406 On the Velocity of Air. 
If a mercurial instrument of the above con- 
struction be preferred, it becomes necessary to 
add the height of the mercurial column to the 
force found as above: thus, if the condensation 
of air be from 12 into g inches, then the addition 
to the force of the internal air in the tube is equal 
4,0r 10 inches of mercury, to which must be 
added the 9 inches raised in the tube, and the 
whole force will be 13 inches of mercury, ex- 
clusive of that of the atmosphere. 
This sort of instrument or guage serves equally 
well for finding the expansive force of any kind of 
elastic fluid, as for measuring the velocities with 
which they issue out of the place of their confine- 
ment. It may be applied to all kinds of bellows, 
to condensed steam, and to the air pump. 
