MAGNETIC 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 Ist 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 Praya, in January 1832, and in September 1836, assign 
almost 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, 
