and hollow Balls on a Magnetic Needle. 7 



of iron bodies, that, although (according to the results of some 

 of my first experiments) the attraction of a solid ball and hol- 

 low shell have the same power on the needle when at rest, 

 yet, if they are both put in rapid motion, their attracting 

 power is no longer equal, notwithstanding their diameters, dis- 

 tance, and velocity of rotation, be precisely the same. This 

 very singular result seemed to furnish the means of submit- 

 ting this theory to the test of an experimentum cruris, and 

 Mr Babbage being in Paris at the time this deduction was 

 made, he wrote to me, at the request of M. Poisson, to make 

 the experiment. 



I accordingly procured one of our largest iron balls, a six- 

 ty-eight pounder, 7.87 inches in diameter, and a hollow sphe- 

 rical case shot, of exactly the same diameter, weighing just 

 one half the former, or thirty-four pounds ; but as I could 

 not conveniently suspend the solid ball in the form I had 

 adopted in my former rotatory experiments, I had another ap- 

 paratus constructed, which is shown in Plate I. Fig. 2, where 

 ABCD is a thick plank secured well to the floor, W a long 

 wheel turning on its axis, and w a small wheel fixed to an up- 

 right axis, to the upper part of which was screwed a short 

 wooden cylinder, having a hemispherical cup exactly the dia- 

 meter of the ball and shell, and into which they were placed 

 when submitted to rotation. The floor was cut away beyond 

 AB, and an upright prop FE was driven securely into the 

 earth, and the table FG fixed to the upper part, projecting 

 over the ball upon which the compass c was placed, which was 

 thus relieved from any shaking from the motion of the ball or 

 shell. This apparatus was fixed on the magnetic meridian, 

 and, in order to increase the apparent effect, the needle was re- 

 duced in directive power by a powerful bar magnet placed in 

 the meridian beyond the table. 



Every thing being thus arranged, the following experi- 

 ments were made : — 



