A.—­The locomotive employed to draw carriages upon railways, consists of a cylindrical boiler filled with brass tubes, through which the hot air passes on its progress from the furnace to the chimney, and attached to the boiler are two horizontal cylinders fitted with pistons, valves, connecting rods, and other necessary apparatus to enable the power exerted by the pistons to turn round the cranked axle to which the driving wheels are attached.  There are, therefore, two independent engines entering into the composition of a locomotive, the cranks of which are set at right angles with one another, so that when one crank is at its dead point, the other crank is in a position to act with its maximum efficacy.  The driving wheels, which are fixed on the crank shaft and turn round with it, propel the locomotive forward on the rails by the mere adhesion of friction, and this is found sufficient not merely to move the locomotive, but to draw a long train of carriages behind it.

130. Q.—­Are locomotive engines condensing or high pressure engines.

A.—­They are invariably high pressure engines, and it would be impossible or at least highly inconvenient, to carry the water necessary for the purpose of condensation.  The steam, therefore, after it has urged the piston to the end of the stroke, escapes into the atmosphere.  In locomotive engines the waste steam is always discharged into the chimney through a vertical pipe, and by its rapid passage it greatly increases the intensity of the draught in the chimney, whereby a smaller fire grate suffices for the combustion of the fuel, and the evaporative power of the boiler is much increased.

131. Q.—­Can you give an example of a good locomotive engine of the usual form?

A.—­To do this I will take the example of one of Hawthorn’s locomotive engines with six wheels represented in fig. 29; not one of the most modern construction now in use, nor yet one of the most antiquated.  M is the cylinder, R the connecting rod, C C the eccentrics by which the slide valve is moved; J J is the steam pipe by which the steam is conducted from the steam dome of the boiler to the cylinder.  Near the smoke stack end of this pipe is a valve K or regulator moved by a handle p at the front of the boiler, and of which the purpose is to regulate the admission of the steam to the cylinder; f is a safety valve kept closed by springs; N is the eduction pipe, or, as it is commonly termed in locomotives, the blast pipe, by which the steam, escaping from the cylinder after the stroke has been performed, is projected up the chimney H. The water in the boiler of course covers the tubes and also the top of the furnace or fire box.  It will be understood that there are two engines in each locomotive, though, from the figure being given in section, only one engine can be shown.  The cylinders of this engine are each 14 inches diameter; the length of the stroke of the piston is 21 inches.  There are two sets of driving wheels, 5 feet diameter, with outside connections.