I 
[ 423 3 
to try any experiments under different circumftances 
of preffure, without changing the apparatus, but ren- 
ders the pump an univerfal engine, for (hewing any 
effedt, that arifes from an alteration in the denfity or 
fpring of the air. Thus, with a little addition of 
apparatus, it (hews the experiments of the air-foun- 
tain, wind-gun, &c. 
This is done in the following manner : The air 
above the pifton being forcibly driven out of the 
barrel at each ftroke, and having no- where to efcape, 
but by the valve at the top ; if this valve be con- 
nected with the receiver, by means of a pipe, and at 
the fame time the valve at the bottom, inftead of 
communicating with the receiver, be made to com- 
municate with the external air, the pump will then 
perform as a condenfer. 
The mechanifm is thus ordered. There is a cock 
with three pipes placed round it, at equal diffances. 
The key is fo pierced, that any two may be made 
to communicate, while the other is left open to the 
external air. One ofthefe pipes goes to the valve at 
the bottom of the barrel j another goes to the valve at 
the top, and a third goes to the receiver. Thus, 
when the pipe from the receiver, and that from the 
bottom of the barrel, are united, the pump exhaufts : 
But turn the cock round, till the pipe from the re- 
ceiver, and that from the top of the barrel, commu- 
nicate, and it then condenfes. The third pipe, in 
one cafe, difcharges the air, taken from the receiver, 
into the barrel and in the other, lets it into the 
barrel. 
