duplicate cylinders, to do duty again, and thus the maximum development of power is obtained, with the minimum expenditure of fuel. The advantages claimed for the improvement are:—
(1.) That the engine, being practically balanced, runs very steadily at a high rate of speed.
(2.) That the power of the engine is distributed over two axles instead of one, as in ordinary non-compound engines, and the strain on the various parts is thus very much reduced.
(3.) The adhesion of two pairs of driving wheels has been obtained, without the use of coupling rods, which become unnecessary.
(4.) The driving wheels may be placed further apart than would be advisable if coupling rods were used, and a larger fire-box can be introduced.
(5.) With independent driving wheels there is less friction in passing round curves, and, if more convenient for the working out of the general design, each pair of wheels may have a different diameter.
The first compound engine was set to work on the London and North- Western Railway in 1881, since which time seventy-three others of the same type have been built, and have run collectively upwards of 11,000,000 of miles. On their first introduction they were met, like all innovations upon recognised methods, by a great deal of hostile criticism, but it is believed that they have lived this down, and are now pretty generally admitted to be a success. An actual trial has shown that, with one of these engines and a train of 321 gross tons in weight, one ton of dead weight can be hauled one mile at a speed of twenty-four miles an hour, with an expenditure of 1·26 oz. of Welsh coal, while to haul the
