PRESSURE INDICATOR. 149 



PRESSURE INDICATOR. 



IF a governor be not used, it is advisable to have a Pressure Indicator attached 

 to the main or mains that leave the works, to serve as a check upon the con- 

 duct of the workmen, whose duty it is to regulate the pressure of gas in them 

 according to the demand at certain hours of the night. It is, in fact, an instru- 

 ment giving the same information to the superintendent as the " tell-tale " of 

 the station-meter. It is thus constructed : A small gasometer about twelve 

 inches diameter is made to move in a tank of water in such a manner that it 

 shall rise or fall according to the pressure in the mains, with which it is con- 

 nected by a small pipe ; a guide-rod, furnished on the top with a pencil, 

 marks the exact amount of pressure upon a sheet of paper coiled round a 

 cylinder. This cylinder is moved round once in twelve hours by a time-piece. 

 It is evident, therefore, that if the paper be divided by horizontal lines cor- 

 responding to the rise or fall of the gasometer by every tenth of an inch in- 

 crease or decrease of pressure ; and if it be divided by vertical lines corre- 

 sponding to the revolutions of the time-piece in twelve hours, it will effect the 

 object required. The gasometer must be formed with an air-vessel inside, so 

 that when it is totally immersed it shall be in exact equilibrium with the exter- 

 nal atmosphere ; and when risen to its full height it shall have a pressure equal 

 to that required to force the gas through the mains ; say the height to which 

 the gasometer rises is equal to ten inches, and the pressure required is three 

 inches ; then if the paper be divided into thirty parts by horizontal lines, each 

 division will indicate one-tenth of an inch. Fig. 33. represents an elevation 

 of one of these instruments. 



A is the gasometer, having double sides, as shown by the dotted lines, which 

 serves as an air-vessel to render the gasometer exactly in equilibrium 

 with the external air when totally immersed in the water. No spe- 

 cific gravity apparatus being attached, it is evident that every different 

 point of immersion will require a different pressure to cause it to rise, 

 and these being known, a correct register is obtained. 



