MAONETIC INTENSITY. 



505 



sistently with this supposition, the loss has been distributed through 

 the first half, or twenty-nine months, of this voyage, in the propor- 

 tion of three-tenths of a second per month in the first ten months^ 

 commencing December 1st 1831 ; two-tenths per month in the 

 next nine months; and one-tenth per month in the remaining nine 

 months. In the last twenty-nine months of the voyage, the 

 intensity of the cylinder is supposed to have been uniform, and 

 the same which it was found to possess on the return to England 

 in 1836. 



It is satisfactory that, with this compensation, the observations 

 at Port Pray a, in January 1832, and in September 1836, assign 

 ahnost identically the same relative magnetic intensity to that 

 station. 



The correction for temperature for this cylinder not having 

 been previously examined, I received it from Captain Fitz-Roy 

 for that purpose, and made with it the following observations. 

 The cylinder, in its own apparatus, was placed in a large earthen 

 jar, glazed at the top, and standing in a larger earthen vessel, into 

 which warm water could be poured at pleasure, and the cylinder 

 was then vibrated alternately in heated air and in air of the natural 

 temperature. These experiments were made at Tortington, in 

 Sussex. 



