurora borealis and australis are explainable, in my opinion, perfectly on the principles 1 have laid down. Evaporation takes place at the equator in greater degree than at any other portion of the earth; and, from the intensity of the sun's rays at the same point, the atmosphere is, by expansion, eight miles higher than at the poles.

When clouds are formed within the tropics, they are carried to the higher regions of the atmosphere, where they are rolled off, down this inclined atmospheric plain, towards the poles. Thus far, this is no theory, but is now demonstrated by meteorologists. An immense mass of cloud is accumulated in the regions toward the poles, where evaporation does not go on to an extent sufficient to cause a hundreth part of that found there. This cloud is much nearer the surface of the earth there than at any other point of the globe, from the causes just named. It is highly surcharged with electricity, a greater quantity of cloud being there condensed in a given space. Although those clouds are so highly charged with electricity, yet we seldom have thunder-storms in the arctic regions; and, from what has been said, this may appear paradoxical; yet, a little reflection will shew its strict accordance with the laws laid down. The clouds are all positive, or nearly so, with respect to each other, and hence there is nothing to disturb their equilibrium,