Although the air-brake can almost talk, it is still not perfect. There are several fortunes to be made yet in improving it. For instance, it is desirable, in descending long and steep grades, that the brake-pressure should be just sufficient to control the speed of the train, and should be steadily applied; otherwise the descent will be by a succession of jerks which may become dangerous. With the automatic the brakes must be occasionally released to recharge the reservoirs, or when the speed of the train is too much reduced; and it is difficult to keep a uniform speed. So far, the means devised to overcome this difficulty and keep a constant and light pressure on the wheels have been thought too costly or complicated for general use. With hand-brakes long trains are controlled by the brakes of but a few of the cars in any one train. It follows that in the descent of grades the braked wheels must often run for miles with the pressure as great as it can be without sliding the wheels. The rim of the wheel is rapidly heated by the friction of the brake-shoe, and the unequal expansion of the heated and the unheated parts of the wheel causes a fracture. This is why so many broken car-wheels are found at the foot of grades--of all places the worst for such an accident to happen. With "straight air," that is, with the pressure from the main reservoir, or the air-pump, going directly to the brake-cylinder, the engineer can apply the brakes to all the wheels of his train simultaneously, and with great delicacy of graduation; and by turning a three-way cock which is placed in the piping of each car, the air can be used "straight." This is regularly done on some mountain-roads. At summits the trains are stopped and the brakes are changed from "automatic" to "straight." This practice is dangerous, however, and is not approved by the best brake-experts, for if a hose bursts, or through some other accident the air in the train-pipe escapes, the brakes are useless. The automatic arrangement by which a reduction of air-pressure in the train-pipe applies the brakes, as previously explained, is much preferred, although no entirely satisfactory means has yet been devised for automatically regulating the air-pressure in the brake-cylinder.