Polytechnic Associatiox. 761 



which the sheets are cut to a uniform size and brushed over with a 

 mixture of birch-charcoal powder and water. From seventy to 100 

 sheets are then brought together in one packet and placed in a heating 

 chamber of peculiar construction, where the temperature is slowly 

 raised for several hours, while oxidation is prevented. The packets 

 having been thus heated are placed under a tilt-hammer. The sheets, 

 after being hammered, are alternately inserted between finished sheets 

 and again subjected to the hammer. This final operation removes 

 the wavy appearance resulting from the first hammering, and pro- 

 duces a smooth and polished surface. "When the sheets have been 

 again cleaned with the wet broom and cut to the standard size, they 

 are ready for the market. The extreme ductility and highly polished 

 surface, by which this sheet-iron is distinguished from all other varie- 

 ties, is said to be chiefly the effect of a peculiar kind of charcoal 

 powder applied as described. The facts set forth by Dr. Percy have 

 been known for some time to American manufacturers of sheet-iron, 

 yet they have not succeeded in giving their product the peculiar dark- 

 blue glaze invariably found on every sheet made in the great iron- 

 works of the Ural Mountains. 



Mr. Dudley Blanchard — It is not at all strange that American 

 genius has not succeeded in producing such sheet-iron as we get from 

 Russia. The secret, and all the secret, is, that there they have cheaper 

 labor. 



Steam Boiler Explosions. 



Mr. Blanchard — I was very much gratified by the report of the 

 experiments on boilers, which we have had before us, from the fact 

 that it places the ordinary common-sense view of the subject in a 

 favorable light ; for the boilers were shown to be burst from over- 

 pressure. One boiler yielded suddenly and at once in a great many 

 places, very much like the " one-horse chaise " described by Dr. Oliver 

 Wendell Holmes in a poem, which was so equally strong in every 

 part that when, at last, it failed, it all went to dust at once. (Laugh- 

 ter.) This boiler was so well constructed that when the pressure rose 

 to a sufficient point, the joints began to leak in a great many places, 

 and it could not be much of an explosion. 



There was one remark made in connection with the report, which 

 I think ought not to have been made ; that a boiler may stand a cer- 

 tain hydraulic pressure, and afterward, when exposed to steam pressure, 

 burst at less than the hydraulic pressure. You might as well say 

 that after filling a jug with two quarts of water, you might afterward 

 find that it would hold but three pints. 



