BOOK VI.

181

is bent down the same distance as it was bent up the last time ; the otherend of it, which also acts as a journal, is straight. This part which protrudesthrough the wood is protected by two iron washers in the shape of discs, towhich are fastened two leather washers of the same shape and size, in orderto prevent the water which is drawn into the box from gushing out. Thesediscs are around the axle ; one of them is inside the box and the otheroutside. Beyond this, the end of the axle is square and has two eyes, inwhich are fixed two iron rods, and to their ends are weighted lumps of lead,so that the axle may have a greater propensity to revolve ; this axle caneasily be turned when its end has been mortised in a crank. The upper partof the box is the shallower one, and the lower part the deeper ; the upperpart is bored out once straight down through the middle, the diameter of theopening being the same as the outside diameter of the column pipe ; thelower box has, side by side, two apertures also bored straight down;these are for two pipes, the space of whose openings therefore is twice asgreat as that of the upper part ; this lower part of the box is placedupon the two pipes, which are fitted into it at their upper ends, and thelower ends of these pipes penetrate into trunks which stand in thesump. These trunks have perforations through which the water flows intothem. The iron axle is placed in the inside of the box, then the two ironpiston-rods which hang from it are let down through the two pipes to the depthof a foot. Each piston has a screw at its lower end which holds a thick ironplate, shaped like a disc and full of openings, covered with a leather, andsimilarly to the other pump it has a round valve in a little box. Then theupper part of the box is placed upon the lower one and properly fitted to it onevery side, and where they join they are bound by wide thick iron plates, andheld with small wide iron wedges, which are driven in and are fastened withclamps. The first length of column pipe is fixed into the upper part of thebox, and another length of pipe extends it, and a third again extends this one,and so on, another extending on another, until the uppermost one reaches thedrain of the tunnel. When the crank worker turns the axle, the pistons inturn draw the water through their discs ; since this is done quickly, andsince the area of openings of the two pipes over which the box is set, is twiceas large as the opening of the column pipe which rises from the box, and sincethe pistons do not lift the water far up, the impetus of the water from thelower pipes forces it to rise and flow out of the column pipe into the drain ofthe tunnel. Since a wooden box frequently cracks open, it is better tomake it of lead or copper or brass.

The fifth kind of pump is still less simple, for it is composed of two orthree pumps whose pistons are raised by a machine turned by men, for eachpiston-rod has a tappet which is raised, each in succession, by two cams ona barrel; two or four strong men turn it. When the pistons descend intothe pipes their discs draw the water ; when they are raised these force thewater out through the pipes. The upper part of each of these piston-rods,which is half a foot square, is held in a slot in a cross-beam ; the lower part,which drops down into the pipes, is made of another piece of wood and isround. Each of these three pumps is composed of two lengths of pipe fixed