The "Rocket" has formed the model on which all subsequent locomotive engines have been constructed; for, though numerous alterations and improvements have been made in details, and though the size of the engines has been greatly enlarged, the principle of construction remains essentially the same. Among the improvements that have been introduced by different inventors, is an increase in the number of the tubes in the boiler, so as to facilitate the generation of steam, some of the engines now made having upwards of 100 tubes, though of smaller diameter than those of the "Rocket." The boilers have also been elongated, to enlarge the evaporating surface and economize fuel. The cylinders are placed horizontally, and they are generally fixed inside the boiler, to prevent the cooling of the steam. The piston-rods are attached to cranks on the axle, placed at right angles to each other; and the engines are generally mounted on six wheels, four of which are driving wheels, made of larger size than the two others, and they are coupled together by connecting arms. The large and powerful engines on the Great Western Railway have, however, only two driving wheels, which are 8 feet in diameter. These engines weigh as much as 31 tons, which is seven times more than the weight of the "Rocket." They are capable of taking a passenger train of 120 tons at an average speed of 60 miles an hour on easy gradients; and the effective power, as measured by a dynamometer, is stated to be equal to 743 horses. The accompanying engraving of one of the recently constructed engines on the Great Western Railway presents a remarkable difference in point of size and general arrangement to the original prototype, from which, however, it does not materially differ in the principle of its construction. [Illustration]