Q26 * LECTURE XIX. 



fFect in d riving it, but it would be absolutely impossible in practice to con- 

 struct macbiuery strong enougb for tbe purpose, and if it were possible, tbere 

 would be an immense loss of force from tbe friction. For example, supposing- 

 a weight of 500 pounds, falling from a beigbt of 50 feet, to drive the pile 

 2 inches at each stroke; then, if the resistance be considered as nearly uni- 

 form, its magnitude must be about 150 thousand pounds, and the same mov- 

 ing power, with a mechanical advantage of 300 to 1, would perform the work 

 in the same time. But for this purpose some parts of the machinery must be 

 able to support a strain equivalent to the draught of 600 horses. In the pile 

 driving engine, the forceps, or tongs, sometimes called the monkey, or fol- 

 lower, is opened as soon as the weight arrives at its greatest height; and at 

 the same time a lever detaches the drum, employed for raising the weight, 

 from the axis or windlass, at which the horses are drawing; the follower 

 then descends after the weight, uncoiling the rope from the drum, and 

 the force of the horses is employed in turning a fly wheel, until the con- 

 nexion with the weight is again restored. (Plate XVIII. Fig 234.) 



When we throw a stone, or a missile weapon of any kind, with the hand, 

 the stone can acquUe no greater velocity than the hand itself, accompanied 

 by the neighbouring part of the arm: so that the whole velocity must be pro- 

 duced in a mass of matter comparatively very large. A sling enables us to 

 throw a stone or a ball much further; for here the stone may be moved with 

 a velocity far greater than the hand that impels it, although the action of the 

 force on the stone is indirect, and the resistance of the air consider- 

 able. An elastic bow, furnished Avith a strong and light string, enables 

 us to apply to an arrow or to a ball the whole force of our arms, unencum- 

 bered with any considerable portion of matter, that requires to be moved with 

 the arrow; hence a very great velocity may be obtained in this maflner. An 

 air gun possesses the same advantage in a still greater degree, and the force 

 of fired gunpowder excels perhaps all others, from its concentrating an im- 

 mense force in the form of an inconceivably hght elastic fluid; of course a ball 

 impelled by this force, becomes a most effectual instrument in penetrating the 

 most refractory substances. We may easily calculate the velocity of an arrow, 

 by comparing its motion with that of a pendulum, if we know the proportion 

 of its weight to the force that bends the bow; including in the weight a small 

 addition for the inertia of the bow and bowstring; the height to which the 

 arrow will rise, being about as much greater than the space through which 



