Evaporation, and Steam-Boiler Ecxploaions. 33 



supply when the engine stops. The water in the boiler goes 

 on boiling until all the air-bubbles are boiled off from the 

 water, and their air is mixed with the steam above. There 

 then ceases to be any evaporating surface, except that on the 

 top layer, which is farthest from the heating surface, and 

 quite inadequate to the consumption of all the heat supplied. 

 Then the mass of water begins to heat up, and it goes on 

 storing up the unconsumed caloric, until the water is far 

 hotter than the head of steam would indicate. The engineer 

 then starts the engine ; this starts the pump, which throws 

 a stream of air-charged water directly into the glowing fluid. 

 The heat instantly finds its outlet by an overwhelming eva- 

 poration on the newly supplied bubble surfaces, and a tumul- 

 tuous ebullition follows. The gathered store of heat flashes 

 off a portion of the water into steam of excessive tension — a 

 tension such as nothing can withstand. The terrific conse- 

 quences are too often witnessed in those fatal catastrophes 

 which have given to our western rivers such a tragic repu- 

 tation. No one can examine a list of western steam-boat 

 explosions without being forcibly impressed with the fre- 

 quency of these accidents just as the boat is starting from 

 the wharf, after a landing. It seems to me beyond doubt 

 that many of these occur just in the manner now stated, 

 and from the deficiency of air-bubbles in the boiler. We see 

 in this reasoning, too, a sufficient explanation of dry steam, 

 or steam hotter than its tension indicates. The heating is 

 then going on faster than the evaporation, and the steam is 

 thus heated as if it were not in contact with the water, or 

 were in a vessel by itself. 



It is not always that the remedy for a danger is as obvious 

 and as easily applied as in this case. It is only necessary to 

 keep the pump in steady, slow operation, while the engine is 

 at rest. It should always be capable of an independent 

 movement, and should constantly, while a boat is fired up, 

 be kept at work, however slowly. By this means air for 

 ebullition will always be supplied, and the accumulation of 

 heat in a sluggish mass of water cannot then go on until 

 the explosion point is reached. The field over which I have 

 thus rapidly traversed is one requiring much patient study 



VOL. LVI. NO. CXI.— JANUARY 1854. C 



