One reason, which seems to have led us into the mistake that the solidity, or firmness of bodies is not caused by the air, has been, that, for the generality, we consider the air, or atmosphere as pressing only downwards; for if we had considered that it presseth equally every way, as well as downwards, (as Boerhaave in his chym. by Shaw, vol. 1. p. 389. has shew by the following experiment) I believe we should not have overlooked that force, or have thought it insufficient for this operation. “Fill three glass vessels, the one of a cylindrical figure, the other conical, the third bellied with a cylindrical neck; let these be filled to the brim with fair water, and covered with a single piece of paper, so as to touch the surface of the water, and by pressing it down with the hand, prevent the external air insinuating between the paper and the water; if the glasses be now inverted, whilst the paper remains close with the palm of the hand, and the hand be afterwards gently withdrawn, the water will still remain in the glasses. The same holds true, though the glasses be held horizontal, or in any other position.” As the cause of the solidity or firmness of bodies is the pressure of the air or atmosphere, so likewise it must depend on the make and size of the pores of such bodies; for bodies whose pores are smallest, must be acted upon with a greater power than those whose pores are largest, or whose pores are so large as not only to admit light, but also common air into them. This Mr. Hauksbee proves by a curious experiment, for having placed two brass hemispheres, of 3 1/2 inches diameter, upon each other, and then extracting the gross air out of them by the air pump, and by these means taking off the resistance of the common air that was within the two brass hemispheres, he says, it required 140 pound weight to separate them; this experiment with that of the two marble slabs before mentioned, is a demonstration of the power that keeps solid bodies from falling to pieces. And even, if these slabs are not so perfectly smooth, yet the weting them with water, which prevents the gross air from entering, will produce the like effect. That solids expand themselves by heat or fire, is proved by heating an iron rod in the fire: in which case, it is always found to be bigger and longer when hot than cold; and it was the opinion of Boerhaave, that cold consolidates all those that are called firm bodies; that is, brings that part, which we call body in them, into a less compass than before, and thus unites the matter thereof more closely together: by which means the cohesion of the whole mass is usually increased; which makes what we call, strength and firmness in bodies.
Having shewn that solid bodies have no power either of attraction or gravitation, and that their firmness or solidity depends on the pressure of the atmosphere; I shall now endeavour to shew, from electrical experiments, how, and by what means they descend towards the earth.
What is called the attraction of the earth seems to be performed in the same manner as that of the glass globe in electricity; the explaining of which will give us a clear idea, by what means heavy bodies are forced towards the terraqueous globe. In accounting for this electrical attraction, Mr. Hauksbee seems to be very clear; for, says he, “if by the heat and rarefaction, consequent upon the attrition, the medium contiguous to the glass be made specifically lighter; then of course, to keep up the ballance, the remoter air, which is denser, must press in towards the tube, and so carry away (in the torrent) the little bodies lying in its way, thither also. The various irregularities in the excitation, or the emission and discharge of the electrical matter or light from the tube (which will be followed with proportional irregularities, in the motion and tendency of the denser air, towards the glass globe, by the hydrostatical laws) may be sufficient to account for the various uncertain motions of the little bodies carried towards the glass globe.” This account of Mr. Hauksbee’s being so very clear, it is a little surprising that he should allow the power of attraction to matter, as in some places he does; for this is no more than in other words, telling us, that the air round the ball is divided and rubbed or ground to pieces by the friction between the glass globe and your hand, and thereby made to expand itself; so the air pressing in to make up that deficiency, forces every thing towards the glass globe, that by its number of particles is not able to withstand the current of air, pressing towards the globe. So in like manner near the surface of the earth, the sun-beams being reflected by the terraqueous globe, must by these means be in a greater quantity near the surface of the earth, than at a distance from it; and so divide, expand and rarify the air near its surface, which rarified or divided air is forced off from the earth on all sides, by the pressing in of the air from above, which must of consequence drive every thing before it, towards the earth. By which we find, that the cause of bodies descending towards the earth, is not from any property either of the earth or of the descending bodies; but that these are forced towards that, by the surrounding air, in its said motion.
To prove that this is the method by which, in electricity, bodies are forced towards the glass globe, I shall bring an experiment or two from Mr. Hauksbee; and as these experiments prove to a certainty, that this is the manner of the attraction in electricity, it will give us little room to doubt, but that the attraction of the earth is performed in the same manner.
Mr. Hauksbee observed, that the electrical effluvia were not only perceiveable by sight; but also, if the hand was held near the tube, seemed to make such sort of strokes upon the skin, as a number of fine limber hairs pushing against it might be supposed to do: and in order to find whether the electrical attraction was regular and uniform, he made the following curious experiment, shewing that all bodies, not too heavy, are forced (or, as is commonly supposed, attracted) to a cylindrical glass, equally all round, if these bodies are, as they term it, within the sphere of its activity.
This experiment seemed to affect Mr. Hauksbee so much, that (speaking of electricity) he says, “it affords us a sort of representation of the great phænomena of the universe.” Page 53.
“For, says he, having observed (in electricity) that light bodies, placed near any part of the rubbed cylinder, seemed to be equally attracted, I contrived a semicircle of wire, which I could fasten at a constant distance, making it encompass the upper semi-cylindrical surface of the glass, at 4 or 5 inches distance. This wire had several pieces of woollen threads fastened to it at pretty near equal distances. The length of them was such, that being extended in a direction towards the center of that imaginary circle, on the surface of the glass, in the plane of which the wire was placed; they would then reach within an inch of the circumference of that circle: but if left to their own liberty, they hung in that parallel portion represented, fig. 1. The cylinder was placed with its axis parallel to the horizon; and in this posture, it was turned swiftly round; and then by the rapid motion and agitation of the surrounding air, the threads were placed into such portions, as are expressed fig. 2. viz. they were all lifted up and bent upwards from the axis of the cylinder.
“All this while, there was only the swift motion of the cylinder round its axis, without any attrition, but now when I came to apply my hand to the lower part of the glass (so swiftly whirled about) and consequently to add attrition to the former motion; the threads presently began to change their direction, and all harmoniously pointed to the center of the circle, in whose plane the wire was placed, as in fig. 3. neither were they at all disordered or flung out of that position, by the wind occasioned by that violent motion (but as if there had been no such hurry of air about them) they still persisted in their central direction; I found I could by shifting the place of the attrition hither or thither, draw the threads towards this or that end of the cylinder; but yet they all still went uniformly converging towards some center in the axis of it; so that they formed themselves into a sort of conical surface.