XXXIX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



June 2LnnH0MAS TELFORD, Esq. was admitted a Fellow 

 i of the Society ; and the reading of a paper " On the 

 theory of the diurnal variation of the needle," by S. H. Christie, 

 Esq. F.R.S. commenced on June 14, was resumed and concluded. 



Mr. Christie having been led to doubt the validity of the moving 

 easterly variation adopted by Canton, but at the same time having 

 observed that the changes in direction and intensity appear always 

 to have reference to the position of the sun with regard to the mag- 

 netic meridian, was led to connect these phaenomena with Professor 

 Seebeck's discovery of thermo-magnetism, and Professor Cumming's 

 subsequent experiments ; and to refer the phaenomena of diurnal va- 

 riation to the effect of partial heating modified, perhaps, by that of 

 rotation and by peculiar influence in the sun's rays. 



In support of this opinion he cites passages from papers by Pro- 

 fessor Cumming and Dr. Traill, whom a similar idea appears also 

 to have impressed. But in place of looking to the stony strata of 

 which the earth's surface consists as the elements of the thermo- 

 magnetic apparatus which this doctrine requires, the author regards 

 them as rather consisting of the atmosphere and the surfaces of 

 land and %vater with which it is in contact. Thermo-magnetic phae- 

 nomena, he remarks, have hitherto only been observed in metallic 

 combinations ; but this may be owing merely to the small scale on 

 which our experiments are conducted. 



To put to the test of experiment whether thermo-magnetism 

 could be excited when the surfaces of two metals instead of touch- 

 ing at one point were in symmetrical contact throughout, the 

 author first employed a compound ring of bismuth and copper, the 

 copper outwards; and he found that to whatever point heat was ap- 

 plied, magnetic powers were developed ; a needle being affected dif- 

 ferently according to the different positions in which the ring was 

 placed with regard to it. After a lapse of two years from this first 

 experiment, the author resumed the inquiry with an apparatus con- 

 sisting of a flat ring of copper having its inner circumferences 

 grooved and united firmly by soldering and fusion to a plate of 

 bismuth cast within it ; the whole forming a circular plate twelve 

 inches in diameter, weighing 119 ounces Troy, which was made to 

 revolve in its own plane. 



Heat was applied by a lamp to a given point in the circum- 

 ference of this plate, and a delicately suspended needle partly neu- 

 tralized, was placed near it, and the deviations observed in all posi- 

 tions of the heated point, which was made to revolve ; the lamp be. 

 ing withdrawn. These experiments led him to conclude that the 

 effect of so heating a portion of the circumference, was to create 

 a temporary polarity in the plate, the law of whicli he explains. 



He* 



