•512 The Kev. T. K. Robinson's Experimental Researches on the 



Z, = 842 . 54 i 1 - < X log-'(7-35439) + t-x log-'(5-78313) 

 -<^x log-' (3-46064)). 



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The fourth column contains the difference between the observed L and 

 that calculated by this formula ; the discordance is considerable, especially in 

 the three first, but not greater than might be expected under the circumstances, 

 and the errors being often effected with contrary signs shows that they are 

 casual. I may also remark, that the current was not reversed in these obser- 

 vations. The difference between the correction given by this formula, and the 

 change of L which I had obtained with the hollow two-inch magnet, showed 

 the necessity of instituting similar experiments for that which I was using in 

 the present series ; and T found that by surrounding it and its slide with a 

 covering of thick cloth, and the keeper with a similar one, I coidd raise the 

 temperature to 220", the limit of the thermometer which I used, by placing a 

 gas flame on the base. The slide is the hottest ; but its temperature, that of 

 the cylinders at their top, and that of the keeper, were taken, and the mean 

 deduced by giving each weight as the length of the piece. 



The first was with the cylinders = 0'- 15, and the spirals (^ ) with i/r=170-79. 

 I obtained 



Table IV. 



whence I similarly deduce 



L = 618-96 [l-t. log-' (7-03890) + 1' . log-' (499503) 

 -<Mog-' (2-43656)}. 



Secondly. With the cylinders = 2'-l, the helices (B) and yjr - 553-75. 



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