STATIONÄRY ENGINES.

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spring is compressed and the regulating valve closed. If a bursttakes place the lower weights drop and instantly close the steamvalve.

The successful working of any pumping engine is dependent ina great degree upon the perfection of the pump valves, which mustbe so arranged as to deliver the water with ease and rapidity, andwithout any concussion in closing. As an illustration of the greatpractical importance of this question, it may be mentioned that whenthe Cornish pumping engine was first used for water-works pur-poses on a large scale, it was on the point of being altogetherabandoned on account of the imperfection of the pump valves.The valves were of large area, and constructed on the old butterflyprinciple, so that, under the heavy pressure at which they wereworked, the concussion caused in shutting was so violent as to occa-sion serious alarm for the safety of the machinery and foundations.The difficulty of constructing a valve which should present a maxi-mum area of discharge with a minimum area of surface exposed tothe concussion of the recoiling load at the termination of eachstroke of the pump appeared for a time insurmountable, but was,however, happily got over by the introduction of the double-beatvalve.

This valve, as already explained, has the upper area con-tracted, and by the difference of the upper beat and the inside ofthe lower one a surface is afforded for the water to act upon inlifting and shutting the valve. The valve having two points forthe water to escape by, a very slight distance of lift gives a largearea for discharge; and the area upon which the recoiling columndescends being only the difference between the upper and lowerareas, and not the entire area of discharge as in the old butterflyvalve, forms a surface insufficient to cause any concussion. Thisvalve also affords, under all circumstances, a means of regulatingthe pressure tending to shut the valve, whatever may be the heightof the column of water or the total pressure of the recoiling column,by adjusting the difference of area of the upper and lower beatsinversely in proportion to the height of the column.

For the ordinary purposes of small lift pumps and colliery enginesthe butterfly valve is serviceable, as there are no expensive faces tobe ground up or deranged by impurities or grit in the water, andit can be readily repaired on the spot. For a higher dass of workthere is no description of valve answers better than the double-beat