this the French call serein: the humidity deposited by mists on trees, and by moist air on windows, generally within, but sometimes without, they call givre.

It is well known that dew is often deposited on glass, when metals in its neighbourhood remain dry; Mr. Prévost has however discovered some new and curious facts relative to this deposition. When thin plates of metal are fixed on pieces of glass, it sometimes happens that they are as much covered with dew as the glass itself: but more frequently they remain dry; and in this case they are also surrounded by a dry zone. But when the other side of the glass is exposed to dew, the part which is opposite to the metal remains perfectly dry. If the metal be again covered with glass, it will lose its effect in preventing the deposition.

These experiments may be very conveniently made on the glass of a window, when moisture is attaching itself to either of its surfaces; Mr. Prévost remarks that it often happens that dew is deposited externally, even when the air within is warmer than without. A plate of metal fixed internally on the window receives a larger quantity of moisture than the glass, while the space opposite to an external plate remains dry: and if the humidity is deposited from without, the place opposite the internal plate is also more moistened, while the external plate remains dry: and both these circumstances may happen at once with the same result. A small plate fixed externally, opposite to the middle of the internal plate, protects this part of the plate from receiving moisture, and a smaller piece of glass, fixed on the external plate, produces again a central spot of moisture on the internal one: and the same changes may be continued for a number of alternations, until the whole thickness becomes more than half an inch. Gilt paper, with its metallic surface exposed, acts as a metal, but when the paper only is exposed, it has no effect. When a plate of metal, on which moisture would have been deposited, is fixed at a small distance from the glass, the moisture is transferred to the surface of the glass immediately under it, without affecting the metal: if this plate is varnished on the surface remote from the glass, the effect remains, but if on the side next the glass, it is destroyed. The oxidation of metals renders them also unfit for the experi When glasses partly filled with mercury, or even with water, are exposed to the dew, it is deposited only on the parts

ment.

which are above the surface of the fluid. But in all cases when the humidity is too copious, the results are confused.

In order to reduce these facts to some general laws, Mr. Prévost observes, that when the metal is placed on the warmer side of the glass, the humidity is deposited more copiously either on itself or on either surface of the glass in its neighbourhood: but that, when it is on the colder side, it neither receives humidity nor permits its deposition on the glass: that a coat of glass, or varnish, destroys the efficacy of the metal, but that an additional plate of metal restores it.

Mr. Prévost was at first disposed to attribute these phænomena to the effects of electricity, but he thinks it possible to explain them all by the action of heat only for this purpose he assumes, first, that glass attracts humidity the more powerfully as its tempe. rature is lower; secondly, that metals attract it but very little; thirdly, that glass exerts this attraction notwithstanding the interposition of other bodies; and fourthly, that metals give to glass, placed in their neighbourhood, the power of being heated by warm air, and being cooled by cold air, with greater rapidity: hence that the temperature of the glass approaches more nearly to that of the air on the side opposite to the metal, and attracts the humidity accordingly more or less, either to its own surface, or to that of the metal. We should indeed have expected a contrary effect; that the metal would rather have tended to communicate to the glass the temperature of the air on its own side: but granting that the assumptions of Mr. Prévost serve to generalise the facts with accuracy, their temporary utility is as great as if they were fundamen tally probable.

Hence dew is perhaps nothing more than a portion of the vapour formerly suspended in the atmosphere, condensed by means of the cold of the evening. It has been observed with surprise that when a number of bodies are exposed together to the dew, some are quite wetted with it while others remain dry. This circumstance probably depends upon the goodness of the body as a conductor of heat. Good conductors will part with their heat more readily, and will therefore evaporate the dew again, whereas it will remain upon bad conductors, which will not so easily part with their heat. If this explanation be the true one, it follows that bodies exposed to the dew and dry, must have a lower temperature than those

which remain moist. The following Table by Dr. Stocke, exhibits the relative quantity of dew remaining upon a variety of bodies*.

If these experiments be accurate, it is obvious that the goodness of the bodies as conductors of heat will not account for the phanomenon. Perhaps we must look for it in the goodness of the bo. dies as conductors of electricity.

Mists are said to consist sometimes of other particles than pure water: these are called dry mists, and they have been supposed to blight vegetables. Such mists are sometimes attended by a smell, resembling that which is occasioned by an electric spark. Rain falling after a dry season deposits, when it has been suffered to stand, some particles of foreign matter which it has brought down from the atmosphere. There must indeed frequently be a multiplicity of substances of various kinds floating in the air; the wind has been found to carry the farina of plants as far as 30 or 40 miles, and the ashes of a volcano more than 200. It only requires that the magnitude of the particles of any substances be sufficiently reduced in size, in order to render them incapable of falling with any given velocity; and when this velocity is very small, it may easily be overpowered by any accidental motions of the air. The diame ter of a sphere of water, falling at the rate of one inch only in a second, ought to be one six hundred thousandth of an inch, which is about the thickness of the upper part of a soap bubble at the instant when it bursts; but the particles of mists are incomparably larger than this, since they would otherwise be perfectly invisible

Phil. Trans. 1742. vol. xli ; p. 112.

as separate drops: the least particle, that could be discovered by the naked eye, being such as would fall with a velocity of about a foot in a second, if the air were perfectly at rest. But it is very probable that the resistance, opposed to the motion of particles so small, may be considerably greater, than would be expected from a calculation, derived from experiments made on a much larger scale, and their descent consequently much slower.

When the particles of a mist are united into drops capable of descending with a considerable velocity, they constitute rain; if they are frozen during their deposition, they exhibit the appearance of a perfect crystallization, and become snow: but if the drops already formed are frozen, either by means of external cold, or on account of the great evaporation produced by a rapid descent through very dry air, they acquire the character of hail, which is often observed in weather much too hot for the formation of snow.

It cannot be doubted but that there is a connexion between the descent of the barometer and the fall of rain; but no satisfactory reason has yet been assigned for the circumstance; nor is it possible to foretel, with certainty, that rain will follow any changes in the height of the barometer that have been observed. The immediate dependence of rain, or of any other atmospherical phænomena, on the influence of the moon, appears to be rendered highly improbable, not only by mathematical calculations of the effects of the moon's attraction, but also by the irregularity of the very observations, which have been adduced in favour of such a connexion. But however uncertain the ultimate causes of rain may be in ge neral, their effects in some places are sufficiently constant to be at. tributed to permanent local circumstances, and in particular to the periodical recurrence of similar winds.

In low and level countries, clouds may often begin to descend from the upper regions of the atmosphere, and may be redissolved by the warmer air below; but when they descend in an equal degree among mountains, they fall on the earth; and besides the quantity of water which they furnish for vegetation, and that which is carried off by evaporation, they afford by means of springs and rivers, a constant supply for the use of man and of other animals in distant parts. The upper regions of the atmosphere are however by no means the principal sources of rain in ordinary climates, since a gage placed on a very high building seldom collects more

than two-thirds as much rain as another standing on the ground be. low and the effects of mountains in collecting rain are perhaps chiefly derived from the ascending currents which they occasion, and by which the air saturated with moisture is carried to a higher and a colder region.

The Abyssinian rains are the causes of the inundation of the Nile; they last from April to September; but for the first three months the rain is only in the night. The inundation, in Egypt, begins at present about the 17th of June; it increases for forty days, and subsides in the same time; but the ancient accounts, as well as some modern ones, assign a longer duration to it. The river Laplata rises and falls at the same times as the Nile. The Ganges, the Indus, the Euphrates, the river of Ava or Pegu, and many other large rivers, have also considerable inundations at regular periods. In many other countries there are seasons at which the rains seldom fail to recur; and sometimes the periodical rains are different in different parts of the same country. Thus the coast of Malabar, which is to the west of the Gate mountains, or Gauts, enjoys summer weather, without rain from September to April, while that of Coromandel, which is on the eastern side, experiences all the rigours of its winter; being at this time exposed to the influence of the north east trade wind. Vicissitudes of a similar nature are also observed on the north and south sides of the island of Jamaica.

[Young. Thomson. Phil. Trans.

It has been a matter of great contest among philosophers by which means water, which is nearly nine hundred times heavier than air, can be rendered capable of ascending into the aërial re. gions. Descartes accounted for it by supposing that by the action of solar heat upon a sheet of water, its superficial particles are formed into little hollow spheres, and become filled with the materia subtilis of space, on a minute substance not unlike the primal atoms of Lucretius, and which Des Cartes conceived to be frequently employed in the formation of clouds. The particles of water thus filled, must necessarily, it was added, from the superior levity of the substance they envelop, ascend through the ambient air till they attain their proper level.

The theory of Dr. Halley was not very different, varying alone