MAGNETIC INTENSITY, FERN ROCK OBSERVATORY. 65 



The value of the induction coefficient 



p r'^Ti sin. Ml — r^r^ sin. u 



Ti sin. Ui — r' sin. u 

 may be put in the following convenient form — 



P= — r^ i where a = — '- — and e=— . 



a — g sm. u i\ 



We find: June 7,1854 p = _0.00'7 



" 8, " —0.003 



" 8, " —0.006 



" 19, " +0.009 



" 19, " —0.003 



" 24, " —0.001 



" 24, " [-+0.033 1 



May 16, 1855 i +0.035 I 



" 17, " [+0.039 J 



" 18, " —0.011 



" 19, " —0.011 



If we take the indiscriminate mean of the above values we find P= + 0.007, 

 and if we reject the three values marked by brackets, P = — 0.004; the latter 

 value is probably nearer the truth than the first one, but both are so small that 

 they may be neglected in the computation of the intensity. 



In the absence of observations, the temperature coefficient for the magnetic 

 moment or q may be assumed =0.0003, a value found for other magnets of the 

 same magnetic moment and size ; with but three exceptions, the temperature cor- 

 rections are small. 



After correcting for difference of temperature, the following results for magnetic 

 moment m and horizontal intensity X have been computed by the formula3 



— =^i r sm. u and m X = -—-. 



