1826.] during the Act of Rotation, 445 



who gave me permission to have a 13 inch mortar-shell fixed to 

 the mandrel of one of the powerful turning lathes worked by the 

 steam engine in the Royal Arsenal. This having been done, and 

 the compass properly placed near the shell, I turned the shell 

 ■slowly round, in order to ascertain whether in this case, as in 

 -Mr. Christie's, there were any effects depending on a change of 

 position ; but if there were any, they were so small in the cast-iron 

 shell as not to have been rendered sensible with the small com- 

 pass I employed. The wheel being now put in geer, the shell 

 commenced its revolutions at the rate of 640 per minute, and 

 the needle was deflected out several degrees, at which it 

 remained perfectly stationary while the ball was in motion ; but 

 it returned immediately to its original bearing as soon as the 

 motion ceased. 



" I now inverted the motion of the shell, and the needle was 

 deflected about the same quantity the contrary way, observing a 

 similar steady direction as in the former case ; but as before, it 

 returned to its original bearing the moment the motion was dis- 

 continued. 



" I afterwards found, that the needle being placed in different 

 situations, its motion was reversed, although the direction of 

 motion in the shell was the same; the amount of the whole 

 deflection also differed very considerably according to the situa- 

 tion of the compass, its direction in some cases having been 

 wholly reversed, while in others no perceptible motion was pro- 

 duced, although the rotation of the shell remained the same both 

 in direction and in speed. 



" I was, therefore, desirous of undertaking a regular set of 

 experiments, in order to reduce the several apparently anomalous 

 results to some certain law of action ; and as the shell in ques- 

 tion was rather too heavy for us to feel a perfect security, as to 

 personal safety, when it was in rapid rotation, and, moreover, as 

 its effects were larger than seemed necessary for the purpose, I 

 now selected a Shrapnell shell of eight inches in diameter, 

 which weighed only 30 lbs. and chose another lathe, whose axis 

 was nearly north and south, that in the former instance having 

 been east and west. I had also a table made with a circular 

 hole in it, which I could place at any height above, below, or 

 about the centre of the ball ; I could also set my compass on 

 any azimuth on the same, and observe the effects of the direct 

 and reversed motion; but after several days' observations, I 

 found the results so complicated, and the needle so much 

 influenced by the iron-work of the lathe and other machinery, 

 that it would"^ be useless to proceed, unless I could contrive to 

 produce the rotation out of the way of any disturbing cause of 

 the kind above-mentioned." 



This Mr. liarlow was enabled to accomplish by means of a 

 machine delineated in Plate XXXIX, and thus described : 



