404 Hydrodynamics. 



wards. The pistons are moved by a cog-wheel in the piece TT, 

 turned by the handle B, of which wheel the teeth catch in the 

 racks of the pistons C, C. PQ is a circular brass plate, having 

 near its centre the orifice K of a concealed pipe that communi- 

 cates with the cavity at D; at V is a screw that closes the orifice 

 of another pipe, and which is turned for the purpose of admitting 

 the external air when required. LM is a glass vessel, from 

 which the air is to be exhausted, and which has obtained the 

 name of receiver, because it receives or holds the subjects on 

 which the experiments are to be made. This receiver is placed 

 on the plate PQ, and is accurately fitted to it by grinding, or by 

 means of moistened or oiled leather. 



When the handle B is turned, one of the pistons is raised and 

 the other depressed ; consequently a void space is left between 

 the raised piston and the lower valve in the corresponding barrel; 

 the air contained in the receiver LM communicating with the bar- 

 rel by the orifice K immediately raises the lower valve by its 

 spring, and expands into the void space ; and thus a part of the 

 air in the receiver is extracted. The handle then, being turned 

 the contrary way, raises the other piston, and performs the same 

 operation in the barrel containing it ; while in the mean time the 

 first mentioned piston being depressed, the air by its spring 

 closes the lower valve, and raising the valve in the piston makes 

 its escape. The motion of the handle being again reversed, the 

 first barrel again exhausts, while the second discharges the air 

 in its turn ; and thus during the whole time the pump is worked, 

 one barrel exhausts the air from the receiver, while the other 

 discharges it through the valve in its piston. Hence it is evi- 

 dent, that the air can never be entirely exhausted ; for it is the 

 elasticity of the air in the receiver that raises the valve, and 

 forces it into the barrel t ; and eaeh operation can only take away 



f Various contrivances have been adopted to facilitate the mo- 

 tion of the valve, and thus to allow the air, when in a state of great 

 rarefaction, to pass from the receiver into the barrels. Smeaton had 

 recourse to a valve presenting a broad surface, and supported on 

 thin bars. Others have proposed to raise the valve mechanically by 

 connecting it with the piston in such a manner that the piston shall 

 exert its action at the moment it reaches the top of the barrel. A 



