jiir Pump. — Maximum of various Air Pumps. 29 



Fig. 7 is the cap fcrcw by which the pipes are fixed. Fig, 8 is the pipe ufed for the 

 condenfing apparatus. 



That this pump may work well, it is neceffary that the bottom of the pifton be perfedly 

 flulh with the bottom of the barrel. — The method of operation is as follows : 



When the pifton is to be raifed, let the plug ^, be opened. — The pifton rifing, expels 

 the air above it through (p. When the pifton is at the higheft, (hut the plug. There will 

 now be no preflure above the pifton, which will greatly facilitate the working. As the 

 pifton rifes, the friction of the collar of leathers h will raife the valve V, and the air in the 

 receiver through the communication E will expand itfclf into the barrel. When the pifton is 

 deprefled, V ftiuts, and the air is expelled through K, to which a pipe as fig. 8 may be 

 attached for condenlation. When the pifton reaches the bottom, no air will be left in the 

 barrel, except the very fmall quantity in the very fmall hole of the valve X, which is very 

 little when compared to the capacity of the barrel. By proceeding in this manner, a very 

 perfecl vacuum will be formed in the receiver. — By taking oft' the receiver and applying . 

 the pipe fig. 8, and attaching it to any veffel, and opening the plug ip, we have a com-- 

 plete condenfing apparatus. If required, the air may be takftn from the receiver and 

 thrown into another veffel. Moiftened leather ought not to be ufed for fixing the re- 

 ceiver, as vapours are conftantly iffuing from it -, a drying lute is better. 



This air pump may be made of a much cheaper conflju£llon than that of the plate,,, 

 which, however, is the more convenient. 



In the obliging letter which accompanied this communication, the author aflures me, 

 that its fimplicity and convenience have been found confiderable by experience. The 

 reader will perceive, by turning to the firft volume of our Journal, p. 128, that the happy 

 contrivance of the wire for lifting the. lower valve is alfo claimed by Cuthbertfon, who 

 in his pamphlet*, page 6, informs us that tlie hint of fuch an apparatus was firft given 

 to him by M. Pacts van Trooftwyk. It is not faid that Ur. Rutherford carried his in- 

 vention into prailice. This merit is due to Mr. Cuthbertfon and Sir G. M. I remem- 

 ber the fame ingenious thought having been alfo ftated by another philofophical gentleman, 

 in 1783, when the air pump of Haasf was much talked of. 



The air pump of Sir George Mackenzie differs in effe£b from that oPMr. Cuthbertfon 

 in the folidity of the pifton, and in not having an oil veffel to the valve through wliich the 

 air is extruded. The air pump of Sir George will ceafe to exhauft, fuppofing every 

 thing elfe perfeft, when the mafs of air in the receiver bears the fame proportion to an 

 equal volume of external air, as the capacity of the bore of the valve-piece X, bears to the 

 interior capacity of the barrel when the pifton is up. Cuthbertfon's pump will have a 

 fimilar limit with relation to the communication pipe as to the upper valve (Philof Jcur-- 

 nal, pi. 7, vol. I, fig. r.) and the capacity of the barrel above the pifton when down. In 

 Prince's air pump, and in the projedt mentioned at p. 131 of the fame volume; if the 



* Defcription of an Improved Air Pump, &c. by John Cuthbertfon, London ; fold by Johnfon. No date, 

 l>ur, as I tbink, publifticd feven or eight years ag9. ' 



t Pliil. Tranf. m,i;cc,lxxiii. 



Talv«s ! 



