Pneumatics.

0 e . J ’ yet is it not fit for common Use, because it canq y stiew the extreme minute Variations of the Air’s,r avity f or the present Time , by reason it is affected byjj 6 Heat as well as Weight of the Air. While theth'^ ree Heat remains the fame, nothing can exceed^. ls Instrument as a Barometer ; but as the Heat of the^' r , v aries, so will the Elasticity of the included Air*p Ic n therefore will cause the Instrument to vary itss ^ity, w hile that of the Air remains the fame, andCa ntiot be of constant Use.

Hj * 9 * I have already hinted that the Common Baro-Ah ,er ’ a ^ ter HI, is the best Instrument to measure theh e f s Gravity; which that it may do to the greatest*I'w' ;° n ’ the following Things are necessary, (r.)

^t the Tube be at least of 4

Inch

^ of an Inch Bore; 4 of

y^^oh is a good Size. (2.) The Tube ought to betfij s 5 vlean, and dry within when fill’d ; in order to30 *be Tube should be hermetically sealed at botha t the Glass-House when made; one End oft*se ' 01 a y be cut off with a File when you intend tothey’ (d-) The Diameter of the Cistern that holdsb e Mercury, in which the Tube is immersed, shouldas ,Hrg e as conveniently may be, that the Mercurycm ln .nray have nearly at all times the fame Altitude ,;0 *fc the Index will not be true. (4.) The Mer-be very pure, and free from any Mixture of~'® a d, or other Metal. (5.) It ought to be purgediiio entirely, as it may be by boiling it, and fill-T| e ^ Tube with it while boiling hot nearly. (6.)Ire a >. u be must be heated hot when fill’d, to avoidby the boiling Mercury. (7.) It should betyilj Ver y hard, to excite the Electric Virtue, which(8,) -J 1 ®! the Particles of Air from the Surface within.Wiej here ought to be a Nonius (as it is call’d) ap-hid^ 0 the Index of the graduated Plate, to measurea fp Ur ately the Rise and Fall of the Mercury.

2o. -r, . .... ...

?stie r p niS Artifice is of singular Use in this and manyN a) . a •• It bears the Inventor Nonius’s Name, and•stjj j u * e and Manner of applying it is as follows.|Wf a upper Part of the Barometer, in which the® of the Mercury is at C. F G is the usual3 Istchcs Extent, from 28 to 31; and D E isK 2 the

* 3 *

PlateXXVII.Fig. 10.