While the action of the feeding apparatus is suspended it falls 

 gradually as the evaporation proceeds. When it has descended to 

 a certain point the feeding apparatus is put in action and the level 

 rises again to its former limit, after which it is again suspended, 

 and so on. This rise and fall of the level of the water in the 

 boiler is, or ought to be, restrained between such limits, that tho 

 level is never either injuriously high or injuriously low. 



When the feeding apparatus works incessantly, the water in tho 

 boiler is kept always at the same level, the arrangements being 

 such that by a self-adjusting mechanism, the quantity of water 

 supplied to the boiler, from minute to minute, is exactly equal to 

 the quantity evaporated. 



6. The importance of keeping the boiler duly supplied with 

 water will be easily understood. So long as those parts of tho 

 boiler which are exposed to the action of the furnace are filled 

 with water the metal can never become unduly heated, because all 

 the heat imparted by the furnace is absorbed by the water in 

 evaporation. But if the level of the water were allowed to sub- 

 side below any part which is exposed to the action of the furnace, 

 the heat acting upon such parts not being taken up by the water, 

 and the steam which in that case would alone be in contact with 

 them, being a slow recipient of heat, the plates of the boiler would 

 soon become red hot, and would consequently be softened, so as 

 no longer to possess the strength necessary to resist the pressure 

 within them, and the boiler would burst. For this reason, it is 

 always of the utmost importance to provide means to ensure such 

 a supply of water as shall prevent the level from ever falling 

 below the highest parts upon which the furnace acts. 



Inconvenience of a different kind would be produced by over- 

 feeding, and consequently by raising the level of the water above 

 a certain limit. When the water in a boiler is in a state of strong 

 ebullition, which it always is in the boilers of engines in full 

 operation, bubbles of steam are produced in great quantities in 

 the lowest parts, these being the parts upon which the action of 

 the furnace is most energetic. These bubbles, rising with violenca 

 to the surface, throw up the water in spray, so that the part of 

 the boiler above the level of the water is filled with a mixture of 

 pure steam and of particles of water in minute subdivision. The 

 latter, however, fall back into the water by their gravity, provided 

 that the space left for the steam have sufficient height. The 

 upper part of that space will then be supplied with pure steam 

 without intermixture with spray. But if the boiler be over-filled 

 with water, so that the space left for the steam have so little 

 height that more or less spray is mixed even with the highest 

 parts of it, this spray will be drawn into the working part of the 





