400 _ On the Velocity of Air. 
To prove whether air gompressed by 39 feet 
of water would be impelled into the atmosphere 
with the above velocity, I have made, amongst 
many more, the following experiments. 
A is a vessel of a known capacity, into the top 
of which is screwed an aperture of a known area. 
The tube Td recurve at d is soldered or screwed 
into the top of the said vessel, The hole ais 
stopped, and water poured into the tube at T 
till it is full, at which time a quantity of water 
will have passed out of the tube at d, and con- 
densed the air in the vessel, more or less as the 
tube T d is longer or shorter. 
At this time a person who has closed the aper- 
ture at d with a finoer of one hand, and held a 
half second pendulum in the other, removes both 
at the same time, while at the same moment 
an assistant opens a cock over the tube 7, which 
‘supplies it with water as fast as it can descend 
into d. The moment that the water appears at 
a, the time piece is stopped, and the time of 
expelling the air is noted, from which, by know- 
ing the capacity of the vessel, the velocity may 
be obtained, 
If the tube Td should be continued near the 
bottom of the vessel A, while it was filling with 
water, the length of the compressing column 
