To combine the two features, engines have been built with the driving-wheels and axles arranged as in Figure 32. The frames are then extended backward, and the water-tank and fuel are placed on top of the frames, and their weight is carried by a truck underneath. This arrangement leaves the whole weight of the boiler and machinery on the driving-wheels, 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 Figure 33. For street railroads, in which the speed is necessarily slow, engines such as Figure 13 (p. 110) are used. To hide the machine from view, and also to give sufficient room inside, they are enclosed in a cab large enough to cover the whole machine. The size and weight of locomotives 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 ten-wheeled passenger engines, built at the Schenectady Locomotive Works for the Michigan Central Railroad, weigh 118,000 pounds, and have 15,666 pounds on each driving-wheel. Some recent eight-wheeled passenger locomotives for the New York, Lake Erie & Western Railroad weigh 115,000 pounds, and have 19,500 pounds on each driving-wheel. At the Baldwin Works, some "consolidation" engines have recently been built which are still heavier than the decapod engines. The following table gives dimensions, weight, price, and price per pound of locomotives at the present time. If we were to quote them at 8 to 8¼ cents per pound for heavy engines and 9 to 22¼ for smaller sizes, it would not be much out of the way. _Dimensions, Weights, and Approximate Prices of Locomotives._