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HISTORY OF ENGINEERING.

Book I.

moved. If the tongue of the lever be placed too far under the weight, and the end be toonear the centre of pressure, it will have no effect; the distance from the fulcrum to the endof the lever must be greater than from the fulcrum to the tongue.

The steelyard or staterm is an instance of this principle, and was in common use amongthe Romans, and our author observes, when the handle of suspension, on which as a centrethe beam turns, is placed nearer the end from which the scale hangs, and on the other sideof the centre, the weight will be shifted to the different weights of the beam ; the fartherit is from the centre, the greater will be the load in the scale which it is capable of raising,and that through the equilibrium of the beam ; thus a small weight, which, placed near thecentre, would have but a feeble effect, in a moment acquires power to raise a very heavy'load.

The rudder turns a ship, though ever so deeply laden, from the action of the lever, butVitruvius also notices that the sails, if only half mast high, will cause the vessel to sailslower than when the yards are hoisted up to the top of the mast, because, not then beingnear the foot of the mast, which is as it were the centre, but at a distance therefrom, theyare acted on by the wind with greater force. For if the fulcrum be placed under themiddle of a lever, it is with difficulty that the weight is moved, and that only when thepower is applied at the extremity of the lever, so when the sails are no higher than themiddle of the mast they have less effect on the motion of the vessel; when, however, raisedto the top of the mast, the impulse they receive from an equal wind higher up causes aquicker motion to the ship. Perrault disputes this doctrine in his Commentary, and properlyobserves that, whether the sails are higher or lower, the motion of the vessel is not affectedby it, for the whole moves together, and there is no fixed point to serve as a fulcrum orcentre of motion ; it is not therefore comparable to a lever, nor can it act as such : it issimply pushed forwards by the wind, and the only advantage in having the sails higher isthat the wind is there stronger, while there is a disadvantage from the head of the shipbeing plunged deeper in the water, which necessarily impedes its course.

Vitruvius continues: “ Oars made fast with rope to the thowls (scahni), when plungedinto tlie water and drawn back by hand, impel the vessel with great force, and cause theprow to cleave the waves, if the blades are at a considerable distance from the centre whichis the thowl.

“ So also, when loads are carried by four or six men on a pole, the weights are so placedin the middle, that each may bear his portion; for if they passed the centre, one set of menwould bear more than the other.

“ Oxen also have an equal draft, when the piece which suspends the pole hangs exactly fromthe middle of the yoke; and when oxen are not equal in strength, by judiciously shiftingthis suspended piece, one may be made to draw more than the other.

“ It is the same in the porter’s levers as in the yokes, when the suspending tackle is notin the centre, and one arm of the lever is longer than the other, namely that to which thetackle is shifted; for, in this case, the lever turning upon the points to which the tackle hasslid, which now becomes its centre, the longer arm will describe a portion of a largercircle, and the shorter a smaller circle.

“ Now, as small wheels revolve with more difficulty than larger ones, so levers and yokespress most on the side which is at the least distance from the fulcrum ; and on the contrary,they ease those who bear that arm which is at the greatest distance from the fulcrum.

“ AM these machines regulate either rectilinear or circular motion, by means of the centreor fulcrum, as also waggons, chariots, drum-wheels, wheels of carriages, screws, scorpions,balista?, presses, and other instruments, which produce their effects by means of rectilinearand circular motions.”

Engines fur raising Water. Tympanum. — The Romans were acquainted with variousmethods for raising water, and probably after Egypt became a province, many of themachines used bv that people were introduced among them, as we have already seen thatVitruvius was well informed upon all the sciences taught in the school of Alexandria, and tohim we are indebted for an account of much that otherwise would have been lost. Thetympanum he describes might have been long in use, and was calculated not to raisewater to any great height, or beyond that of the radius of the wheel, but to lift a largequantity in a small period of time. A shaft or axis turned in a lathe, or made cylindricalby hand, was hooped with iron at each end, to prevent it splitting. This axis was madeto turn on tops of posts cased with iron; into this were fitted eight arms or spokes, for thepurpose of supporting the rim of the tympanum, which was thus formed ; the horizontal facewas closely boarded; around it were small apertures about 6 inches in width, to admit thewater. When the tympanum was used, it was moored like a vessel, having another wheelattached to the side of it on which a number of men could tread, by which means it wasturned round ; the water received through the apertures in front of the wheel was elevatedby the arms or division thus raised beyond the horizontal position ; it flowed towards theaxis, at the end of which it ran into a trough prepared to conduct it either to gardens, orto dilute salt in pits.