Page:Popular Science Monthly Volume 90.djvu/731

 Popular Science Monthly

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���How the caterpillar crosses a trench. It noses its way down and up, following the contour of the ground. It is steered by the rear wheels or by making one caterpillar go faster than the other

��imaginary wheel of the same diameter. Only that portion of the assumed wheel in contact with the ground acts as the lever and it is this portion that is repro- duced in the front end of a caterpillar belt.

Why Don't the Tanks Stick in the Mud?

This large area of the belts in contact with the ground, as shown in the accom- panying sketches, also brings out the in- herent property of the caterpillar which has made it possible for the tanks to traverse soft, muddy ground — the large bearing area of the caterpillar shoes. With the caterpillars thirt%^ inches wide and with about fifteen or twenty feet of length in contact with the earth at any time, the pressure on the ground is said to be less than three pounds to the square inch. This is a great deal less than that exerted by a horse or a two-hundred-pound man. This large supporting area explains why the tanks do not sink down and become mired in places where an ordinary wheeled armored motor car would be hopelessly buried down to the hubs.

The tanks are of various sizes and designs, each later model being an improvement over its predecessors. The first ones had a maximum speed of from six to eight miles an hour. This was too slow, and the Germans soon found a way to destroy them. Before a tank could get back to shelter after a raid, the German observers were able to wireless its position to their artillery which promptly dropped shells on on it. The latest tanks are lighter than their predecessors, have gasoline engines of over five-hundred horsepower and travel at the remarkable speed of eighteen miles an hour!

Although varying in size and details.

��all tanks have the common characteristic of being divided into three main compart- ments between the two side caterpillar frames. The first is the observation com- partment in which the driver and his helper are perched high above the ground to direct the movements of the huge steel beast. In the middle is the ammunition room from which the guns carried in the two side turrets are fed. At the rear is the engine room. From two to four gasoline engines are used, these driving the rear axle and its integral sprockets over which the caterpillars run. The latter run over an idler pulley or sprockets at the extreme front ends and are supported by means of rollers attached to the upper portion of the frame on each side when passing over the top. This movement of the caterpillar belts is exactly analogous to that of the ordinary- variety of garden insect with the same name which similarly lays down his own track by humping his back continu-. ously and regardless of the land surface.

How the Tanks Are Guided

The tanks are steered by a pair of small ordinar>' wheels at the rear. These are supported in a piv^ot on a frame extended from the rear. They are merely for steering, and support none of the weight of the tank except when bridging wide trenches or dips in the surface. Steering can also be accomplished by making one caterpillar go faster than the other by manipulating clutches on the driving mechanism.

The tanks have been developed entirely by the British Admiralty, the idea of using the large round end of the caterpillar being credited to E. H. Tennyson D' Eyncourt, Director of Naval Construction.

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