AMERICAN INSTITUTE. 579 



will not burst a cannon if it, is burned gradually, but a small 

 quantity would if ignited suddenly. 



These explosions generally occur when the boiler has been at 

 rest for a few minutes; the steam is then rapidly taken from it; 

 when set in motion, the hot surface is covered with particles of 

 water, thrown in the form of spray, this by expansion forms a 

 quantity of steam, which, while endeavoring to make its escape 

 will burst a boiler of very great strength, in spite of safety valves 

 or fusible metal. The accumulation of heat must be prevented 

 by not overloading the boiler, and by never entirely closing the 

 safety valve, when the engine is resting. This small quantity of 

 steam constantly escaping will keep the water in the boiler always 

 in motion, and prevent damage. Low water in a boiler is dan- 

 gerous. 



Incrustation in a boiler diminishes the capacity, wastes fuel, 

 and eventually destroys it. It is caused by substances precipi- 

 tated by water, all of which contain to a greater or less degree 

 carbonate of lime, sulphate of lime with other alkaline earths ; 

 also sahs of the oxides of lead, manganese, and iron, which are 

 precipitated and cover the interior of the boiler. It is very diflB.- 

 cult to find a perfect remedy against this great evil : lumps of 

 charcoal thrown in the boiler prevent the formation to a certain 

 extent, by condensing the substances within its pores. Potatoes, 

 Indian meal, clay free from sand, a small portion of molasses re- 

 newed every two months, soda or potash, lUO grains monthly in a 

 boiler 17 feet long. 



Boilers may be made by crossing the joints and double rivetting, 

 that will bear 34,0U0 pounds to the square inch. Flat surfaces 

 in boilers are not so objectionable as has been supposed, but may 

 be made by staying the strongest part of the construction. 



Sufficient care is not generally taken in the preparation of the 

 bottoms of boilers, which should be in the torm of an anchor, 

 and kept entirely free from incrustation. 



When a want of water occurs in a boiler, and the flues are red 

 hot, the fire must be at once extinguished, and the engine kept in 

 motion, without on any account touching the safety valve, as by 

 so doing the steam would escape quickly, set the water in ebul- 

 lition and cause it to fall like rain on the red hot surface, and im- 

 mediately expand into volumes of steam, or its constituent gases, 

 and thus produce an explosion. 



Boilers are certainly dangerous, as terrible accidents have 

 occurred when there was neither steam nor water in the boiler. 



