Page:Popular Science Monthly Volume 92.djvu/572

 Why Tanks Are Giant Caterpillars

Armor? The Caterpillar has it. Traveling treads? The Cater- pillc^r has them tcx). Machine guns? It has a poison squirt-gun

By John Walker Harrington

��THE motion of the most formidable and terrifying of modern war ma- chines has often been compared with that of the lowly larva from which comes the radiant butterfly. This famed cruiser of the battlefields might never have been, but for the invention of the farm trac- tor of Benjamin Holt with its caterpillar tread. Through the courtesy of Captain Haig, of the British Army, who is here dem.onstrating the pride of the English arms, the writer was permitted to spend nearly an hour within the Britannia, and at every point he was more and more im- pressed with the idea that not only does the tank resemble the caterpillar in movement, but that there are strange likenesses in structure, in armor, and even in control between the two objects. The tank is a high-powered, armored automobile differing from the war motor- car in that it moves not on wheels but on two steel belts traveling on the heavy metal frames on either side of its diamond- shaped body. The belts consist of shoes ingeniously linked together in endless chains. Each shoe has a flange, with which the tank can lay a firm hold on the ground. The belts are fitted to heavy sprockets. The rear sprockets are con- nected by gearing with the powerful engine in the back of the tank. The front sprockets are idlers over which the belts glide. There are also wheels which rest on the upper surfaces of the belts. At the top of the frames are rollers over which the belts pass. The tank is really laying down twin tracks or a railroad of its own.

The body of the average caterpillar consists of thirteen segments, four of which belong to his thorax or, dropping into mechanical terms, his fore compart- ment, while nine are assigned to the abdominal section. The number of segments varies with the species. The chest portion has three pairs of true legs, so culled because they are well jointed, easily controlled and muscular.

��They are protected with horny sheaths and are in effect armored. With these true legs the caterpillar can steer him- self, help himself along a twig, or seize leaves.

The pro-legs, or false legs, appear on at least five of the segments, duly paired. In their structure they resemble the shoes of the tank belts to some extent and they perform the same functions. They are fleshy un jointed protuberances rather than limbs. At the bottom of each one are minute hooks which are used auto- matically in giving the animal a hold on the surface he is traversing. They are for clasping, and in fact the rear pair are so modified as to be called claspers. Now, if a caterpillar could keep his pro- legs or shoes moving over his head and over his tail in an endless chain arrange- ment, his resemblance to the tank as far as the locomotion details are concerned would be perfect.

Some of the caterpillars have such a rapid, undulating movement, that it is hard at first to analyze its elements. The caterpillar actually walks by extending and contracting the fleshy segments of his body, the power being transmitted mostly to his pro-legs.

Any one who has seen the fuzzy larvae of the tussock moth going up a tree trunk will reahze that the caterpillar is happy at any angle. The same principle of construc- tion illustrated in that insect permits the tank almost to stand on end without los- ing balance.

For the sake of simplicity, the wheels at the rear of the tank by which it was once steered have been discarded and the direction is given by running the two belts at different speeds. The landship is rudderless. The caterpillar can twist his segments at the jointures.

The observation facilities and guide centres of both are in their forward com- partments. The commander of a tank and the driver sit well forward in the Juggernaut, looking out of very narrow

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