116 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVII.) 
or at the equinoxes, and the least in December and June, when the sun is at the 
tropics. The force is the least towards noon, when I suppose that the air above is in 
the worst condition of conduction, and would cause a magnet in it to show more 
power. But how that may affect the curves beneath on the surface of the earth, where 
they are compressed together, is doubtful, and the whole matter of intensity is too 
uncertain and has too many bearings for me to consider it usefully here. 
3069. I hope soon to give further experimental data for the purpose of illustrating 
and testing the view of the physical cause of the magnetic variations which I have put 
forth, namely, those I expect to obtain by the differential balance, and others con- 
cerning the sensible influence of oxygen in causing, under different conditions, deflec- 
tion of the lines of magnetic force. 
Royal Institution, 
November 16, 1850. 
