626 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



Regnault's scale. A small vessel was used, capable of sustaining' a pres- 

 sure of one hundred and fifty pounds to the inch, with a thermometer at- 

 tached. Each of the gauges were tried in succession; and, after two or 

 three trials, the true pressure seemed to be the nearest to Shaw & Justice's. 

 At the request of Mr. Shaw, one of your committee made a journey to Phi- 

 ladelphia, and compared the gauge with both the temperature and the co- 

 lumn of mercury. The steam vessel used was capable of sustaining a 

 pressure of three hundred pounds to the square inch. We could only make 

 the comparison up to two hundred and twenty pounds, as the column of 

 mercury was but forty-two feet in height. The height of the column was 

 measured in feet and inches, and the gauge of Messrs. Shaw & Justice was 

 found j^erfedly correct wilh the mercury column. The principle on which this 

 gauge is constructed is a combination of the mercury column and piston 

 with a difference of the area of the two sides of the piston on which the 

 steam and mercury column presses, very much as the weight and lever 

 added to Papin's safety valve. The greater the difference of areas the 

 shorter is the mercury column, and unlike the mercury and air gauge, the 

 scale is as open at the highest pressures as it is at the lowest. Too much 

 credit cannot be given to Messrs. Shaw & Justice for the erecting of a mer- 

 cury column (the only one of this height in this country), for testing and 

 marking their gauges correctly. It is proposed to have one at the Metropolitan 

 Police head-quarters, in this city, to try the gauges used by the boiler- 

 inspectors of the Sanitary Police, and also to try such gauges as they find 

 are out of order. 



In conclusion, the committee would say, that they think the gauge of 

 Messrs. Shaw & Justice the most complete and perfect of any gauge now 



before the engineering world. 



WARREN ROWELL, 



JOHN K. SIMPSON, 



J. K. FISHER. 



New York, March 31, 1864. Committee. 



The following illustration will give a clear idea of the gauge examined 

 by the committee: 



Description of the Vertical Section shown 'of Shaiv & Justice's Patent 31er- 

 curial Pressure Gauge. — A is a brass cup containing the iron disc B, which 

 is recessed on its lower side to form the mercury chamber a. 



C is a double headed piston or plunger, with its upper and larger head, 

 covered by gum diaphragm b, and receiving the pressure of the mercury in 

 the chamber a, and into which it passes when the pressure is applied on the 

 small head of the piston/. 



D is a cap screwing into the cup A, and impinging on iron disc or mer- 

 cury chamber B, and the gum diaphragm b, thus making a perfectly tight 

 joint at its point of contact d, and preventing any escape of mercury into 

 the lower part of the cup A. 



E is Si jam-nut with an orifice (for the steam) which presses against a 

 perforated button or jing g, which in turn impinges on gum diaphragm c, 

 making an air and steam-tight joint at point of contact. 



/is the lower and smaller head of the piston, which receives the steam 

 pressure, and in its upward motion forces the mercury in chamber a, through 



