60 ANNUAL OF SCIENTIFIC DISCOVERY. 



transmission of heat is greater in cast iron than in wrought iron 

 of the same thickness, and hence greater economy in the genera- 

 tion of steam. From the construction of this boiler its shape can 

 be suited to any place. The advantages of this boiler are, that it 

 is secure from destructive explosions ; that its parts, simple, few 

 in number and small, are easily put together, repaired, or taken 

 apart; that its strength depends on the material and its form, 

 without the necessity of stays or braces ; that it is not liable to 

 corrosion or scale ; that it affords a free circulation of the water 

 and the external heat. For an account of experiments on the 

 strength and durability of this boiler under very severe tests, see 

 " Journal of the Franklin Institute," February, 1867. 



Millers American Boiler. This, as explained before the Massa- 

 chusetts Institute of Technology, in 1867, is also a cast-iron boiler 

 made in the form of tubes of moderate diameter, put together in 

 series, or in compartments of any desired size or shape. He uses 

 cast iron for the reason mentioned above, as any imperfection in 

 form or material will give timely warning by cracking, even under 

 low pressure, when fire is applied, and when it does break it pro- 

 duces a fracture without explosion ; hence, the giving way of one 

 compartment acts as a safety-valve, relieving all other parts from 

 pressure. 



The great desideratum seems hitherto to have been to make a 

 boiler exceedingly strong, as if that were the preventive of explo- 

 sions. The strength of its containing-chamber no more makes 

 steam safe than it would gunpowder or nitro-glycerine ; the true 

 way is to prevent the conditions of explosion, and not to strengthen 

 the boiler. The strength of a boiler is the strength of its weakest 

 point, and, as boilers are generally made of wrought iron, no one 

 can tell where the weakest point is. A safety-valve is no safe- 

 guard against explosion in an ordinary steam or rather water 

 boiler ; but the case is different in a generator of steam, one that 

 makes dry steam without superheating, simply changing the 

 cohesive force of water into the repellant force of steam, leaving 

 the water a dense mass free from steam. Boilers should be 

 made to make steam, and not to boil water. By apptying heat below 

 a high column of water, the vapor must force itself through the 

 dense medium and diffuse itself through tlie whole mass, not 

 escaping until the whole is charged with as much steam as it can 

 contain; and this is rendered more difficult by tubes placed at 

 right angles with the rising steam. 



Mr. Miller's improvement, in addition to the material and mode 

 of construction of his boiler, consists in increasing the circulation, 

 causing thin films of water to circulate rapidly over the heating 

 surface, the steam, as soon as made, passing to the steam space : 

 in this way the most power is got from the fuel, and the less is 

 the danger from explosion. No boiler containing only dry steam 

 and dense water can explode, if proper safety-valves be used. 

 His boiler is made on the principle that every atom of steam shall 

 be carried directly into the steam- chamber, without forcing its 

 way through superincumbent water; the heat is also prevented 

 from reaching the main body of water by an intervening shield. 



