by rods with the cranks attached to the driving-wheels or axles. These cranks are placed at right angles to each other, so that when one of them is at the "dead-point" the piston connected with the other can exert its maximum power to rotate the wheels. This enables the locomotive to start with the pistons in any position; whereas if one cylinder only were used it would be impossible to turn the wheels if the crank should stop at one of its dead-points.

If steam is admitted to the cylinders and the wheels are turned, one of two results must follow,-either the locomotive will move backward or forward according to the direction of revolution, or the wheels will slip, as they often do, on the rails. That is, if the resistance of the cars or train is less than the friction, or "adhesion" of the driving-wheels on the rails, the engine and train will be moved; if the adhesion is less than the resistance the wheels will turn without moving the train.

The capacity of a locomotive to draw loads is therefore dependent on the adhesion, and this is in proportion to the weight or pressure of the driving-wheels on the rails. The adhesion also varies somewhat with the weather and the condition of the wheels and rails. ordinary weather it is equal to about

In

one-fifth of the weight which bears on the track; when perfectly dry, if the rails are clean, it is about onefourth, and with the rails sanded about one-third. In damp

or frosty weather the adhesion is often considerably less than a fifth.

It would, then, seem as though all that is needed to increase the capacity of a locomotive to draw loads would be to add to the weight on its driving-wheels, and provide engine power sufficient to turn them-which is true. But it has been found that if the weight on the wheels is excessive both the wheels and rails will be injured. Even when they are all made of steel, they are crushed out of shape or are rapidly worn if the loads are too great. The weight which rails will carry without being injured depends somewhat on their size, but ordinarily from 12,000 to 16,000 pounds

powerful engine was required than that shown above, another pair of wheels would be added, as shown on page 184 and e 185. Or, if you wanted a still more powerful engine than these, another pair of driving-wheels would be added, as shown in d [p. 185]. In this way the ten-wheeled and the mogul engines have been developed from that shown on page 183. The mogul locomotive [p. 184, b] has three pairs of driving-wheels, but only one pair of truck-wheels. The engravings d and ƒ [pp. 185 and 187], represent consolidation and decapod types of engines, which have four and five pairs of driving-wheels.

From the last three illustrations it will be seen that when so many wheels are used, even if they are of small diameter, the wheel-base must necessarily be long, so that a limit is very soon reached beyond which the number of drivingwheels cannot be increased.

Improvements in the processes of manufacturing steel, which resulted in the general use of that material for rails and tires, have made it possible to nearly double the weight which was carried on each wheel when they were made of iron.

years ago iron rails weighing 56 pounds per yard were about the heaviest that were laid in this country. Now steel rails weighing 72 pounds are commonly used, and some weighing 85 pounds have been laid on roads in this country, and others weighing 100 pounds have been laid on the continent of Europe.

Of late years urban and suburban traffic has created a demand for a class of locomotives especially adapted to that kind of service. One of the conditions of that traffic is that trains must stop and start often, and therefore, to "make fast time," it is essential to start quickly. Few persons realize the great amount of force which must be exerted to start any object suddenly. A cannon-ball, for example, will fall through 16 feet in a second with no other resistance than the atmosphere. The impelling force in that case is the weight of the ball. If we want it to fall 32 feet during the first second, the force exerted on it must be equal to double its weight, and for higher speeds the increase of force must be in the same proportion. This law applies to the movement of trains. To start in half the time, double the

rangement leaves the whole weight of the boiler and machinery on the drivingwheels, and at the same time gives a long wheel-base for steadiness. This plan of engine was patented by the author of this article in 1866, and has come into very general use-since the expiration of the patent. In some cases a two-wheeled truck is added at the opposite end, as shown in g [p. 187]. For street railroads, in which the speed is necessarily slow, engines such as j [p. 189] are used. To hide the machine

have steadily been increased ever since they were first used, and there is little reason for thinking that they have yet reached a limit, although it seems probable that some material change of design is impending which will permit of better proportions of the parts or organs of the larger sizes. The decapod engines built at the Baldwin Locomotive Works, in Philadelphia, for the Northern Pacific Railroad, weigh in working order 148,000 pounds. This gives a weight of 13,300 pounds on each driving-wheel. Some