12 $

Pneumatics.

Force with an equal Momentum ; andcause what we call the Renitency,

PlateXXVII.Fig. 9.

a String going over a Pulley C D, and is balanced 1another Weight H hanging freely in the Air. A§ .Surface of the Mercury at A B is very large, and • jat G very small, the Motion of the Quicksilver, a *consequently of the Ball G, will at Bottom be ve jconsiderable; but as the Weight G moves up a jdown, it turns the Pulley C D, and that aor Index K L, by the Divisions of a large grad ua •Circle MNOP; by which means the minutestations of the Air are plainly shewn, if the Instru 1 * 1 ^,be so very accurately made that the Friction of th e ,veral Parts be inconsiderable. This is one of thecurious Inventions of Dr. Haoke. q

13. These are the principal Contrivances hsth e j jinvented for enlarging the Scale of Variation in h^yMercurial Barometers. There are otherTnventi°'’s f/compound Barometers, viz , such as are made of 1 ^cury and Water, or other Liquors for that P ur Pje,but they are so difficult to make, so faulty when i:r Y t oand so troublesome to use, that it is not worth wh (1 ^describe them. However, as the Reader may h^^!Idea of one of the best Sort, I (hall here give hit 11which owes its Invention to the Reverend Mr. Ro^together with his Demonstration of its Theory.

14. A B C is a compound Tube seal’d at A» wopen at C, empty from A to D, fill’d with

from thence to B, and from thence to E with V jylet G, B, H, be in an horizontal Line, then it i S Pfrom the Nature of the Syphon, that all the coMs^jtFluid contain'd in the Part between H and Gi $ever be in Equilibria with itself be the Weight ^ ^Air what it will, because the Pressure at H and qo'always be equal. Whence his evident, that f h e ylumn of Mercury D H is in Equilibria with the C°‘.of Water G E, and a Column of Air of theBase conjointly, and will therefore vary with th e ot