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pending piece so that one side of the yoke shall helonger than the other, and so relieve the weakeranimal. It is the same in the porters’ levers as inyokes, when the suspending tackle is not in the centre,and one arm of the lever is longer than the other,namely that towards which the tackle has shifted; forin this case if the lever turn upon the points to whichthe tackle has slid, which now becomes its centre, thelonger arm will describe a portion of a larger circle, andthe shorter a smaller circle. Now as small wheels re-volve with more difficulty than larger ones, so leversand yokes press most on the side which is the leastdistance from the fulcrum, and on the contrary theyease those who hear that arm which is at the greatestdistance from the fulcrum. Inasmuch as all these ma-chines regulate either rectilinear or circular motion bymeans of the centre or fulcrum, so also waggons, chariots,drumwheels, wheels of carriages, screws, scorpions,balistse, presses, and other instruments, for the samereasons produce their effects by means of rectilinear andcircular motions.

CHAPTER IX.

OF ENGINES FOR RAISING WATER; AND FIRST OFTHE TYMPANUM.

I shall now explain the machines for raising water, andtheir various sorts. And first the tympanum, which,though it raise not the water to a great height, yet liftsa large quantity in a small period of time. An axis isprepared in the lathe, or at least made circular by hand,hooped with iron at the ends ; round the middle whereofthe tympanum, formed of planks fitted together, is ad-justed. This axis rests on posts also cased with ironwhere the axis touches them. In the hollow part of thetympanum are distributed eight diagonal pieces, going