CONDITIONS THAT INCREASE TRAIN RESISTANCES.
In a calm day the atmospheric resistance is very slight under a speed of twenty miles an hour. To a fast train, atmospheric resistance becomes an important obstruction. The atmosphere acts on the train in various ways, that are hard to calculate with any degree of accuracy,—head resistance to the locomotive, which is presumably equal to the exposed area of the front of the engine and cab in square feet multiplied by the air-pressure due to the speed; then, various parts of the cars present surfaces that the air strikes against, and increases the resistance; the raised and projecting roofs of passenger coaches offer an ample area for the wind to hold the train by; and every opening between the cars permits the wind to obstruct, to some extent, each individual car. Where wind is blowing freely in a direction to strike the train on the side, the resistance is greatly increased; the retardation being due to the wind pushing the car sidewise, so that the wheel-flanges rub against the rail, and also to the wind obtaining a strong hold on the front of each car. In the case of a freight train, the resistance is greatly increased when the doors of cars are left open; for every car in that condition acts like a parachute to reduce speed. Freight trains arranged with box cars and flat cars mixed, obtain more than a fair share of obstruction from the atmosphere; for every box car that has a space opened in front by a flat car, gets nearly the full pressure of the wind in its front. It pays in coal to incur some trouble and delay in putting box cars together. That also enables the brakemen to get along the train more rapidly than where the cars are mixed.
In the experiments already alluded to on the Erie Railway, it was found, in the absence of wind, that the first car of a freight-train produced atmospheric resistance equal to a surface of sixty-three feet, multiplied by the air-pressure due to speed; and that each subsequent car offered a resistance of twenty per cent of that due to the first car.