. Science of railways . d is and always has been the weightof mercury (quicksilver) expressed in pounds pres-sure per square inch. One of the first pressure gauges used was a simpleU-shaped glass tube partly filled with mercury; thepressure admitted to one side lowering the level in thatside and raising it in the other. The differencebetween the two levels determined the pressure—21-32 inches (approximately) being equal to one poundper square inch at a temperature of 60 degrees Fahr. When it was not convenientto graduate directly uponthe glass tube, recourse washad to a metal tube with afloat
. Science of railways . d is and always has been the weightof mercury (quicksilver) expressed in pounds pres-sure per square inch. One of the first pressure gauges used was a simpleU-shaped glass tube partly filled with mercury; thepressure admitted to one side lowering the level in thatside and raising it in the other. The differencebetween the two levels determined the pressure—21-32 inches (approximately) being equal to one poundper square inch at a temperature of 60 degrees Fahr. When it was not convenientto graduate directly uponthe glass tube, recourse washad to a metal tube with afloat and independent scalein some convenient loca-tion. The engraving shows such a U-shapedtube with float transferringthe indications to a scalepjq^ by means of a cord over Mercury Column and Gauge. pullcVS. While the mercury gauge has been greatly per-fected it is, as before stated, stiU the standard gaugeto this day. The most accurate test is a perpendiculariron tube, immersed in a sealed pot of mercury at its 190. G e<Xcje LOCOMOTIVE APPLIANCES, 191 base and running high in the air (see Fig. 1). Thepressure from the test pump is apphed to the surface of
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