that there are some deviations from the usual practice.



NATURE AND USES OF A VACUUM.

6. _Q._--The pressure of a vacuum you have stated is nothing; but how can
the pressure of a vacuum be said to be nothing, when a vacuum occasions a
pressure of 15 lbs. on the square inch?

_A._--Because it is not the vacuum which exerts this pressure, but the
atmosphere, which, like a head of water, presses on everything immerged
beneath it. A head of water, however, would not press down a piston, if the
water were admitted on both of its sides; for an equilibrium would then be
established, just as in the case of a balance which retains its equilibrium
when an equal weight is added to each scale; but take the weight out of one
scale, or empty the water from one side of the piston, and motion or
pressure is produced; and in like manner pressure is produced on a piston
by admitting steam or air upon the one side, and withdrawing the steam or
air from the other side. It is not, therefore, to a vacuum, but rather to
the existence of an unbalanced plenum, that the pressure made manifest by
exhaustion is due, and it is obvious therefore that a vacuum of itself
would not work an engine.

7. _Q._--How is the vacuum maintained in a condensing engine?

_A._--The steam, after having performed its office in the cylinder, is
permitted to pass into a vessel called the condenser, where a shower of
cold water is discharged upon it. The steam is condensed by the cold water,
and falls in the form of hot water to the bottom of the condenser. The