266 Description of the Machine of Ctesibius by Vitruvius . [Book III.

ment; its flow from the discharging orifice being merely rendered uniformor nearly so.

In the ordinary use of forcing pumps a constant instead of an inter-rupted flow of water from the discharging orifice, may be a matter ofno importance ; but when those of large dimensions are required to raiseit to great elevations, air vessels are not only valuable but indispensableadjuncts; for the elastic fluid within them forms a medium for graduallyovercoming the inertia of the ascending liquid columns, and thereby pre-vents those jars and shocks which are incident to all non-elastic substancesin rapid motion, when brought suddenly to a state of rest. A column ofwater moving with great velocity through a pump, produces, when in-stantly stopt, a concussion like that of a solid rod of the same length, whenits end is driven against an unyielding object; but with an air-vessel, theeffect is like that of the same rod when brought in contact with a bale ofcotton or caoutchouc. Less force is required also to work pumps thathave air-vessels, because in them the column of Water in the dischargingpipe is continued in motion during the ascent of the piston, hence it hasnot to be moved from a state of rest on the piston’s return. When twoor more cylinders are connected to one discharging pipe, one air-vesselonly is required, as in fire-engines, water-works, &c.

It is this kind of forcing pump that is generally adopted in water-worksfor the supply of towns and cities ; the piston rods being moved bycranks or levers attached to water wheels: sometimes they are driven bywindmills, steam-engines, and by animals. The cylinders are commonlyused perpendicularly as in the figure, but they are sometimes worked inan inclined and also in a horizontal position.

The celebrated pump of Ctesibius was constructed like that representedin the last figure, except that it had two cylinders. It seems to have beenalmost identical in its construction with our fire-engines. “ It remains now[says Vitruvius ] to describe the machine of Ctesibius which raises watervery high. This is made of brass; at the bottom a pair of buckets [cylin-ders] are placed at a little distance, having pipes like the shape of a forkannexed, meeting in a basin in the middle. At the upper holes of thepipes within the basin, are made valves, hinged with very exact joints;which, stopping the holes, prevent the efflux of the water that will bepressed into the basin by the air. Upon the basin a cover like an invertedfunnel is fitted, which is adjoined and fastened to the basin by a collar,riveted through, that the pressure of the water may not force it off: andon the top of it, a pipe called the tuba, is affixed perpendicularly. Thebuckets [cylinders] have valves placed below the lower mouths of thepipes, and fixed over holes that are in their bottoms : then pistons turnedvery smooth and anointed with oil, being inclosed in the buckets [cylin-ders] are worked with bars and levers from above ; the repeated motionof these, up and down, pressing the air that is therein contained with thewater, the holes being shut by the valves, forces and extrudes the waterthrough the mouths of the pipes into the basin; from whence rising to thecover, the air presses it upwards through the pipe; and thus from the lowSituation of the reservoir, raises it to supply the public fountains.” Bookx, cap. 12. Newton’s Trans.

The machine as thus described is a proof of the progress which the an-cients had made in hydraulics : the whole appears to have been of themost durable materials, and of the best workmanship. Although the figuresof this and other machines which Vitruvius inserted in his work are lost,there is little -difficulty in realizing its construction from the text. Transla-