383 Dr. Tyndall on the Laws of Magnetism. 



the attraction of No. 1 is in those cases precisely Jive times that 

 of No. 3. The same coincidence is exhibited in the following 

 table : — 



Ball No. 1. Ball No. 3. 



W. tan^S. tan/S. W. 



During the latter experiments each ball was separated by a 

 fixed distance of gjijtli of an inch from the pole. We see that the 

 corresponding magnetic powers are nearly equal throughout, but 

 that the corresponding weights are in the constant ratio of 5:1. 



§6. 



Proposition IV. — To determine the geiwal relation between 

 force and distance, that is to say, the laiv according to which the 

 magnetic attraction decreases when the distance is ina'eased^. 



34. There was some little difficulty in applying our mode of 

 experiment in the present case. Did we resort to the method of 

 laymg on weights or of pouring in shot, nothing could be appa- 

 rently easier than to determine this law. We should simply 

 have to preserve the magnetic power constant ; to place, by means 

 of the interposed leaves, various intervals between the sphere and 

 magnet, and to determine in each case the weight necessary to 

 bi-eak the hold. We have, however, already stated our objec- 

 tions to this method. The plan of proceeding will perhaps be 

 rendered plainer by reference to a method sometimes adopted to 

 determine the melting-point of wax or fatty matter. A little 

 glass tube containing a portion of the matter is dipped into water 

 of a known temperature — it does not melt. It is now dipped 

 into water of a higher temperature — it melts. It is evident that 

 the melting-point lies somewhere between these two temperatures, 

 and that by approximating the temperatures of the fluids, the 

 exact melting-point can at length be obtained. Our proceeding 

 with the magnet was precisely similar. A magnetic power due 



* Some little difficulty may be encountered in tlie attempt to separate 

 the third proposition from the fourth. This will vauisli when it is con- 

 sidered, that in the former case a constant force (a weight) operated against 

 the magnet, and the question was one heX,vie.&a. magnetism swA. distance; in 

 the latter case, the magnetism is preserved constant, and the question is 

 one between weiffkt and distance. 



