280 Sir Is a ac Newton’s BookII.

the laft place, when the chord is returned into the fituationI K L, the particles of air from / to «A fhall acquire their pre-fent rangement, and the motion of the air be extended asmuch farther. And the like will happen after every com-pleat vibration of the firing.

13. Concerning this motion of found, our authorfhews how to compute the velocity thereof, or in what timeit will reach to any propofed diflance from the fonorousbody. For this he requires to know the height of air, hav-ing the fame denfity with the parts here at the furface ofthe earth, which we breath, that would be equivalent inweight to the whole incumbent atmofphere. This is tobe found by the barometer, or common weatherglafs. Inthat inftrument quickfilver is included in a hollow glafscane firmly clofed. at the top. The bottom is open, butimmerged into quickfilver contained in a veflel open to theair. Care is taken when the lower end of the cane is immer-ged, that the whole cane be full of quickfilver, and that no airinfinuate itfelf. When the inftrument is thus fixed, the quick-filver in the cane being higher than that in the veffel, ifthe top of the cane were open, the fluid would foon finkout of the glafs cane, till it came to a level with that inthe veflel. But the top of the cane being clofed up, fothat the air, which has free liberty to prefs on the quick-filver in the veflel, cannot bear at all on that, which is with-in the cane, the quickfilver in the cane will be fufpendedto fuch a height, as to balance the preflure of the air onthe quickfilver in the veflel. Here it is evident, that the

1 weight