ment, where there was no other alteration than allowing the cloud to move freely, then the point was not struck. I imagine, if Mr. Wilson's large artificial cloud at the Pantheon, which was 155 feet long and sixteen in diameter, had been properly insulated, and there had been several cylinders properly mounted to have charged it, he would have found the striking distance, and his other experiments, very different from what he did, particularly those where his substitute was fixed about one inch and a half from his artificial cloud,

I must beg to intrude a little more on your time to remark on that part of Mr. Wilson's paper, where from his experiments he seems to conclude, that the lightning at Purfleet first struck on the point of the rod of the conductor, and then, by a lateral part of that stroke, struck the cramp on the coping stone. I believe, if he had examined the situation of the stone, and the place where the cramp was struck, he would have found, that if the lightning had struck on the point of the conductor, that to have produced that effect on the stone, it must, after it had struck on the point, and passed down a quantity of metal, have struck from the metal up into the air, then down again on the cramp, and then again to the metal it had left; for the small dent or hollow made by the lightning was on the upper surface of the stone, and yet the metallic communication to the earth continued from the point under the stone which was struck. It appears more probable to me, from the trifling damage it did, that the charged cloud had passed over the pointed conductor, and had been exhausted of a great part of its electricity in passing; and that after it had passed, it was attracted down lower by a ridge of hills that was beyond, and that the cloud being out of the influence of the point to prevent its striking, the end of the cloud might strike at an angle in the cramp, and so to the metallic part of the conductor, which was only about seven inches below. I shall conclude with observing, that Mr. Henly and myself had the pointed rod of the conductor at Purfleet taken down to examine the point; but we found no appearance on it that showed that it had been struck.

On the Cause of Thunder. By Mr. HENRY EELES.

MR. EELES's opinion on the cause of thunder is, that it is by electrical explosions among the clouds, the fire of lightning and electricity being of the same nature, as had been long before proved by the experiments of Mr. Franklin in America. After the explosion, then the echo of it from the other clouds is the cause of the continued or distant rumbling noise. Mr. Eeles adds, that he intends afterwards to show, that this fire is a most considerable agent in nature. First, that the ascent of vapour and exhalation is principally owing to it, and that our atmosphere, by that means, is kept more homogeneal than is generally supposed, and fitter for respiration, vision, &c., and that clouds of heterogeneous matter are kept

suspended at their usual height merely by this fire, Secondly, that this fire is the cause of the reflection, refraction, and inflexion of light. Thirdly, that it is the cause of that secondary attraction and repulsion, which Sir Isaac Newton has taken notice of. Lastly, he will give some hints of the great use of this fire in animal life, and in vegetation.

Lismore, Ireland, June 18, 1752.

Letters of HENRY EELES, Esq., concerning the Cause of the Ascent of Vapour and Exhalation, and those of Winds; and of the general Phenomena of the Weather and Barometer.

1755.

Ir is agreed, that the ascent of vapour and exhalation through the air may be effected in two ways, by impulse, and an alteration of their specific gravity. That vapour does not generally ascend by impulse, may be proved by many familiar experiments, viz.: put boiling water into a vessel; then empty it, and hold the vessel with the aperture downwards ; the vapour, which is afterwards expelled from the vessel, must be in a direction downward; but we find, that as soon as it has got but a very little below the rim of the vessel, it has its direction altered, and ascends by the laws of specific gravity. The same thing may be observed in all boiling vessels, where the vapour is emitted in a direction downward; or, in cold weather, when the vapour of a man's breath may be seen, let him breathe downward, and the direction of his breath will be presently altered, as in the former case. Since then vapour ascends without any other impulse than that which is incident on all bodies ascending by the laws of specific gravity, it is necessary to inquire, how the specific gravity of vapour is altered, to cause its ascent. This is generally supposed to be done by filling vesicles of water with rarefied air, till the diameter of the vesicle be ten or more times the diameter of a drop of water, composed of the same constituent particles; and that the vesicle, by this means, becomes specifically lighter than air. But Mr. Eeles thinks that this cannot be done so easily as it has been generally imagined; and when done, it will not be sufficient for the purpose; which he infers from the following considerations. First, the great difficulty in forming those vesiclés, especially of the particles of dry bodies carried off by exhalation, and filling them with rarefied air, while the exterior air remains condensed. Secondly, that there is not any allowance made for the weight of the included air. Thirdly, the constituent particles of water are but very little, if at all altered in their specific gravity. Fourthly, that this thin vesicle can never be a sufficient boundary between the exterior condensed air and the interior air, so exceedingly rarefied.

Rejecting these popular opinions, as to the cause of the ascent of vapours and exhalations, Mr. Eeles observes, that it now remains to

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inquire, by what means this may be done; since neither impulse, rarefaction of the air, nor any formation of their parts by expansion, seem sufficient for the purpose. There appears to him but one way of altering the specific gravity of the particles of vapour and exhalation, to render them lighter than air, which is by adding to each particle a sufficient quantity of some fluid, whose elasticity and rarity are exceedingly greater than that of air. That the fluid or fire of electricity is such, will be easily granted; but how far it is adapted to this purpose, we must enquire from experiments. For the purpose is great no less than all vegetation and animal life depending on the ascent and descent of vapour and exhalation.

Mr. Eeles says he has made some experiments, by which it appears that all fumes arising from fire, whether blazing or otherwise, and all steams arising from boiling or warm waters, and from all other fluids, and the breath of man, and of all other animals, and all the effluvia thrown off by perspiration, are strongly electrified. But he now only mentions a few. First, that desultory motion, by which it flies off from an electrified body to any number of non-electrics, which are brought within the sphere of its activity and affection, till it be equally diffused through all. Secondly, that the sphere of its activity is increased by heat. Thirdly, that this fire does not mix with air. Fourthly, that it intimately pervades water, and many other bodies, covering their superficies to a certain distance; which distance is not in proportion to the bulk of the body electrified, but in proportion to the state of activity of the electrical fluid. Fifthly, this electrical fluid readily joins with any fire which fumes, or rather with the blaze or fumes of any fire; but will not mix or fly off with the fire of red hot iron, or any other metal which does not fume.

Now, to shew that this electrical fire or fluid is the principal cause of the ascent of vapour and exhalation, we need only prove that it attends all vapour and exhalation, and that in such quantity, as is ́necessary to render them specifically lighter than the lower part of the atmosphere. He does not undertake to determine, by what cause vapour and exhalation are detached from their masses, whether by the solar or culinary fire, or by the vibrations of the electrical fluid rendered more active by those fires; though he thinks the latter. But it is evident, that they are emitted in exceedingly minute distinct particles, and that these particles must pass through that electrical fluid which surrounds the surface of the mass; and that, by that means they must be equally electrified with the mass; that is, they must be covered with the electrical fluid to as great a distance from their superficies as the mass is covered; which must always be in proportion to the state of activity of the electrical fluid.

In which state, when they have passed the surrounding fluid, they must be repelled by it; and also repel each other; and if each particle of vapour, and its surrounding fluid, occupy a greater space than the same weight of air, they must be fitted to ascend till they come in equilibrium with the upper and rarer part of the

atmosphere, where they must float, till their specific gravity is altered. As it is very difficult to assign the magnitude of each particle of vapour and exhalation, and that of the surrounding fluid; and to show that both, taken together, occupy a greater portion of the space than the same weight of air; we can only apply to experiment, to show that it is possible that it may be so; and that will show, that in all probability it is so; since it is evident, that every particle must be endued with a portion of this electrical fire or fluid; and that there is not any other sufficient cause assigned for their ascending.

It is evident, that on electrifying any light matter, such as down, or the downy parts of feathers, their specific gravity is much lessened; and that, by holding another electrified body under them, they may be driven upwards at pleasure. It is also evident, from experiment, that the more you divide the parts of such bodies, the more of their specific gravity they will lose by being electrified; and by dividing them into very minute parts, that they ascend to a considerable height after they are electrified. Hence he thinks it highly probable, that the exceedingly small particles of vapour and exhalation may be, and are, sufficiently electrified, to render them specifically lighter than the lower air; and that they do ascend by that means. And that they will ascend proportionally higher, as the surrounding fluid is proportionally greater than the particle which is carried up.

Mr. Eeles then endeavours to show that the ascent and descent of vapour and exhalation, attended by this fire, is the principal cause of all our winds. It being admitted that wind is only air put into motion, many have been the conjectures how that motion is caused. Among which, the motion of the earth, and the air's being rarefied by the sun, seem to stand first. The trade winds being most regular, and occupying a considerable part of the globe, it has been thought proper first to account for them, from the afore-mentioned causes. But he thinks that these causes, by themselves, are not suffi cient for the motion of those winds, and much less so for the irregular motion of all the other winds. If the apparent motion of the air was occasioned by the diurnal revolution of the earth from west to east, by the air's being left behind, the motion must be found more regular, and very different from what it is; for in that case the greatest motion must be at the equator, and from thence lessen gradually to the poles; and must be continued always equally one way, both day and night, and at all seasons. But we find quite the contrary the most gentle gales blowing at the equator and between the tropics pretty steadily, one way all day long, and dying away at night; while high winds and storms, blowing all manner of ways, are found in the higher latitudes. Neither does he think that the sun's rarefying air can simply be the cause of all the regular and irregular motions found in the atmosphere; but he

thinks the cause is the ascent and descent of vapour and exhalation, attended by the electrical fire, or fluid.

Now, all the vapour and exhalation, raised in the torrid zone, being buoyed up by the electrical fire, must add a column to the air, though of a different matter, at least one thousand times greater than the vapour and exhalation taken up; which column must necessarily force the adjacent part of the incumbent air upwards; and must as necessarily be reacted on by the incumbent air, to restore the equilibrium of the whole air. And as it cannot be readily forced down again, it must float off, at that altitude, toward those parts where little or no addition has been made to the atmos.. phere; and by that means must propel the air on the horizontal level with it, and that below it, as it is itself propelled by the weight of the incumbent air. And that motion must be from the equator, where the greatest quantity of vapour, &c., is raised, towards the poles, and partly to the west; as the column of vapour is always rising from east to west, as the earth turns toward the sun. For here we must confess, that the sun is the great agent in detaching vapour and exhalation from their masses; whether he acts immediately by himself, or by his rendering the electric fire more active in its vibrations; but their subsequent ascent Mr. Eeles attributes entirely to their being rendered specifically lighter than the lower air, by their conjunction with this electrical fire. The fire, which surrounds the vapour, beginning to condense, and the vapour to subside, in passing the tropics, becomes a greater pressure on the air beneath, and by that means forces some part back into the tropics, in the place of that air protruded by the ascent of the vapour, &c., and the remainder in a direction toward the poles. The common rotation of the air in coming in below, to supply the place of that part carried up by any fire, may explain this motion. To show how this motion must tend to the west, we must consider, that the column of air, raised by the ascending vapour, &c., is at its greateat altitude to the east, and therefore must press that air to the westward, which is continually protruded by the vapours, &c., beginning to ascend from east to west; and the compressed air at the tropics must tend to the westward, till their forces meeting make the motion entirely to the west. The air itself being rarefied, and carried up by the reflection of the intense heat of the sun, may be a considerable additional cause of these trade winds: but never can be the sole cause of all the erratic winds. To account for all the irregular winds within the tropics, he says, that where such happen, it must be by means of some tracts of land, which rise to a greater height above the horizontal level, than vapours generally do; by which the motion of the vapours is stopped, and the vapour accumulated by succeeding vapour, and the air, on which they float, is of consequence pressed into a new direction. And from hence may also be explained the cause of the rains, particularly so called in the sea language.