The third type was the Whippet. The tracks were nearly as long as those of a heavy Tank, but the body had been reduced to a small cab perched at the back, rather as an urchin rides a donkey. It was armed with two machine-guns, managed by a crew of three men, and developed a speed of seven miles an hour. Whippets were designed for use as raiders and in conjunction with cavalry. In practice, however, the cavalry was seldom able to act with them. Partly in consequence of this, partly owing to the state of open warfare being of such short duration, the Whippets, though having brilliant feats to their credit (see the exploits of “Musical Box,” Chapter XIII), remained creatures of promise rather than of achievement. III As a rule Male Mark V. Tanks were used against Pill-Boxes and other “strong points,” while the special work of Female Tanks was to deal with hostile infantry (for example, by sitting astride and thus enfilading their trenches), and then to finish the process of flattening the enemy’s wire which the Male Tanks had begun. All three types of Tank were capable of going across country. That is to say they could, for example, follow a pack of hounds anywhere, except perhaps in the Fens. Ditches, heavy plough, banks, walls, hedges, or fences could all be negotiated. Tanks could also go over many obstacles--notably over wire--where the Field, even were they willing “to take a windmill in the harbour of the chase,” must go round. But as a moment’s reflection will show, there must remain in every country certain features which will prove absolute barriers to the progress of Tanks. Chief among these are canals and deep rivers (unless spanned by strong bridges), very steep railway cuttings, railway embankments, marsh, or woods in which the trees are too strong to be pushed over, and too dense-set to be steered through. Besides these natural, or at least civilian, obstacles, there will be inevitable military obstacles in any country that has been fought over. For example, old half-blown-in trench systems make ground “awkward,” and Tanks operate at extreme disadvantage in country like that round Ypres, which was by 1917 a continuous network of water-logged shell and mine craters, with no original ground left at all. Again, by the close of hostilities the number of anti-Tank devices employed by the Germans was very considerable. They paid the new arm the compliment of an intricate system of defence and counter-offence which included concealed Tank traps made on the model of elephant-pits, formidable double-traversed trenches, a branch of special anti-Tank artillery, heavily reinforced concrete stockades, and an elaborate system of land mines. With so many obstacles to avoid or to negotiate, with their fate often hanging upon a prompt and accurate use of their guns, the crew inside the Tank were doomed by the conditions under which they fought to an almost incredibly limited view of the surrounding world.