11. Q.—­Is a barometer sometimes applied to the condensers of steam engines?

A.—­Yes; and it is called the vacuum gauge, because it shows the degree of perfection the vacuum has attained.  Another gauge, called the steam gauge, is applied to the boiler, which indicates the pressure of the steam by the height to which the steam forces mercury up a tube.  Gauges are also applied to the boiler to indicate the height of the water within it so that it may not be burned out by the water becoming accidentally too low.  In some cases a succession of cocks placed a short distance above one another are employed for this purpose, and in other cases a glass tube is placed perpendicularly in the front of the boiler and communicating at each end with its interior.  The water rises in this tube to the same height as in the boiler itself, and thus shows the actual water level.  In most of the modern boilers both of these contrivances are adopted.

12. Q.—­Can a condensing engine be worked with a pressure less than that of the atmosphere?

A.—­Yes, if once it be started; but it will be a difficult thing to start an engine, if the pressure of the steam be not greater than that of the atmosphere.  Before an engine can be started, it has to be blown through with steam to displace the air within it, and this cannot be effectually done if the pressure of the steam be very low.  After the engine is started, however, the pressure in the boiler may be lowered, if the engine be lightly loaded, until there is a partial vacuum in the boiler.  Such a practice, however, is not to be commended, as the gauge cocks become useless when there is a partial vacuum in the boiler; inasmuch as, when they are opened, the water will not rush out, but air will rush in.  It is impossible, also, under such circumstances, to blow out any of the sediment collected within the boiler, which, in the case of the boilers of steam vessels, requires to be done every two hours or oftener.  This is accomplished by opening a large cock which permits some of the supersalted water to be forced overboard by the pressure of the steam.  In some cases, in which the boiler applied to an engine is of inadequate size, the pressure within the boiler will fall spontaneously to a point considerably beneath the pressure of the atmosphere; but it is preferable, in such cases, partially to close the throttle valve in the steam pipe, whereby the issue of steam to the engine is diminished; and the pressure in the boiler is thus maintained, while the cylinder receives its former supply.

13. Q.—­If a hole be opened into a condenser of a steam engine, will air rush into it?

A.—­If the hole communicates with the atmosphere, the air will be drawn in.

14. Q.—­With what Velocity does air rush into a vacuum?