OF ARTS AND SCIENCES. 



375 



[Note. — The reader is requested to attend particularly to the 5th and 6th col- 

 umns of this table, as these give the errors of the instrument, and of the computa- 

 tions.] 



Now how is all this ? Why, Captain Rodman in his computations 

 took, not the coefficient of the true tenacity of 27,000, as laid down by 

 Barlow, but the tenacity which his instrument gave for the cylinder 

 one inch thick. Thus taking at 38,313 what should have been 13,500. 

 Not only so, but the law of the inverse square, which he used, and 

 which is Barlow's rule, makes it impossible that any of his last instru- 

 mental measures should be nearly right, even if the tenacity be in- 

 creased from 27,000 up to 76,626, which is the number that he in fact 

 uses. For, according to this law, a cylinder of two inches' internal 

 diameter, made of cast-iron, or any unmalleahle material, of 76,626 

 pounds' tenacity, can never be made thick enough to sustain a pressure 

 of 76,626, or at least it cannot be done until some one shall contrive to 

 make 1 X^=l-l-^jOrl multiplied by 1 equal to 1 added to 1, a 

 feat which Captain Rodman's instrument must have performed in giv- 

 ing several of the results above tabulated. 



I do not think it will be necessary to pursue this instrument any 

 further to show its utter worthlessness. It must be given up, and the 

 whole family of results born of it must go with it.* 



* I shall be excused for citing one other case, although it may be thought, by 

 some, unnecessary. Captain Rodman gives, at page 197, in his table, the force 

 produced by firing 12.67 pounds of powder in an 11 -inch gun, behind a shot of 

 186.4 pounds' weight, at, in one instance, 100,000 pounds. Now this must have 

 given a bursting pressure upon the walls of the gun of 1,100,000 pounds to every 

 inch of its length ; consequently, if the gun was 11 inches thick, the iron to sustain 

 this pressure must, if Barlow's formula be true, (and Captain Rodman makes no 



