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 vacuum above the water, the water being only like so much iron or lead lying at the bottom.

9. Q.—­Is the vacuum in the condenser a perfect vacuum?

A.—­Not quite perfect; for the cold water entering for the purpose of condensation is heated by the steam, and emits a vapor of a tension represented by about three inches of mercury; that is, when the common barometer stands at 30 inches, a barometer with the space above the mercury communicating with the condenser, will stand at about 27 inches.

10. Q.—­Is this imperfection of the vacuum wholly attributable to the vapor in the condenser?

A.—­No; it is partly attributable to the presence of a small quantity of air which enters with the water, and which would accumulate until it destroyed the vacuum altogether but for the action of the air pump, which expels it with the water, as already explained.  All common water contains a certain quantity of air in solution, and this air recovers its elasticity when the pressure of the atmosphere is taken off, just as the gas in soda water flies up so soon as the cork of the bottle is withdrawn.