On the Velocity of Air. 405 
of the tube B, which will force the water into the 
glass tube DE, and, make the air in its upper part 
of the same density, deducting from the com- 
pressing force, the altitude of the water raised 
above D, which however will be of little or no 
importance ; if the gauge is placed in a horizontal 
position, with the glass tube downward, there will 
be no difference of density. 
The computation for the force in the case 
where a tube hermetically sealed at the top is 
adopted in the instrument, will be effected by 
considering that the space occupied by any 
elastic fluid is inversely as its force.—Thus, let 
the tube be 12 inches long, and suppose the 
water to be raised 1 inch; then it will be 11: 12:2 
the force of the atmosphere : the force of the. 
air in the tube:: 1 : 1¢;——Hence a scale may be 
adapted to the instrument, to express the force 
of condensation over and above the common 
atmospheric pressure; which force is signified 
in the instance above by the fraction s:, unity 
being the atmospheric pressure. If we de- 
note the atmospheric pressure by go inches of 
mercury, or 32 feet of water, then the force 7%, 
in the above example will be expressed by 2,727 
inches of mercury, or 2.91 feet of water; and 
the like for any other instance, 
