424 HENRY A. BOWLAND 



But in neither of these methods does Him seem to have recognized 

 the principle of the work transmitted by a shaft being equal to the 

 moment of the force multiplied by the angle of rotation of the shaft. 

 In designing his apparatus, he evidently had in view the reproduction 

 in circular motion of the case of friction between two planes in linear 

 motion. 



Since I designed my apparatus, Puluj 42 has designed an instrument 

 to be worked by hand, and based on the principle used by Him. He 

 places the revolving axis vertical, and the friction part consists of two 

 cones rubbing together. But no new principle is involved in his appa- 

 ratus further than in that used by Him. 



In my apparatus one of the new features has been the introduction 

 of the Joule calorimeter in the place of the friction cylinders of Him 

 or the cones of Puluj. At first sight the currents and whirlpools in 

 such a calorimeter might be supposed to have some effect; but when 

 the motion is steady, it is readily seen that the torsion of the calorimeter 

 is equal to that of the shaft, and hence the principle must apply. 



This change, together with the other new features in the experi- 

 ments and apparatus, has at once made the method one of extreme 

 accuracy, surpassing all others very many fold. 



2. GENEBAL DESCRIPTION 



The apparatus was situated in a small building, entirely separate 

 from the other University buildings, and where it was free from dis- 

 turbances. 



Fig. 6 gives a general view of the apparatus. To a movable axis, ab, 

 a calorimeter similar to Joule's is attached, and the whole is suspended 

 by a torsion wire, c. The shaft of the calorimeter comes out from the 

 bottom, and is attached to a shaft, ef, which receives a uniform motion 

 from the engine by mean's of the bevel wheels g and Ji. To the axis, 

 ab, an accurate turned wheel, M, was attached, and the moment of 

 the force tending to turn the calorimeter was measured by the weights 

 o and p, attached to silk tapes passing around the circumference of the 

 wheel in combination with the torsion of the suspending wire. To this 

 axis was also attached a long arm, having two sliding weights, q and r, 

 by which the moment of inertia could be varied or determined. 



42 Pogg. Ann., clvii, 437. 



"Joule's latest results were published after this was written, and I was not aware 

 that he, had made this improvement until lately. The result of his experiment, how- 

 ever, reached me soon after, and I have referred to it in the paper, but I did not see 

 the complete paper until much later. 



