WHAT A SPEED OF A MILE A MINUTE MEANS.

New Zealand Graphic, Volume XVII, Issue XI, 12 September 1896, Page 343

WHAT A SPEED OF A MILE A MINUTE MEANS.

At sixty miles an hour, says a writer in the Lalies Home Journal, the resistance of a train is four times as great as it is at thirty miles —that is, the fuel must be fonr times as great in the one case as it is in the other. But at sixty miles an hour this fuel must be exerted for a given distance in half the time that it is at thirty miles, so that the amount of power exerted and steam generated in a given period of time must be eight times a« great at the faster speed. This means that the capacity of the boiler, cyclinders and the other parts must be greater with a corresponding addition to the weight of the machine. Obviously, therefore, if the weight per wheel, on account of the limit of weight that rails will carry, is limited, we soon reach a point when the driving wheels and other parts cannot be further enlarged, and then we reach the maximum of speed. The nice adjustment necessary of the various parts of these immense engines mav be indicated by some figures as to the woik perfirmed by these parts when the locomotive is worked at high speed. Take a passenger engine on any of the big railroads. At sixty miles an hour a driving wheel five and a half feet in diameter revolves five times every second ; now, the reciprocating parts of each cylinder, including one piston, piston tod. cross head and cmnecting rod, weighing about 650 pounds,

must move back and forth a distance equal to the stroke, usually two feet, every time the wheel revolves, or in a fifth of a second. It starts from a state of rest at the end of each stroke of the piston and must acquire a velocity of 32 feet per second in onetwentieth of a second, and must be brought to a state of rest in the same period of time. A piston 18 inches in diameter has an area of 254 i square inches. Steam of 150 pounds' pressure per square inch would, therefore, exert a force on the piston equal to 38,175 pounds. This force is applied alternately on each side of the piston ten times in a second.