CrIAP. V.] 



THE STEAMENGINE. 



403 



keeps in equilibrium by a counterpoise. The motion of the pulley, 

 caused by the variations of the level of the water, communicates 

 itself to a needle, which indicates in this way the height of the water 

 in the boiler. 



In the magnetic gauge of M. Lethuilier-Pinel, which is now much 

 employed in France, the motion of the float shows itself by means 

 of a rod which raises or lowers a horse-shoe magnet : in front of the 

 poles of this magnet, a magnetized needle, movable under the in- 

 fluence of their attraction, passes over the divisions of a graduated 

 scale which marks the level of 

 the water in the boiler. When 

 this level sinks to an unusual 

 and dangerous degree, the magnet 

 carries with it the arm of a lever 

 that opens a valve, previously 

 closed by a spring. The steam, 

 which emerges freely from the 

 boiler into the tube containing 

 this mechanism, escapes and 

 whistles outside, and so warns 

 the stoker of the danger. 



The safety appliances of a 

 steam-engine are not confined to 

 the water gauges, since the causes 

 of explosion do not arise exclu- 

 sively from the insufficiency of 

 water in the boilers. Under certain 

 circumstances the steam might 

 acquire an elastic force surpassing 

 the limits of pressure for which the boiler has been constructed. To 

 prevent this, safety-valves are used, the ordinary arrangement of 

 which is represented in Fig. 276 at s, s. 



How then can we ascertain at each instant, during the working of 

 the engine, the variations of the pressure of the steam ? The instru- 

 ments which furnish this indication in atmospheres are known by 

 the name of pressure-gauges. 



The pressure-gauges employed are not all based upon the 

 same principle. Some are simply siphon barometers, whose long 



D D 2 



FIG. 277. Lethuilier-Pinel's magnetic gauge. 



