wind, it will affect bodies more powerfully than when it subsides into a calm; and fire, by parity of reason, ought to produce its effect sooner when it is more agitated. We shall also see, in the proper place, that it is the nature of fire, when in a rising state, to expand or rarefy bodies; by means of which it prepares for itself a more easy admission into their pores but, on the contrary, when it is decreasing, bodies are thereby contracted in all their dimensions, their pores become narrower, and thus the cold helps as it were to shut the door against itself. This leads us to consider that other property of heat, by which fire is most generally known. It is vulgarly supposed, that where there

The vessel being thus prepared, its lower half was dipped into boiling water for the space of six seconds, by means of which the air in the lower cavity being rarefied, expanded itself through the tube IG, and drove the water forcibly up the tube PQ into the reservoir R. After which the vessel being placed in cold water, the water began to descend from the reservoir, but did not regain its former station in less. than 18 or 20 seconds. The trial was many times repeated, and always with nearly the same event. My experiment gave the ratio of 5 to 14, this of Mr. Amontons gives that of 6 to 18: and if we consider that the two experiments were made at very different stages of heat and cold, the re sults are perhaps as near as they ought to be.

tle

is most heat, there is most fire: but heat is rather owing to a particular motion of fire, than to the quantity of it. When the air is stirred into wind, its quantity is much the same as before, and its weight differs but litupon the barometer. Its effects as wind are owing to its motion; and these effects are most violent when the wind moves in contrary directions, as in the whirlwind, and in some particular hurricanes. So it is in the sea, when the wind is one way and the tide another; or when two streams meet and form an eddy. It is on this principle that fire affects bodies with heat. So long as the rays of the sun proceed without disturbance and preserve their parallelism, but little heat arises from them: but when they are thrown together in various directions, and returned upon themselves by the action of the air, or by attrition, or by a variety of reflexions, or by the refractions of a common burning glass throwing them into a focus, so as that they work together with an expansive force, then they never fail to affect us with a sense of heat: and if their agitation is both complicated and violent, they will tear in pieces and dissipate the bodies they act upon. Near the surface of the earth the rays of the

sun

sun produce a much greater heat, on account of their various reflexions, first from the matter of the atmosphere, and then from the earth itself, but more especially if the ground is hard and dry, or of a light colour. Nothing reflects the heat of the sun more intolerably than sand; and the reason is plain, if we consider that the particles of sand are little angular bodies with many sides and polished surfaces, so as to throw the rays that fall upon them into a multitude of directions, and thereby keep up and increase the agitation. Most people have opportu nities of observing how much the heat is increased when the sun's light is reflected from the ground and from the walls of buildings in a public street: and so great is the tumult which arises on this principle, that it may be perceived with the naked eye when the air is viewed obliquely near a south wall, or over a field of ripe corn, or along the fallow ground; and this vibratory motion is most perceptible in a season of drought, when it is generally understood as a prognostication that the drought will continue for some time. When this motion is not visible to the naked eye, but the horizon and the objects near it appear steady and distinct, even then a good telescope,

03

telescope, which magnifies about sixty times, will shew the light to be in such agitation, that the horizon of the earth will look like the horizon of the sea in a storm, and the objects placed upon it will seem as unsteady as a vessel tossed upon the waves: but whoever would see this tumult in the greatest perfection, must view it about the middle of the day, in the summer time, when the sun is most powerful. As we ascend higher in the air, we get above this disorder which prevails near the surface of the earth, and find a more quiet, and consequently a colder region: insomuch that if a mountain rises to more than a mile in perpendicular height, the rays of the summer sun have not power enough to dissolve the snows which lie upon the head of it. The vibratory motion with which light and fire are affected when they give us a sense of heat, may easily be distinguished by the sight when objects are viewed through a heated medium. If the flame of a candle is interposed between the eye and the objects viewed by it, in the day time, those objects which are seen a little above the point of the flame will seem to have an undulatory or dancing motion; because the rays of light which come across the heated

medium,

medium, are disturbed and thrown out of their rectilinear course by the motion communicated to them by the flaming body. The same fluctuating motion in the visual rays is still more easily observed over a dish of burning charcoal: but it is never so conspicuous as over a lime-kiln, when the fire hath subsided, and the stones have acquired their full heat. The objects then viewed through the heated vapour appear to tremble in such a manner as to shew us, that the rays of light are disturbed in their progress, and that the motion of fire, when it gives heat, is not progressive, like that of the rays of light, but vibratory; and this gives the reason why light passes instantaneously through transparent bodies, while heat makes its way more slowly and by degrees. The most violent blast of air is not found to affect the rays of light when it blows across them: but if the matter of fire strikes upon them, when it is so agitated as to give heat, they are staggered in their course, and no longer give us a steady sight of those objects from which they are either emitted or reflected, refracting telescope is pointed toward a star of the first magnitude, and a candle is held near the farther end of it, so as to illuminatę 0 4

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