OF PNEUMATICS. 



20/ 



pump, place the glass receiver M upon the leather, so 

 that the hole i in the plate may be within the glass. 

 Then, turning the handle F backward and forward, 



the air will be pumped out of the receiver ; which will 

 then be held down to the plate by the pressure 

 of the external air, or atmosphere. For, as the han- 



By the use of the air-pump we may, as will be presently shewn, ex- 

 haust a vessel of its air, and thus compare the changes wrought upon 

 various substances by being kept from air, with the same bodies when 

 exposed to the action of the atmosphere : so that we thus acquire a 

 knowledge of the effects of that fluid upon bodies in general. 



