On a New Double-acting Air-pump with a single Cylinder. 297 



proved above, equal to j ^dt), in the case of each element, the 



theorem expressed by these equations is true of the thermo- 

 electric forces in the single elements for all ranges of tempera- 

 ture, provided the absolute temperatures of the hot and cold 

 junctions be the same in the different elements. The second 

 equation, by successive applications of which the first may be 

 derived, is the simplest expression of a theorem which was, I 

 believe, first pointed out and experimentally verified by Becquerel 

 in researches described in the second volume of his Traite 

 d'Electricite. 



140. For brevity, we shall call what has been denoted by 

 ^(B, C) the thermo-electric relation of the metal B to the metal C ; 

 we shall call a certain metal (perhaps copper or silver) the stand- 

 ard metal ; and if A be the the standard metal, we shall call 

 ^(A, B) the thermo-electric j)Ower of the metal B. The theorem 

 expressed by the last equation may now be stated thus : — The 

 thermo-electric relation between two metals is equal to the differ- 

 ence of their thermo-electric poioers, which is nearly identical with 

 Becquerel's own statement of his theorem. 

 [To be continued.] 



XXXVII. On a New Double-acting Jir-pump with a single 

 Cylinder. By T. Tate, F.R.A.S.* 



THE chief points of novelty in this air-pump consist in a 

 double piston acting in a single cylinder, and in the supe- 

 rior system of valves. This double piston with a single cylinder 

 gives to the instrument all the properties of an ordinary air- 

 pump with two cylinders : this new instrument, in fact, may be 

 regarded as a single-barrelled pump, capable of performing its 

 work with only one-half the usual motion. 



The annexed diagram represents a section of the cylinder, &c. 

 of my new air-pump : CD the cylinder ; A and B solid pistons 

 rigidly connected by a rod, and moved by the 

 piston-rod AH, passing through a stuffing- 

 box S ; V and v valves lifting outwards ; R an 

 open pipe at the middle of the cylinder leading 

 to the receiver from which the air is to be ex- 

 hausted. The distance between the extreme 

 faces of the pistons is about fths of an inch 

 less than one-half the length of the cylinder, 

 this fths of an inch being the space requisite 

 for clearing the exhausting pipe R. The pis- 

 tons are each about 1| inch in thickness, and 

 • Communicated bv tbe Author. 



Phil. Mag. S. 4. Vol. 11. No. 73. April 1856. 



