264 HISTORY OT 



it is mixed. Care, however, must be taken not 

 to heat this instrument too violently; for then 

 the enclosed air would become irresistible, and 

 burst the whole, with perhaps a fatal explosion. 



There are numberless other useful instruments 

 made to depend on the weight, the elasticity, or 

 the fluidity of the air, which do not come within 

 the plan of the present work ; the design of which 

 is not to give an account of the inventions that 

 have been made for determining the nature and 

 properties of air, but a mere narrative of its ef- 

 fects. The description of the pump, the forcing- 

 pump, the fire-engine, the steam-engine, the sy- 

 phon, and many others, belong not to the natu- 

 ralist, but the experimental philosopher : the one 

 gives a history of nature, as he finds she presents 

 herself to him ; and he draws the obvious pic- 

 ture: the other pursues her with close investi- 

 gation, tortures her by experiment to give up 

 her secrets, and measures her latent qualities 

 with laborious precision. Much more, therefore, 

 might be said of the mechanical effects of air, 

 and of the conjectures that have been made re- 

 specting the form of its parts ; how some have 

 supposed them to resemble little hoops coiled up 

 in a spring ; others, like fleeces of wool ; others, 

 that the parts are endued with a repulsive quali- 

 ty, by which, when squeezed together, they en- 

 deavour to fly off, and recede from each other. 

 We might have given the disputes relative to 

 the height to which this body of air extends 

 above us, and concerning which there is no 

 Agreement. We might have inquired how much 



