breaking off the small seal'd neck of the Bubble (without at all stirring the sights, object, or glass) and admitting the external Air, you will find your self unable to see the utmost ends of the object; but the terminating rayes A E and A D (which were before refracted to G and F by the rarified Air) will proceed almost directly to I and H; which alteration of the rayes (seeing there is no other alteration made in the Organ by which the Experiment is tryed, save only the admission, or exclusion of the condens'd Air) must necessarily be caused by the variation of the medium contain'd in the Glass B; the greatest difficulty in the making of which Experiment, is from the uneven surfaces of the bubble, which will represent an uneven image of the object.
Now, that there is such a difference of the upper and under parts of the Air is clear enough evinc'd from the late improvement of the Torricellian Experiment, which has been tryed at the tops and feet of Mountains; and may be further illustrated, and inquired into, by a means, which some whiles since I thought of, and us'd, for the finding by what degrees the Air passes from such a degree of Density to such a degree of Rarity. And another, for the finding what pressure was requisite to make it pass from such a degree of Rarefaction to a determinate Density: Which Experiments, because they may be useful to illustrate the present Inquiry, I shall briefly describe.
Fig. 1.I took then a small Glass-pipe A B, about the bigness of a Swans quill, and about four foot long, which was very equally drawn, so that, as far as I could perceive, no one part was bigger then another: This Tube (being open at both ends) I fitted into another small Tube D E, that had a small bore just big enough to contain the small Pipe, and this was seal'd up at one, and open at the other, end; about which open end I fastned a small wooden box C with cement, so that filling the bigger Tube, and part of the box, with Quicksilver, I could thrust the smaller Tube into it, till it were all covered with the Quicksilver: Having thus done, I fastned my bigger Tube against the side of a wall, that it might stand the steadier, and plunging the small Tube cleer under the Mercury in the box, I stopt the upper end of it very fast with cement, then lifting up the small Tube, I drew it up by a small pully, and a string that I had fastned to the top of the Room, and found the height of the Mercurial Cylinder to be about twenty nine inches.
Then letting down the Tube again, I opened the top, and then thrust down the small Tube, till I perceived the Quicksilver to rise within it to a mark that I had plac'd just an inch from the top; and immediately clapping on a small piece of cement that I had kept warm, I with a hot Iron seal'd up the top very fast, then letting it cool (that both the cement might grow hard, and more especially, that the Air might come to its temper, natural for the Day I try'd the Experiment in) I observ'd diligently, and found the included Air to be exactly an Inch.
Here you are to take notice, that after the Air is seal'd up, the top of the Tube is not to be elevated above the superficies of the Quicksilver
