260 HISTORY OF 



may be compressed into a cavity not larger than 

 the eye of a needle. In short, no bounds can be 

 set to its confinement or expansion ; at least, ex- 

 periment has hitherto found its attempts indefi- 

 nite. In every situation it retains its elasticity ; 

 and the more closely we compress it, the more 

 strongly does it resist the pressure. If to the in- 

 creasing the elasticity on one side by compression, 

 we increase it on the other side by heat, the force 

 of both soon becomes irresistible ; and a certain 

 French philosopher supposed, * that air thus con- 

 fined, and expanding, was sufficient for the explo- 

 sion of a world. 



Many instruments have been formed to mea- 

 sure and determine these different properties of 

 the air, and which serve several useful purposes. 

 The barometer serves to measure its weight ; to 

 tell us when it is heavier, and when lighter. It 

 is composed of a glass tube or pipe, of about 

 thirty inches in length, closed up at one end ; 

 this tube is then filled with quicksilver ; this done, 

 the maker, clapping his finger upon the open end, 

 inverts the tube, and plunges the open end, ringer 

 and all, into a basin of quicksilver, and then 

 takes his finger away : now the quicksilver in the 

 tube will, by its own weight, endeavour to des- 

 cend into that in the basin ; but the external air, 

 pressing on the surface of the quicksilver in the 

 basin without, and no air being in the tube at 

 top, the quicksilver will continue in the tube, 

 being pressed up, as was said, by the air on the 



* Monsieur Amonton*. 



