Page:Philosophical Transactions - Volume 024.djvu/477

Grains; being all of the sort generally sold at Toy-shops. The Plane whereon they dropt was a round flat piece of solid Glass, about an inch thick, and 3⁠1/2⁠ over; whose upper surface was well Grownd and Polisht. It was fix'd in a Tin frame, contriv'd on purpose to keep its lower surface from being contiguous to the Plate or Leather, on which the Recipient was plac'd; (which I mention purposely, lest any Gentlemen, who may have the curiosity to repeat these Experiments, should fall into an error I very narrowly escap'd) for when first I made these Experiments I us'd a Stone Plane laid carelesly on the Leather that cover'd the Plate, over which the Recipient being plac'd, and the Air exhausted, the descending Marbles would not rebound so high by an inch, as when the Experiment came to be made on the same Plane in common Air; which unexpected Phænomenon gave great umbrage that it might proceed from some unheeded cause, and several reasons were alledg'd against the candour of the Experiment; the principal of which was, that the swelling of the Leather in vacuo raised the incumbent Plane, causing it to lye more hollow and soft than it did in the open Air. In order therefore to set the Experiments in a true light, I procured the prementioned Glass Plane, on which, when I came (as before) to drop the Marbles, their Resilition then was something more in Vacuo than in common Air (which put it past all doubt, that the Error proceeded from the cause suggested.) Those dropt in common Air had likewise some small advantage in their rebound, above those let fall in Air condensed. The Condensation being but one Asmosphere, not daring to venture more, the breaking of the Recipient being very hazzardous. The Resilition of the Marbles from the Plane in Vacuo was about 10⁠1/2⁠ inches, which was something more than ⁠3/4⁠ of their descent. In Condensed Air their rebound was about 10 inches. Accordingly we must account their Resilition in Common Air to be the medium of the other two, it beingdifficult