_Answer._ Various kinds of tank locomotives, that is, locomotives which have no separate tenders, but carry the water-tanks and fuel on the frame and wheels of the locomotive itself, have been devised and are to some extent used. Plate XIII represents a locomotive of this kind on which the tanks are placed on each side of the boiler, and the fuel on a Bissell truck at the back end. A similar truck is placed at the front end, so that a locomotive of this kind can run equally well either way. The lateral movement of the two trucks also gives great flexibility to the wheel-base, so that such an engine will adjust itself easily to the curvature of the track. If, however, the two pairs of truck-wheels should both stand on an elevated part of the track, and the driving-wheels on a depression, the latter would evidently not carry as much and the truck-wheels would carry more of the weight of the engine than they did on a level part of the track. If the reverse condition of things should occur, that is, if the driving-wheels should be on an elevation and one or both pairs of the truck-wheels on a depression, then the latter would bear less weight than they did and the driving-wheels more. For this reason, in order to distribute the weight evenly on all the wheels, it is necessary to equalize the weight on the truck and driving-wheels, by connecting them with equalizing levers, similar to those which were described in answer to Question 301. These levers distribute any undue weight which may come on one wheel to that next to it. This is important, because if the driving-wheels bore less weight at some times than at others, their adhesion and their capacity to draw loads would be reduced in like proportion. It is evident, however, that if the water-tank or fuel is carried on the driving-wheels, there will be a greater weight on them when the tank is full than when it is empty, and that therefore there will either be so much weight on the wheels at one time as to be injurious to the rails, or else there will be too little for adhesion at another. Of course cases are conceivable, and doubtless exist in practice, where more adhesion is required to start a train and haul it during the first part of the “run” than will be needed during the latter part. In such cases doubtless the variable character of the weight might be an advantage instead of the reverse, but for ordinary practice a variable load on the driving-wheels would have the disadvantages which have been described. For this reason tank locomotives have been built like that represented in Plate XIV. In this it will be seen that the weight of the water-tank rests on a four-wheeled truck at the back end. A Bissell or two-wheeled truck is, however, placed in front in the same position as in the engine represented in Plate XIII, and carries a portion of the weight of the boiler and machinery.