280

Description of a Pum/p from Besson.

[Book III.

destroy the vacuum upon which the etficiency of the machine depends.The same remarks apply to these that were rnad'e ob atmospheric pumpswith two pistons, at page 227.

There is a pump with two pistons in Besson’s Theatre des Instrumenswhich shows that such devjces were known in the 16th Century. It con-sists of a square trunk four or five feet in length, and the bore five or sixinches across, immersed perpendicularly in water at the bottom of awell; its lower end bejng open and the upper one closed, except at thecentre, where an opening is left and covered by a valve. A square piston,with its valve opening upwards, is fitted to work in the trunk from belowby a rod connected to its under side, as in No. 126. A lever passesthrough the lower part of the trunk, (through slits made for it in two op-posite sides,) one end of which is secured to a piece of timber walled in thewell, by a pin, on which it moves ; and the other end extends to the op-posite side of the trunk, where it is hooked to a chain that reaches fromthe pump brake at the top of the well. The lower end of the piston rodis connected by a bolt to that part of the lever that is within the trunk.This apparatus forms the lifting or forcing part of the machine. A com-mon pump tree or bored log extends from the place to which the wateris to be raised, to the top of the trunk, and die junction with the lattermade perfectly tight: an upper box or piston with its rod is fitted to workin the tree like an ordinary wooden pump, while the valve on the trunkanswers the purpose of a lower box. This rod is attached to the brake onone side of the fulcrum and the chain that is connected to the lever andlower rod to the opposite side, so that as one piston rises the other de-scends and a constant stream of water is discharged above.

This is the oldest pump with two pistons that we know of, and it hasone advantage over others, viz : in raising water without changing its di-rection. We at first intended to insert a figure of it, but the apparatusfor working it is too complicated for populär Illustration. Although mo-tion is imparted to the piston as noticed above, it is not done directly, butby means of such an enormous amount of complex anduseless machinery as would excite amazement in a mo-dern mechanician. There is an assemblage of rods andleyers, tongs and lazy tongs, chains, right and left hand-ed screws, a heavy counterpoise and a massive pendu-lum, &c., all of which are required to be put in motion be-fore the pistons can be moved. A figure of such a pumpwould possibly interest some readers as a matter ofcuriosity, for certainly a rarer example of the wasteof power could not well be imagined : it presents asclumsy and “ round-about” a mode of accomplishing avery simple purpose, as that of the genius who, in tap-ping a cask of wine, neyer thought of inserting thespigot into the barrel, but attempted to drive the barrelon the spigot.

Sometimes pumps with solid and hollow pistons arecombined as in No. 130, a contrivance of Mr. Treve-thick. The cylinder of a forcing pump communicateswith that of an atmospheric one ; both piston rods areconnected to a cross-bar and rise and fall together. Whenthe pistons are raised the water above that in the long(or atmospheric) cylinder is discharged at the spout, andthe space below them is filled by the atmosphere forc-ing up fresh portions through the suction pipe. Wheu

No-130. Trevethick’sPump.