Lifting Power of the Electro-Magnet. 



oil 



manipulation is easier tlian in mine, and the accuracy superior, but the concus- 

 sion is greater. The hollow cylinders receive copper vessels, which may be 

 filled with hot water or ice, and their magnitude is sufficient to preserve a 

 nearly uniform teniperatui-e for some lime. Tlie experiments were, unfortu- 

 nately, much disturbed by the j)erpetual passage of carriages through the street, 

 which caused the loss of many results ; indeed we could only work during the 

 night, and even then had much disturbance. The temperature was measured in 

 the middle of the keeper, the middle of the base, and the top of the cylinders, 

 and the mean taken, i^was kept at 0-4734, giving -f^ 143-91 ; J" is the mean. 



Table III. 



The third column shows that the value of Z diminishes as the temperature 

 increases ; becomes a minimum at about 75° ; it then increases to the highest 

 temperature in the Table ; aftQr that it would certainly again decrease ; but 

 we could not easily, with tliis arrangement, pass 127". If there be no subse- 

 quent maxima and minima, these facts imply a formula such as 



L = L'\l-at^-he-ct'\, 

 which, in fact, is the form given when the coefficients are determined by 

 Cauchy's method of interpretation. 



By this, reckoning t from 60^, is derived 



