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lation that they follow the law of inverse proportion, the same as 

 those of larger dimensions, the strengths diminishing as the lengths 

 are increased. 



The same law of resistance is maintained in 6-inch tubes, giving, 

 for a tube 30 inches long, 55 Ibs., and for one 59 inches long only 

 32lbs. on the square inch, as the pressure of collapse. Again, in 8- 

 inch tubes we have, in a long series of experiments, 32 Ibs. per square 

 inch in a tube 39 inches long, and 39 Ibs. in one 30 inches long. In 

 the same manner all the experiments on tubes of 10 and 12, and up 

 to 18 inches in diameter may be compared, and the law of resistance is 

 in like manner shown to hold true in every case. Discrepancies to 

 a certain extent do certainly occur ; but they are comparatively small, 

 and, as they appear to follow no law, are evidently to be accounted 

 for from defects in the construction of the tubes inseparable from 

 such a mode of research. 



2. The strength as affected by the diameter. 



A precisely similar law is found to hold in relation to the diameter. 

 Tubes similar in other respects vary in their resistance to collapse 

 inversely as their diameters ; and with a view of testing this law, we 

 may place the calculated pressure beside that derived from experi- 

 ment, as under : 



Resistance of tubes to collapse 5 feet long. 



The above variations are slight when compared with the resisting 

 powers of the tubes ; they are doubtless caused by the varying rigid- 

 ity of the iron, or by defects in the cylindrical form. Similar results 

 follow in the experiments on tubes 2 feet 6 inches long ; and although 

 some slight variations occur, they are nevertheless not more than 

 might have been anticipated within the ordinary limits of error. 



3. The strength of tubes as affected by thickness. 



In these experiments it is found that the tubes vary in strength 

 according to a certain power in the thickness ; the index of which, 



