OF ARTS AND SCIENCES. 873 



within 1,000 pounds in cases, which he supposes to have been of press- 

 ure, of 90,000 or 100,000 pounds. He seems, indeed, to have had no 

 suspicion of any error when his instrument gave indications of force 

 wholly incompatible with the strength of the materials to which it was 

 applied. Thus, at page 197, he takes the force which acted upon it at 

 100,000 to the square inch. Now the instrument in this case was held 

 to the gun by a screw, formed into the cast-iron body of the gun, 1^ 

 inches in diameter and 1^ inches deep. This gives an area of the plug 

 of the instrument of 1.75 square inches, which received the full press- 

 ure of the fired powder. The pressure upon the end of the instrument 

 then was 1.75 X 100,000 = 175,000 pounds, or about 80 tons. He 

 must be a very bold engineer who would sleep under a weight of 10 

 tons suspended over him by a bolt tapped into a hole, in a cast-iron 

 plate of 1^ inches in diameter and 1^ inches deep ; and yet it does not 

 seem to have occurred to Captain Rodman that 80 tons' pressure must 

 have driven his instrument from its place. 



But let us go a step further. Captain Rodman, at page 192 et seq., 

 relates a course of experiments which may be given in short hand as 

 follows. He made 18 cylinders, of three different kinds of iron. They 

 were all one foot long, and all bored out through their entire lengths with 

 calibers two inches in diameter. They were then turned ofi" on their 

 outsides, so as to leave them of six different thicknesses, varying from 

 each other from half an inch up to three inches ; the variation being, of 

 course, by increments of half an inch. Each kind of iron gave one cyl- 

 inder of each thickness. The mean strength of the iron, as ascertained 

 from specimens of each casting, being 26.866, say 27,000 pounds per 

 square inch. These cylinders were then subjected to a bursting force 

 produced by firing gunpowder confined within them. This was done 

 for the purpose of ascertaining the power of cast-iron of these different 

 thicknesses to withstand the force thus produced. The force was 

 measured, by Captain Rodman's instrument, of course, and the first 

 two columns in the following table give the strength of each thickness of 

 metal according to Captain Rodman's mean : — 



^ 37842 25541 



1 38313 38313 

 1^ 63384 46057 



2 80229 51085 

 2J 92270 54732 



3 93702 57468 



