CHAP. IX.—STEAM NAVIGATION.
Resistance of Vessels in Water.
Experiments on the Resistance of Vessels.
Influence of the size of Vessels upon their Speed.
Structure and Operation of Paddle Wheels.
Configuration and Action of the Screw.
Comparative Advantages of Paddle and Screw Vessels.
Comparative Advantages of different kinds of Screws.
Proportions of Screws.
Screw Vessels with full and auxiliary Power.
Screw and Paddles combined.
Oscillating Paddle Engines.
Direct acting Screw Engine.
Portable Steam Engines.
Steam Fire Engines.
Construction of Engines.
Erection of Engines.
Management of Marine Boilers.
Management of Marine Engines.
Management of Locomotives.
CLASSIFICATION OF ENGINES.
1. Q.—What is meant by a vacuum?
A.—A vacuum means an empty space; a space in which there is neither water nor air, nor anything else that we know of.
2. Q.—Wherein does a high pressure differ from a low pressure engine?
A.—In a high pressure engine the steam, after having pushed the piston to the end of the stroke, escapes into the atmosphere, and the impelling force is therefore that due to the difference between the pressure of the steam and the pressure of the atmosphere. In the condensing engine the steam, after having pressed the piston to the end of the stroke, passes into the condenser, in which a vacuum is maintained, and the impelling force is that due to the difference between the pressure of the steam above the piston, and the pressure of the vacuum beneath it, which is nothing; or, in other words, you have then the whole pressure of the steam urging the piston, consisting of the pressure shown by the safety-valve on the boiler, and the pressure of the atmosphere besides.
3. Q.—In what way would you class the various kinds of condensing engines?