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. In ordinary weather it is equal to about one-fifth of the weight which bears on the track; when perfectly dry, if the rails are clean, it is about one-fourth, and with the rails sanded about one-third. In damp or frosty weather the adhesion is often considerably less than a fifth. [Illustration: Fig. 25.--Turning Locomotive Tires.] 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 or weight, but ordinarily from 12,000 to 16,000 pounds per wheel is about the greatest load which they should carry. For these reasons, when the capacity of a locomotive must be increased beyond a limit indicated by these data, one or more additional pairs of driving-wheels must be used. Thus, if a more powerful engine was required than that shown in Figure 14 (p. 113), another pair of wheels would be added, as shown in Figures 26, 27, and 28. Or, if you wanted a more powerful engine than these, still another pair of driving-wheels would be provided, as shown in Figure 30. In this way the Mogul, ten-wheeled and consolidation engines have been developed from that shown in Figure 14. The Mogul locomotive (Fig. 27) has three pairs of driving-wheels, but only one pair of truck-wheels. The engravings shown in Figures 30 and 31 represent consolidation and decapod types of engines which have four and five pairs of driving-wheels. [Illustration: Fig. 26.--Six-wheeled Switching Locomotive. By the Schenectady Locomotive Works.] From the illustrations, Figures 28, 30, and 31, 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 driving-wheels cannot be increased.