94 Mr. Patten's Air Pump, Gazometer, ^c. 



be a quantity of air left in the globe, and should there be 

 an expansion, the mercury would be thrown out at the 

 aperture ; but a great objection to it is, that any meth- 

 od that could be proposed to open or shut the cock except 

 by hands, would be either inconvenient or complicated, 

 and after all the " improvement," seems to be an awkward 

 alteration of the very cock that is now upon the pump ; for 

 let that cock be as represented in fig, 2nd, (PI. 2, Vol. 

 VIII, No. 1) if now the plug be turned " half round," (see 

 the Prof's, description,) so that the back of the plug is 

 against the hole a in the bulb, then there is only a com- 

 munication between the receiver R and the globe K ; 

 — now let an exhausition be made, then turn the plug 

 "one quarter round," with its hole a downwards, the air 

 now from the globe can escape only into the atmosphere 

 through the hole a drilled in the bulb where it should 

 be, and not through a useless and crooked one that inter- 

 feres with the screw that holds the plug tight — it is therefore 

 apparent that as yet the only way of closing the aperture is 

 by valves, and if constructed with the least accuracy, I can 

 conceive no more efficient or secure way of doing it, and 

 their weight in this case is a matter of perfect indifference 

 as it is impossible from the manner of opening and shut- 

 ting them, that they should interfere with the exhaustion. 

 If I have not already intruded too far upon your patience, 

 I transmit for insertion, a modification of the other pump, 

 or rather an adaption of the principle to the ones of the 

 usual construction ; it was cotemporaneous with the first 

 but I did not send it at that time, as I considered the other 

 the most perfect; the only objection however to this is 

 that it has an air tight collar, and the vapor of the oil is not 

 removed ; but it has its advantages — there is no glass that 

 can be broken — it takes but an ounce or two of mercury — • 

 and there are no caps to be cemented to the glass, a diffi- 

 culty that you feared might be experienced. ABCD rep- 

 resents an iron barrel with a solid piston E, the piston rod 

 works through an airtight collar at F ; at G is a valve open- 

 ing inward at H and I valves opening out ; K is the plate 

 of the pump connected with the top and bottom of the bar- 

 rel by the tube anaa — the top of the barrel is made with a 

 rim round it for the purpose of holding an ounce or two of 

 mercury — -through the piston rod a small diagonal hole is 



