^E 



the new Double-adiny Air-pump wilh a Single Cylinder. 361 



valves are constructed after the manner of No. 2, but in which 

 the pistons are placed a little more apart, so that neither of them 

 ever passes the exhaustion orifice E, but at the end of every 

 stroke the interior face of one of the pistons just arrives at the 

 corresponding edge of the exhausting orifice. It is scarcely ne- 

 cessary to observe, that this form of the instrument possesses the 

 same property as that of No. 2 with respect to the oeconomy of 

 dynamic effect ; but it will be seen that its exhausting power is 

 the same as that of the common pump. 



With the view of showing the applicability ^^ || 

 of the advantages of the double piston acting 

 in a single cylinder to the valve system of the 

 common pump, I have constructed the form, 

 which I shall call No. 4, represented in the 

 annexed cut, where the arrangement of the 

 pistons, A and B, is the same as in No. 3. 

 In this case all the valves lift inwards ; the 

 orifice E, at the centre of the cylinder, leads 

 into the atmosphere ; and the pipes C and D 

 lead to the receiver. The exhausting power, 

 as well as the dynamic effect, of this pump is 

 the same as that of an ordinary double-bar- 

 relled pump. At the same time it must be 

 observed, that this form of the double piston ' "^ 



acting in the single cylinder, enables us to apply the moving 

 pressure in the most simple manner. 



I have found the friction of a single piston, ] i inch in dia- 

 meter, to be 2^ lbs., and that of the double piston with the 

 stuffing-box of the piston-rod to be 3^ lbs. ; with these experi- 

 mental data I have found the work applied to the different pumps 

 necessary to exhaust the air from a receiver containing 108 cubic 

 inches, to be as follows : — 



The work of No. 1 (requisite to produce the given exhaustion) 

 is nearly one-half that of the common pump. 



The work of Nos. 2 and 3 is nearly one-third that of the com- 

 mon pump. 



Taking all circumstances into consideration, I am persuaded, 

 that, for most purposes, the pump with the solid pistons (No. 1) 

 is the most advantageous form of the instrument. 



The following is an investigation of the formulae employed in 

 calculating the work expended in exhausting the receivers of tlie 

 different ])umps. 



1. To find the work expended in exhausting the receiver of 

 the common air-pumj). 



Let U„= the work in the /tth double stroke; u = the work 

 expended in overcoming the pressure E of the external air; i<, = 



Phil May. S. 4. Vol. 11. No. 73. May 1856. 2 B 



