ATTWOOD'S MACHINE. This appara tus was invented by George Attwood, a fellow and tutor of Trinity College, Cambridge, to wards the close of the last century. It is intended to demonstrate the law of the velocity of falling bodies.
Suppose weights of six and seven pounds hang over a pulley, the weight and friction of which we neglect for the present ; if both weights were six pounds, the machine would not move: therefore, the moving pressure is the one pound by which the one weight ex ceeds the other. This if it had only its own mass to move, or if it fell freely, would generate 32i- feet of velocity per second ; but before this system can move, 0 -1- 7, or 13 lbs. must be stirred by 1 lb., and there will only be the 13th part of 321 feet of velocity produced in one second, that is, about 21 feet. Therefore, in one second, the heavier weight will fall only 11 foot ; and in five seconds, 25 times as much, or 30 feet. And the velocity acquired may be reduced in any proportion, by making the weights more nearly equal.
Attwood's machine is a pulley, the pivots of which, instead of being placed in a block, are sustained on friction wheels, to diminish the friction. Two weights are hung over this by a string, and the mass moved consists of the two weights, the pulley, and the friction wheels.
The length described in any time is mea sured by a vertical scale of feet, placed close to the line of motion of one of the weights. To measure the velocity acquired at any point the moving pressure (the excess of one weight above the other) must be taken off, in order that there may be no fresh accession of velo city, or that the system may proceed only with the velocity acquired. This is effected by making the larger weight in two parts, one part equal to the smaller weight, and the other of course to the excess or moving pressure. The latter is so formed that it cannot pass through a certain ring, while the former can. By fixing this ring to any required point of the scale of feet, the moving pressure is taken off when the larger weight passes through it. Attwood's machine is not wholly satisfactory ; but of the four principles-1, the law of uni formly accelerated motion : 2, the constancy of the retardation caused by the having to communicate every acceleration also to the pulley and friction wheels ; 3, the constancy of the retardation arising from friction; 4, the smallness of the resistance of the air to small velocities —this machine may be made to prove any one to a spectator who admits the other three.