262
Single- Valve Forcing Pumps.
[Book III.
A forcing pump differs but little from a syringe: the latter receives andexpels a liquid through the same passage, but the former has a separatepipe for its discharge, and both the receiving and discharging orifices arecovered with valves. By this arrangement it is not necessary to removea pump from the liquid to transfer the Contents of its cylinder, as is donewith the syringe, but the Operation of forcing up water may be continuouswhile the instrument is immoveable, A forcing pump, therefore, is merelya syringe furnished with an induction and eduction valve—one throughwhich water enters the cylinder, the other by which it escapes from it.Of the process or reasoning which led to the application of valves to thesyringe, history is silent; but as has been retnarked in a previous chap-ter, their employment in bellows or air forcing machines, probably openedthe way to their introduction into water forcing ones. The ordinary bel-lows has but one valve, and the simplest and most ancient forcing pumpshave no more. One of these is shown at No. 116. It represents a syringe
having the orifiee at the bottom ofthe cylinder covered by a valveor clack, opening upwards ; anda discharging pipe connected tothe cylinder a little above it: whenplaced in water the orifiee of thispipe is closed with the finger, andthe piston being then drawn up,the cylinder becomes charged, andwhen the piston is pushed downthe valve closes and the liquid isforced through the pipe. In thismachine the finger performs thepart of a valve by preventing airfrom entering the cylinder whenthe piston is being raised. Suchpumps made of tin plate were for-merly common, and were used towash Windows, syringe plants andgarden trees, &c. The figure isfrom plate 67 of “L’Exploiter desMines,” in Arts et Metiers, and is described (page 1584) as a Dutchpump, “ pour envoyer commodement de l’eau dans les differents quartiersde l’attelier.”
No. 117 is another single-valve forcing pump from the second volumßof a Latin treatise on Natural Philosophy , by P. P. Steinmeyer, Friburgh,1767. It is secured in a cistern, the surface of the water in which is al-ways kept above the small openings made through the upper part; sothat when the piston is drawn up, as in the figjire, the liquid flows in andfills it; and on the descent of the piston the water is forced up the as-cending pipe, the valve preventing its return. This is a very simpleand efficient forcing pump ; and having no induction valve and the pistonbeing always under water, it is not very liable to derangement. It has,however, its defects ; for in elevating the piston the whole weight of theatmosphere above it has to be overcome, a disadvantage that in large ma-chines would not be compensated by the saving of a valve. As the pistonhas to pass the holes in the upper part of the cylinder, its packing wouldbe injured if their inner edges were not rounded off. This pump has beenerroneously attributed to a modern European engineer : see the LondonRegister of Arts, v, 154, and Journal of the Franklin Institute, viii, 379.
No. 116.
No. m.
Single-Valve Forcing Pumps.