of all known Substances to the Magnetic Injluence. 333 



inches asunder, we have an extent, it is observable, of influence 

 very nearly equal to the sum of the forces of both bars taken se- 

 parately. For all practical purposes, we may therefore infer, 

 that two well-tempered magnets, placed parallel to each other, 

 and a few inches asunder, will exert the same force as the sum 

 of their separate influences ; and that at all distances beyond 

 twice the length of the bars, the angle of deviation produced 

 by the two, so combined, will be almost precisely the same as 

 the amount of their individual deviations, or twice as great as 

 either separately. 



By means of a set of ten equal bars (\^^^ inches long), be- 

 longing to a straight-bar compound magnet, I made a very ex- 

 tensive series of experiments on the relative influence on the 

 compass of several bars in juxtaposition or contact, when arran- 

 ged in a great variety of ways. I shall only state briefly the 

 results obtained with six of these bars. Their separate directive 

 power on the compass, at the distance of 4 feet, was 3° 45', 

 4° 15', 4°3(y, 4°l(y, 4°3(y, and 3° 50.' When placed in the 

 arrangement a. Fig. 3, ^th of an inch apart, their united power 

 produced a deviation of 15° 54' ; in the manner of b, 16° 40' ; in 

 the manner of c (all at the same distance from the compass, as in 

 case b, but disjoined), 15° 0' ; of d, 16° 10' ; of e, 16° 35' ; off, 

 (the same form as e, but touching laterally), 16° 43' ; of g^ 

 13° 30', &c. &c. 



After the experiments, according to the arrangements from 

 a to n. Fig. 3 Plate VI., were completed, the six bars were 

 placed in lateral contact, as at o, when their power, which the 

 instant before, in the arrangement a, was 15° 54', was now re- 

 duced to 12° 30' whilst in contact^ and being restored to the 

 position a, was 13° 54', exhibiting a permanent injury by the 

 contact equivalent to the difference of the tangents of 15° 54' 

 and 13° 54'. The powers of the different bars in juxtaposi- 

 tion, as in Fig. a, before and after contact, were : — 



