PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 625 



Tyne, the lever and weight, (or as it was then called the steel-yard) was 

 added to the safety valve of Papin, and this arrangement has never been 

 improved, and in exactly this form is an essential part of every steam-boiler 

 at the present time. But still there was wanted some instrument that 

 would show the varying pressures before the safety-valve was lifted. Mr. 

 Wolf, the inventor of the high and low pressure engine, added to the stem 

 of the safety-valve, a quadrant that acted on an arm, like one arm of a 

 governor : but it was too complicated to answer any common purpose. At 

 what time the siphon mercury gauge was introduced is not mentioned in 

 the history of the steam-engine, but an old engine builder, familiar with 

 steam-engines in England, says he has seen them on old engines con- 

 structed before Watts's time. The siphon gauge, for a limited jiressure, is 

 all that is needed, and is universally in use for low pressure at the present 

 time. The mercury column is in fact the true measure of all pressures up 

 to hundreds of pounds, but for such pressures the great height of the mer- 

 cury column is too unwieldy for ordinary purposes. The other gauges In use 

 now are the mercury and air gauge, in which the air acts as a spring, by 

 compression, and the metal spring gauge. The mercury and air gauge 

 when properly made and in good order is up to the pressure of 80 lbs., 

 quite practical, but above that pressure the scale becomes so contracted 

 that it requires the closest inspection to observe the pressure indicated. 

 The metallic spring gauges are on various plans, involving the same prin- 

 ciple, that is, sorae]elastic metal that yields to pressure in connection with 

 some gearing and a dial and pointer. Each gauge is marked by a mercury 

 and air gauge as a standard. But the spring becomes gradually weakened 

 by the constant pressure, and unless frequently tested they become very 

 unreliable. To be sure, they err on the safe side, for boilers with steam in 

 them ; but when a metal spring gauge is used, with a force pump, for 

 testing a boiler, its imperfection may lead to disaster. 



The gauge of Messrs. Shaw &^Justice, which was referred to your com- 

 mittee, has been diligently examined, and its operation on steam boilers, 

 at two different establishments, noted by some of the committee, from day 

 to day, for a period of ten or twelve weeks. At the Metropolitan Mills, in 

 this city, it has been, constantly compared with a mercury and air gauge, 

 and with a metal spring gauge. The pressures, indicated by the several 

 gauges, varied from twelve to thirty pounds. At the highest pressure, 

 they carry at the mills, which is between seventy and eighty pounds, the 

 gauges differed thus: 



Shaw & Justice's tl lbs. 



Mercury and air 83 " 



Metal spring 101 " 



On comparing them one day, when the mill was stopped for an hour or 

 two, and the steam pressure in the boilers was allowed to run down, the 

 gauges were noted at all pressures down to the following: 



Shaw & Justice's 16 lbs. 



Mercury and air 21 " 



Metal spring 2*1 



The gauges varied at this pressure from five to eleven and a-half pounds. 

 Another test was made, that of temperature and pressure, according to 

 [Air. Inst,] 0* 



