ions. Engines, as I have already said,
are sometimes made double acting, though unprovided with a crank; and there
would be no difficulty in so arranging the valves of all ordinary pumping
engines, as to admit of this action; for the pumps might be contrived to
raise water both by the upward and downward stroke, as indeed in some mines
is already done. But engines without a crank are almost always made single
acting, perhaps from the effect of custom, as much as from any other
reason, and are usually spoken of as such, though it is necessary to know
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
water, which would else be accumulated in the condenser, is continually
being pumped out by a pump worked by the engine. This pump is called the
air pump, because it also discharges any air which may have entered with
the water.

8. _Q._--If a vacuum be an empty space, and there be water in the
condenser, how can there be a vacuum there?

_A._--There is a