natural Cause of the Propofition is this; that the hot Particles of the Sun's Rays, or any Fire, are greater plenty with the Air nearest us, than in the upper Air. These things being laid down, it is manifeft that the Air of the higher Countries being lefs ftored with Vapours, has, in proportion to it's Denfity, more Elasticity in it thanthat which comes next to the Earth, whence the reciprocal Ratio of Gravity, which is in the Air next the Earth, does not hold here; and further, that Vapours and Exhalations have not fuch Elafticity as Air, but that this is much more rarefied and extenuated. But the excellent Mr Fontenelle, Secretary of the Society, explains thefe Phænomena in a quite different Manner, in his Hiftory of the Academy, Anno 1708. He propofes fome Experiments performed by the famous Mr De la Hire, and others, from which he infers, that the elaitic Force of the Air is increased when it is mixed with Moisture, or when compounded of Air and aqueous Vapours, the Rarefaction will be greater, than from pure Air; and that therefore on the Tops of Mountains the Air is found more rarefied, because many Vapours are carried thither for producing of Rain. The Experiments are thefe : They took a flender glafs Siphon, one of whofe Legs ended in a large hollow Sphere, being open at the other. This the Siphon was full of common Air, and expofed to the external Air coming into the Siphon. The Globe and Siphon was plunged into hot Water, found by previous Experiments to be of the fame degree of Heat as boiling Water, and confequently caufing the fame degree of Rarefaction; tho' the Fire underneath were greater or less. the this When the Air included in Globe was rarefied with degree of Heat, it would be gradually thrust out at the other end of the Siphon; 'till at length the Globe being heated to the utmoft, there was left a very fmall quantity of Air, highly rarefied, that poffeffed the whole Cavity. Then the Water being removed from the Fire, the Air, as it gradually cooled, which before poffeffed the whole Globe, being gradually contracted by the Cold, gave way to the Water that entered at the Orifice of the external Leg, and at length, when the Water became entirely cold, it was contracted into a very small Space, whilst the reft of the Globe remained filled with Water. Now by comparing the Space, poffeffed by the Air, cooled and reduced to it's natural State, and the whole Cavity of the Globe which it had at the utmost Heat, it appears how much the Air was rarefied with that degree of Heat. This the moft fubtile in the World, and inconftant Motion, and while these are mixed with the At This Experiment was firft made in clear Weather, again in a moift and rainy Seafon; and at a third time, a little Water was left adhering to the inner Surface of the Globe. And it was obferved that the Air condenfed at the end of the Experiment, in the firft Cafe poffeffed of the Globe, in the fecond poffeffed but, and in the third Whence Mr Fontenelle concludes, that the Air was more dilated in the fecond Cafe, but particularly in the third, than in the firft Cafe; and therefore as the Air is the more dilated the more moist Vapour is mixed with it; hence he concludes it probable, that for the fame Reason, there is a greater Rarefactionon the Tops of Mountains, because the Air that furrounds them is mixed with a greater quantity of Vapour. But there are two Confiderations that render the Argument inconclufive. For firft in the two later Experiments, as aqueous Vapours were plentifully mixed with the Air, it might happen that when the Air was condensed, and the Water entered thro' the Siphon into the Globe, thefe Vapours might again return to Water, and mixing with the other Water partly by the Force of Condenfation, and partly by the mutual Attraction there is betwixt the Particles of Liquors, leave but little true Air included in the very fmall Space. mosphere, Whence it might feem, that the quantity of Air which rarefied with the fame degree of Heat poffeffed the whole Cavity, was lefs in the two later, than in the former Cafe; and therefore more dilated, fo as to poffefs the whole Space. Again, allowing that the Air was more rarefied in the later Cafes, yet as this was effected by the means of Heat, I do not fee how it follows that because the Vapours mixed with the Air, and agitated by Heat, are more rarefied than Air without Vapours, therefore these Vapours without Heat, fhould have a greater Elafticity than pure Air. We fhall here add a Table of M. Caffini, junior, made from the foregoing Obfervations, and exhibiting the Height of the Air from the Surface of the Sea, correfponding to the Sinkings of the Barometer; as alfo the Spaces increasing in arithmetical Proportion, wherein the Height of the Air increafes almoft half a French League, whilft the Barometer finks in twelfths of an Inch, at a time when, being placed on the Surface of the Sea, it ftands at about 28 French Inches or 293 of English. I use the French Meafures, being unwilling, by reducing them to the English Feet, to disturb the beautiful Series of Proportions by fmall fractional Parts; tho' thefe may, by the help of the |