ORRIN <a aiid 
- 
Mr. Patten’s Air Pump, Gazometer, ¥e. 93 
ny sa ersons; if I remember rightly, one was Prof. 
Grisc ie ess Prof. Dana can go behind these 
po the ‘credit ” will I believe rest where it belongs, 
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 
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 obec & of the instrument 
roposed by me was to gain a vacuum in a receiver as 
nearly torricellian as the elasticity of A 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 anpeeat ass 
iom that two bodies cannot at the same time od 
same space; if therefore the globe K, (see plate ‘Vol. iil, 
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 isa communica- 
tion with the receiver R; but in order to drive out afl the 
air, it is necessary that the ASME should rise complete- 
ly up into the valve S. Now it was to insure its doing this 
that the cap O was made ih 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 exp n. 
The valve S Seenaiean as will be seen by esa floats 
con 
tirely up to the plug of the stop cock, there would of course 
