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Momentum of Running Water.
[Book IV.
When the males of sheep or goats prepare to butt, they always reeedebackwards to some distance ; and then rushing impetuously forward, (ac-cumulating force as they go,) bring their foreheads in contact with a shockthat sometimes proves fatal to both. The ancients, perhaps, from witness-ing the battles of these animals, constructed military engines to act on thesame principle. A ponderous beam was suspended at the middle by chains,and one end impelled, by the United efforts of a number of men at the op-posite end, against walls which it demolished with slow but sure effect.The battering end was generally, and with the Greeks and Romans uni-formly, protected by an iron or bronze cap in the form of a ram’s head;and the entire Instrument was named after that animal. It was the mostdestructive of all their war machinery—no building, however solid, couldlong withstand its attacks. Plutarch , in his life of Anthony, mentions oneeightyfeet in length.
The action of the ram is familiär to most people, but it may not beknown to all that similar results might be produced by a liquid as by a solid—that a long column of water moving with great veloeity might be madeequally destructive as a beam of wood or iron—yet so it is. Waves of thesea act as water-rams against rocks or other barriers that impede theirprogress, and when their force is increased by storms- of wind, the mostsolid structures give way before them. The old lighthouse on the Eddy-stone rocks was thus battered down during a storm in 1703, when theengineer, Mr. Winstanley, and all his people, perished.
The increased force that water acquires when its motion is accelerated,might be shown by a thousand examples : a bank or trough that easilyretains it when at rest, or when slightly moved, is often insufficient whenits veloeity is greatly increased. When the deep lock of a canal is openedto transfer a boat or a ship to a lower level, the water is permitted to de-scend by slow degrees : were the gates opened at once, the rushing masswould sweep the gates below before it, or the greater portion would becarried in the surge quite over them—and perhaps the vessel also. Asluggish stream drops almost perpendicularly over a precipice, but the mo-mentum of a rapid one shoots it over, and leaves, as at Niagara, a widespace between. It is the same with a stream issuing from a horizontaltube—-if the liquid pass slowly through, it falls inertly at the orifice, butif its veloeity be considerable, the jet is carried to a distance ere it touchesthe ground. The level of a great part of Holland is below the surface ofthe sea, and the dykes are in some parts thirty feet high : whenever aleak occurs, the greatest efforts are made to repair it immediately, ahd forthe obvious reason that the aperture keeps enlarging and the liquid massbehind is put in motion towards it; thus the pressure is increased and,if the leak be not stopped, keeps increasing tili it bears with irresistableforce all obstructions away. A fatal example is recorded in the ancienthistory of Holland :—an ignorant burgher, near Dort, to be revenged ona neighbour, dug a hole through the dyke opposite the house of the latter,intending to close it after his neighbor’s property had been destroyed;but the water rushed through with an accelerating force, tili all resistancewas vain, and the whole country became deluged. The ancients werewell aware of this accumulation of force in running waters. Ällusiönsto' it are very common among the oldest writers, and various maxims oflife were drawn from it. The beginning of strife, says Solomon, “ is aswhen one letteth out water”—the “ breach of waters”—“ breaking forthof waters”—“rushing of mighty waters,” &c. are frequently mentäoned, toindicate the irresistable influence of desolating evils when once admitted.
That the force which a running stream thus acquires may be made to