GENERAL DESCRIPTION OF A LOCOMOTIVE ENGINE. QUESTION 63. _What are the principal parts of an ordinary locomotive engine?_ _Answer._ A boiler for generating steam and a pair of high-pressure steam engines, which are all mounted on a suitable frame and wheels adapted for running on a track consisting of two iron or steel rails. QUESTION 64. _How is the power of high-pressure engines applied to locomotives?_ _Answer._ By connecting the engines with the wheels so as to give the latter a rotary motion. QUESTION 65. _When they revolve what will occur?_ _Answer._ Either they will slip on the track, or the locomotive will move either backward or forward, according to the direction the wheels are turning. QUESTION 66. _What will determine whether the wheels will slip or the locomotive move?_ _Answer._ The friction or _adhesion_, as it is called, between the wheels and the track. If this adhesion is greater than the resistance opposed to the movement of the locomotive, the latter will overcome the resistance; but if the latter is greater than the friction, the wheels will slip. QUESTION 67. _Upon what does the amount of friction or adhesion of the wheels depend?_ _Answer._ Chiefly on the weight which they bear, but to some extent upon the condition of the rails. Under ordinary circumstances, the adhesion of the wheels of a locomotive is in direct proportion to the weight they carry. QUESTION 68. _Why are two cylinders employed on locomotives?_ _Answer._ Because if only one was used, it would be impossible or very difficult to start the engine, if it should stop on one of the dead points. QUESTION 69. _How is this difficulty overcome by the use of two cylinders?_ _Answer._ By attaching the two cranks to the same shaft or axle, and placing them at right angles to each other, so that when the one is at a dead point the other is in the position where the steam can exert the maximum power on the crank. QUESTION 70. _How are the cranks of an ordinary locomotive made?_ _Answer._ They are cast in one piece with the wheels that drive the locomotive, which are therefore called _driving-wheels_. In this country the centre portion of such wheels, or _wheel-centres_ as they are called, is always made of cast iron, with tires of wrought iron or steel around the outside, and is fastened to the axles of the locomotive. The shaft of a locomotive engine is called the _main driving-axle_, and the wheels attached to it the _main driving-wheels_. QUESTION 71. _How are the cylinders and driving-wheels of a locomotive usually placed?_