Mr. Patients Air Pump, Gazomeler, ire. 93 



ny different persons ; if I remember rightly, one was Prof. 

 Griscom. Now unless Prof, ''■ -ana can go behind these 

 dates, the "credit" will I believe res! where it belongs, and 

 the insinuated charge of borrowing will return to the source 

 from which it originated. With regard to to the proposed 

 " improvement " I regret the Prof, did not take the trouble 

 so examine the modus operandi of an instrument, the "de- 

 tails" of which he considers to "coincide" so nearly with 

 his own ; had he done so, it would have saved him the 

 trouble of constructing in place of the valves, a substitution 

 that is altogether useless. The object of the instrument 

 proposed by me was to gain a vacuum in a receiver as 

 nearly torricellian as the elasticity of the air would permit; 

 the obstacles to this had always been the weight of the 

 valves, and the difficulty if not impossibility of making a 

 piston fit so nicely into a barrel that there should be no 

 air below or around it; and lastly to prevent the vapor of 

 the oil necessarily used, from interfering with the exhaus- 

 tion. The principle proposed to overcome all these obsta- 

 cles at once, was to make one fluid not easily vaporable, 

 expel another, and was founded upon the self apparent ax- 

 iom that two bodies cannot at the same time occupy the 

 same space; if therefore the globe K, (see plate Vol. VIII, 

 No. 1.) is full of mercury, there is evidently no air in 

 it, and when the mercury descends by taking off the pres- 

 sure, the vacuum is torricellian unless there is a communica- 

 tion with the receiver R; but in order to drive out all the 

 air, it is necessary that the mercury should rise complete- 

 ly up into the valve S. Now it was to insure its doing this 

 that the cap O was made with a view to contain a surplus 

 quantity of mercury, and to supply any waste or contrac- 

 tion or to receive any that should be driven out by expansion. 

 The valve S therefore, as will be seen by inspection, floats 

 upon the mercury in the cap O, and almost the whole must 

 return into the globe before the valve can close, it then 

 does it so effectually that no air can possibly enter ; upon 

 the next return of the mercury not only all the air is ex- 

 pelled from the globe, but likewise the surplus mercury, 

 and is retained in the cap. Now this object could not be 

 attained by the proposed " improvement," for if by the 

 contraction or waste the mercury should not ascend en- 

 tirely up to the plug of the stop cock, there would of course 



