502 



THE STEAM-ENGINE. 



water would continually fall by reason of the excess of the evaporation over 

 the feed ; and if it were greater, the level would rise by the accumulation of 

 water in the boiler. If, therefore, the quantity of water-space allowed in the 

 boiler be five times the volume of water evaporated per hour, the quanti- 

 ty introduced by the feed per hour, whether continuously or at intervals, must 

 be of the same amount. Since the process of evaporation is continuous, 

 the variation of level of water in the boiler will be entirely dependent on 

 the intervals between the successive feeds. If the feed be continuous, 

 and always equal to the evaporation, then the level of the water in the 

 boiler will undergo no change ; but if, while the evaporation is continu- 

 ous, the feed be made at intervals, then the change of level of water in the 

 boiler as well as its change of temperature, will be subject to a variation pro- 

 portional to the intervals between the successive feeds. It is manifest, there- 

 fore, that the feed should either be uninterrupted or be supplied at short inter- 

 vals, so that the change of level and temperature of the water in the boiler 

 should not be considerable. 



Different methods have been, from time to time, suggested for indicating 

 the level of the water in the boiler. We have already mentioned the two 

 gauge-pjpes used in the earlier steam-engines, and which are still generally 

 continued. There are, however, some other methods which merit our atten- 

 tion. 



A weight F, fig. 56, half immersed in the water in the boiler, is supported 



by a wire, which, passing steam-tight through a small hole in the top, is con- 

 nected by a flexible string or chain, passing over a wheel W, with a counter- 

 poise A, which is just sufficient to balance F when half immersed. If F be 

 raised above the water, A being lighter will no longer balance it, and F will de- 

 scend, pulling up A, and turning the wheel W. If, on the other hand, F be 

 plunged deeper in the water, A will more than balance it, and will pull it up, 

 so that the only position in which F and A will balance each other is, when F 

 is half immersed. The wheel W is so adjusted, that when two pins placed on 

 its rim are in the horizontal position, the water is at its proper level. Conse- 

 quently it follows, that if the water rise above this level, the weight F is lifted 

 and A falls, so that the pins come into another position. If, on the other hand, 

 the level of the water fall, F falls and A rises, so that the pins assume a differ- 

 ent position. Thus, in general, the position of the pins becomes an indication 

 of the quantity of water in the boiler. 



Another method is to place a glass tube, fig. 57, with one end T entering 

 \ the boiler above the proper level, and the other end T' entering it below the 

 1 1 proper level. It must be evident that the water in the tube will always stand 



