62 



WRITINGS OP JOSEPH HENRY. 



[1832 



o'clock, an uncommonly high degree of magnetic intensity, it now 

 showed an intensity considerable lower than usual. 



Observations were also made on the 20th and 21st, but no 

 disturbance was again noticed; the intensity had resumed 

 its former state. 



The following table exhibits the observed times of three 

 hundred vibrations, with the mean temperature and aspect 

 of the weather during each observation : 



The above observations may be reduced approximately to 

 the uniform temperature of 60°, by the formula, 

 T=T[1 ±0.0001 65(<'±/)l,* 



{T being time, t temperature in degrees of Fahrenheit,) 

 which was deduced from experiments on a similar needle. 

 The relative intensities may also be readily calculated, since 

 they are reciprocally as the squares of the times of the vibra- 

 tions. In this way, by assuming as unity the time observed 

 on the 20th, we have the following results: 



From the mean of several observations made with this 

 needle in April, I consider its time of three hundred vibra- 

 tions for this month, and in an undisturbed state of terres- 



* This formula was obtained by Hansteen. 



