1001 



as like that of a relatively paramagnetic mass of air; and when its 

 position was reversed, its action was representative of that of a heated 

 or relatively diamagnetic mass of air. Bringing this helix into the 

 vicinity of small magnetic needles, suspended either freely, or so as 

 to show declination or inclination, the planes of action or indif- 

 ference as regards the power of deflecting the lines of force and the 

 needle were observed. When the needle can move only in one 

 plane, there are four quadrants, formed (in the case of the de- 

 clination needle) by the intersection of the planes of the magnetic 

 equator and meridian. When in these planes there is no deflection 

 at the needle, but when in the quadrants there is, and in opposite 

 directions in the neighbouring quadrants. 



As the lines of force are held in and by the earth, so these experi- 

 ments were repeated with a needle in near vicinity to a magnet, and 

 the difference of efi^ect is pointed out : then the extent to which these 

 results are applicable to those of the earth is considered, and their 

 utility in guiding the inquirer. 



The effect of heated air having been considered in the last paper, 

 that of cold air is now taken up; and after considering its action in 

 causing a contraction or drawing together of the terrestrial lines of 

 magnetic force, according to the principles of conduction before 

 enunciated, the author considers generally where the regions of cold 

 which travel round the earth every twenty-four hours will be in 

 the northern and southern hemispheres, and how they will grow up 

 and diminish in extent and importance as the sun moves north and 

 south during the year. After which he applies these considerations, 

 and the results of the experiments with the ring helix, to the expli- 

 cation of the changes of the needle as they are given by observations 

 at St. Petersburgh, Greenwich, Hobarton, Toronto, Cape of Good 

 Hope, St. Helena and Singapore. In doing this, he endeavours to 

 explain the night action, the early morning eff'ect, the contrary 

 course of the needle for the same hours in different months, the dif- 

 ference of local time dependent on the distribution of land and 

 water, the cumulative effect of preceding months, and the con- 

 tinual effect, especially in the tropical regions, of the higher tempe- 

 rature of the northern hemisphere above that of the south. In all 

 these points the author sees such an agreement between the natural 

 results and those which are suggested by the assumed physical cause 

 of the magnetic variations, as to give him a growing confidence in 

 the truth of the views he has put forth. 



November 30, 1850. 



At the Anniversary Meeting^ 



The EARL OF ROSSE, President, in the Chair. 



Mr. Edward Solly, on the part of the Auditors of the Treasurer's 

 Accounts, reported, that the total receipts during the past year, in- 



