THE BOILEE, 



evaporation is produced and maintained. The boiler is formed of 

 plates of metal, of suitable thickness, rivetted together, so as to 

 be steam-tight, that is to say, so that steam cannot be forced be- 

 tween them. 



The manner in which the plates are rivetted together is shown 

 in fig. 1, the edges of the plates being laid one upon the other 

 and their surfaces forced Fig. 1. 



into steam-tight contact by y <j<* 



rivets r r' passing through ^.^.....^.^nn,,,^^:^^^ 

 holes punched in them, the y T' 



heads of the rivets being formed by the hammer while the iron is 

 still soft by heat. 



The appearance of the rows of rivets along the edges of the 

 plates composing the boiler is shown in the general view of a 

 waggon-boiler in fig. 7. 



4. The material of the boiler is most commonly wrought iron. 

 Copper is sometimes though very rarely used. It has an advan- 

 tage over iron, inasmuch as it is a better conductor of heat, and 

 is less liable to become incrusted by lime and other earthy matter, 

 which is always held in solution by the water, and precipitated 

 in the process of evaporation. It is also more durable than iron, 

 but is excluded, save in, rare and exceptional cases, because of its 

 greater cost. 



Cast iron, though cheaper than wrought iron, would be inadmis- 

 sible for several reasons, one of which is its brittleness. If 

 explosion happened it would fly in pieces, the fragments becoming 

 destructive missiles. In case of explosion wrought iron would be 

 ripped and torn. The one is tough, the other brittle. 



5. The boiler is a reservoir not only for water but for steam. 

 The steam, being much lighter, bulk for bulk, than water will always 

 ascend in bubbles through the water, and will collect in the upper 

 parts of the boiler. The space within the boiler, therefore, may be 

 conceived to be divided at a certain level between the water and 

 the steam. All the space below that level is appropriated to the 

 water, all above it to the steam. 



But according as the water is converted into steam, the quantity 

 contained in the boiler being proportionally diminished, this level 

 would fall continually lower and lower. That, however, is pre- 

 vented by a FEEDING APPARATUS, which generally consists of 

 forcing pumps, of adequate power, by which as much water is 

 driven into the boiler as is converted into steam by the furnaces. 

 This feeding apparatus is, in some cases, worked only from time 

 to time to replenish the boiler, in other cases the supply is con- 

 tinual. In the former case, the level which separates the steam 

 from the water alternately rises and falls within certain limits. 



B 2 3 



