Description of a new Air-pump, vdth an investigation of the 

 limit of Exhausting Power in various constructions of that In- 

 strument. By T. R. Robinson, D. D. M. R. I. A. Professor of 

 Astronomy at Armagh. 



Read October, 24, 1825. 



1 HE improvement of the Air-pump has long been an object of 

 attention to the votaries of Physical science ; and particulary of late 

 years, as the recent discoveries in chemistry have made its use in- 

 dispensable to every experimenter who veishes to attain that ac- 

 curacy in operating on gases, which is now essential. A great 

 number of ingenious contrivances have accordingly been devised to 

 augment its power, but I am not aware that the actual value of 

 them has ever been theoretically determined. The common treatises 

 of pneumatics contain theorems assigning a geometrical progres- 

 sion to the densities of the air after successive strokes, but these are 

 false except in a case which can never be found in practice, and are 

 also useless, as we are only concerned with the ultimate exhaustion. 

 Perhaps therefore, the following results may not be unacceptable, 

 though obtained by the simplest possible calculation. 



The Air-pump which is in common use, was invented by Hooke : 

 the elasticity of the air in its receiver, lifts a leather valve, and 

 opens a communication between the receiver and the pump barrel, 

 partially exhausted by the ascent of its piston. This force is op- 

 posed by the elasticity of the air previously contained in the barrel, 

 and by that of the valve itself; when they balance it the valve can- 



