2o2 Messrs* Babhage and Her schel, and [Oct. 



subordinate poles in the shell, whose places would shift by 

 rotation. The compass, therefore, in this case cannot be neu- 

 tralized by a magnet ;* but we must have recourse to some 

 neutral system, such as that described in the foregoing pages, 

 in its place, or it may be left unneutralized. It ought too to be 

 so small, or so remote, as not to produce induced polarity in the 

 fihell, which would react on itself when the sphere is set in 

 motion, and destroy the success of the experiment. 



The effect of a solution of continuity in the revolving bodies 

 ootnes next to be considered. It is difficult ; but the difficulty 

 is not a consequence of our principles of explanation, but of our 

 ignorance of the very complicated laws which regulate the dis- 

 tribution and communication of magnetism in bodies of irregular 

 figure. So far, however, as the operation of the general principle 

 xan be traced, its results are consonant to observation. 



In the first place, it is obvious that where one or more slits 

 sre cut in a metallic plate, over which the pole of a magnet is 

 revolving, that immediate and free communication between 

 particle and particle, on which probably the rapid, and certainly 

 the intense developement of magnetism depends, is destroyed. 

 The induced pole (by which we mean now the whole of that 

 space in which sensible magnetism is developed, and which is, 

 of course^ a spot of sensible, and probably considerable magni- 

 tude — of a -figure more or less elongated according to the velo- 

 city of the r/iotion) — instead of travelling regularly round, retain- 

 ing a constant magnetism and force, will now be in a perpetual 

 state of change. Instead of being carried uniformly across the 

 sht, it will die away in intensity, and shrink into a point in 

 dimension on the hinder side, and be again renewed on the side 

 ill advance, but at first not in its full intensity ; so that it is not 

 merely the diminution of surface arising from the abstraction of 

 a part of the metal, but a much more considerable defalcation of 

 magnetic force which takes place on either side of the slit, that 

 operates. Now this operation is always to weaken the drag 

 between the magnet and the disc, and no reason, a priori, can 

 Jbe assigned why this efiect should not take place to any extent. 



The validity of this reasoning is shown by taking the extreme 

 case in which the substance acted on is in the state of powder. 

 Each particle of this becomes necessarily a feeble magnet, and 

 its north and south poles, being at the same distance (almost 

 precisely) from the pole of the magnet, counteract each other's 

 action. The extreme feebleness of their magnetism prevents 

 the particles from affecting each other by induction across the 

 intervals which separate them ; so that each acts as an indivi- 

 dual, and destroys in great measure its own effect. The moment, 



• In Mz. Barlow's experiments, the large and powerful bar-magnets used to neutral- 

 ize the earth's action pn the compass-needle, cannot be without some disturbing influence 

 i>f this kind. 



