Many other insects work on the same principle. Certain Termites, for example, construct tunnels of clay, in order to conceal them on their travels, and have the art, even in the hottest and driest weather, of mixing their clay with some liquid which renders it, when dry, nearly as hard as stone. Indeed, there have been instances where the Termites have attacked the wooden beams of houses, and literally transformed them into beams of stone. Then there are many Ants, notably several species of South America, which cover their approach by tunnels, and never venture into the open air. GRAVITY AS A PROPULSIVE AGENT. The two figures on the accompanying illustration will almost speak for themselves. We have already seen how the same force of gravitation which causes the avalanche to thunder down the precipice may be utilised as a means of projecting missiles in time of war. When, however, the stones or beams were once sent on their destructive mission, they were out of the control of those who launched them. We now come to a modification of the force of Gravity, by which the missile, if we may so term it, is kept under control, its power increased or diminished at will, and its point of attack shifted according to the requirements of the moment. [Illustration: Ram.] [Illustration: Head of Battering-Ram.] Before the invention of artillery, the Battering-ram was by far the most formidable engine that could be brought against a fortified place. The principle of the Battering-ram was simple enough. A long and heavy beam, generally the trunk of a tree, was suspended by ropes at the centre of gravity, so that it could be swung backwards and forwards. Although a simple beam was an effective weapon, its value was much enhanced by loading the thickest end with a heavy mass of metal, usually iron, and, when there was time for adornment, roughly modelled into the form of a ram’s head. Generally the Battering-ram was mounted on an elevated platform, and the soldiers who worked it protected by a roof, which was called by the name of Testudo, or Tortoise. The force of this weapon was tremendous, and no wall, however strong, could resist it. Sometimes the beam was considerably more than a hundred feet in length, being composed of several pieces bolted and banded together with iron. It may easily be imagined that such a weapon as this must have been a most terrible one, and, indeed, the whole success of the siege practically depended upon it. The assailants did their best to bring the Battering-ram into position under the walls, and the besieged did their best either to keep it away, or to neutralise its effects by catching it with nooses, dropping large stones upon it so as to break or dismount it, or, if they could not succeed in either of these attempts, they deadened the force of its blows as well as they could by interposing large sacks of wool between the wall and the head of the ram.