The Screw.

141

Chap. 16.]

No. 63.

its passage through the screw was altogether an illusion. On the contra-ry, it is uniformly raised by the contmual elevation of that part of the tubewhich is immediately behind the liquid, and which pushes it up in a man-ner analagous to that represented by the following diagram.

Suppose A Y, the edgeof a wide Strip of clothor tape, secured at bothends, at an angle withthe horizon, as repre-sented, and upon whichthe boy’s marble orball at P, can roll. Ifwe hold the pen withwhich we are writingunder the tape between P Y, and raise that part into the position indicatedby the dotted lines; the ball would necessarily be pushed forward to E ;and if the pen were then drawn towards B on the line D B, the ball wouldbe carried up to A, and without deviating in its path from the line Y A.Tf A Y were the under side of a flexible pipe or gutter, containingwater at E in place of the ball, it is obvious that it would also be raisedto A, in a like manner. By the same principle water is raised in the screw,and we may add, in much the satne way, for the rotation of the screw ismerely another mode of effecting the same thing, which we have suppos-ed to be done more directly by the pen, i. e. by producing a continual ele-vation of the plane immediately behind the ball or the water. The pathof the latter through a screw is the same as that of the ball, while thecurves assumed by the tape, as in the dotted lines, represent sections ofthe helix, and the lines D B, A Y, of the cylinder within which it isformed.

All the ancient machines hitherto examined, have come down from pe-riods so extremely remote, that not a single circumstance connected withtheir origin or their authors has been preserved. The screw is the firstmachine for raising water, whose inventor, or alleged inventor, has beennamed; and yet, from the imperfect and mutilated state of such ancientwritings that incidentally mention it, and the loss of others which treatedprofessedly on it, the question of its origin is far from being settled. Al-though it is said to have been invented by Archimedes and has long beennamed after him, there are circumstances which render it probable thatDiodorus Siculus and Atheneus were mistaken when they attributed itto the great philosopher of Syracuse . Had the account of this machinewhich Archimedes himself wrote, been preserved, there would have beenno occasion to reason on its origin or its author ; but unfortunately this, aswell as his description of pneumatic and hydrostatic engines, “ concerningwhich he wrote some books,” are among those that have perished.

There is no reason to believe that Archimedes himself ever claimed itsmvention; and his countryman Diodorus , who lived two hundred yearsafter him, and upon whose authority chiefly it has been attributed to him,admits that it was invented by him in Egypt ; thus allowing it to have beendevised in that country, whence the Greeks derived all or nearly all thatWas valuable in their philosophy and their arts. Every person knowsthat Egypt was the grand school for the nations of old, in which the learn-ed men of other countries were instructed in every branch of philosophy—for the cultivation of which the Egyptians were celebrated even in the