520 The Rev. T. R. Robinson's Experimental Researches on the 



From these is deduced by Caucht's interpolation — 



L ^ 212-43 \l-txL-' (6-31639) + fx Lr' (4-28451) - «' x Ir^ (2-19549) j, 



in which t= 7—60. The fourth column shows the excess of the observed 

 above the calculated value. The coefficients differ widely from those for a 

 2— inch magnet of the same length, for which the logarithms would be — 



7-23121; 5-33385; 2-92184. 



A series was then taken of the L corresponding to various values of •^. 

 For the three highest, N, N' were used collaterally with 0, O'. 



Table XXX. 



The ratio of this magnet to the 2-inch one may be derived from No. 65 

 in Table xiii. In that the arms are 8'-l, nearly the same as here, and the effi- 

 ciency of the helices is not very different, their diameters being 2'-26 and 3'-93. 

 The 1^ is 547 -65, which I find, by interpolation, would give with the small 

 magnet 207-94. The actual Z = 765-12; so, with four times the section, it is 



