702 TRANSACTIONS OF THE 



iron, as this is a rotten and brittle material, and becomes blistered 

 in casting and is liable to sudden fracture. Copper is better than 

 iron for this purpose, but having less cohesive power than iron, 

 a greater thickness of metal is necessary to produce an equal 

 strength. Copper never flies when the boiler bursts, but merely 

 tears asunder and causes far less destruction. Boilers should, for 

 many reasons, be cylindrical, and have spherical heads. This 

 form withstands internal pressure best, the strain being equal on 

 all points of the circumference. Sometimes we see fire-tubes of a 

 prismoidal form introduced into high pressure boilers, a plan 

 fraught with the greatest danger. Boilers are generally of too 

 large a size. The greater the contents of a boiler, the greater 

 surface it must offer to the pressure of the steam, and the greater 

 consequent danger it must be subject to. It is customary to give 

 to boilers of great size a proportionate thickness of metal, but this 

 does not help the case much, for experience has shown that thick 

 plates, especially of cast metal, are more liable to crack by the 

 action of fire than thin ones, because the temperature of their too 

 sides, exposed to fire without and water within, does not quickly 

 assimilate, whereby unequal expansion and contraction ensues, 

 and thick vessels tend more to retard the transmission of heat to 

 the water than thin ones, which fact seems to have escaped the 

 notice of engineers. Another objection against the high pressure 

 engine is, that in the use of high pressure steam much heat is 

 wasted, and consequently a greater expenditure of fuel is required 

 than for low pressure. The fallacy of this opinion has been fully 

 shown, not only in our country but in France. A current heated as 

 high as 400 degrees Fahrenheit is necessary to produce a sufficient 

 draft in most chimneys; low pressure engines must then be sub- 

 ject to a greater waste than high pressure, in order to secure the 

 draught in the furnace. 



Mr. Christian, of Paris, to whom we are much indebted for 

 many researches in the theory of steam, found that equal quanti- 

 ties of water were evaporated in equal times, by the same fire, 

 under various pressures and temperatures. Engines will work 

 with the least consumption of fuel wlien the throttle valve is 

 almost closed, as it raises the pressure in the boiler higher than 



