74 



On the Strength of Cylindrical Steam Boilers. 



table at p. 27 of his "Young Steam Engineer's Guide," he has given 

 calculations for seventeen different diameters of boilers, with the 

 power which, at each diameter, the steam would exert " to break 

 every ring of one inch wide in any one place," and " the thickness 

 of the sheets of good iron necessary to hold the power." His table 

 is formed on the supposition that sheet iron will bear 64,000 lbs. to 

 the square inch, and would consequently lead to considerable excesses 

 if strictly applied in practice. To six of the diameters he has an- 

 nexed the "power exerted on the heads to burst them out, in pounds 

 weight." These he has calculated in the usual manner, by multi- 

 plying the area by the pressure per inch. Opposite to three of the 

 numbers just mentioned, he has added "the strength of the boiler to 

 hold the head on, in pounds weight." These he has calculated on 

 the supposition that the metal had equal tenacity in all directions. 

 On this supposition, and on the principles above developed, each of 

 those three numbers should have been exactly double of that against 

 which it stands in the preceding column. Neither of the three is so, 

 precisely; but the first and third come as near it as could be expect- 

 ed, considering that the thickness is expressed only in hundredths of 

 an inch, while the second is too small by more than a million of 

 pounds. These errors would not, I apprehend, have occurred had 

 the author adverted to the general principle above developed, in re- 

 gard to strength required of the metal in the two directions. 



The following extract from the table just alluded to, will illustrate 

 the preceding remarks : a column of corrected results has been added. 



The very general use, in this country, of strong cast iron heads, 

 fastened to the wrought iron cylinders by broad flanches extending 

 some inches widiin the latter, there riveted and subsequently further 

 secured by a strong wrought iron hoop, driven on when hot and 

 shrunk by cooling, — appears to obviate the necessity of examining 

 the question in regard to the best form and necessary thickness of 



