Diurnal Variation of the Needle. 357 



To put to the test of experiment whether thermo-magnetism 

 could be excited when the surfaces of two metals, instead of 

 touching at one point, were in symmetrical contact throughout, 

 the author first employed a compound ring of bismuth and cop- 

 per, the copper outwards ; and he found, that, to whatever point 

 heat was applied, magnetic powers were developed ; a needle be- 

 ing affected differently 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 in- 

 quiry with an apparatus consisting of a flat ring of copper, ha- 

 ving its inner circumferences grooved and united firmly, by sol- 

 dering and fusion, to a plate of bismuth, cast within it ; the 

 whole forming a circular plate, twelve inches in diameter, weigh- 

 ing 119 ounces Troy, which was made to revolve in its own 

 plane. 



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

 ference of this plate, and a delicately suspended needle partly 

 neutralized, was placed near it, and the deviations observed in 

 all positions of the heated point, which was made to revolve, the 

 lamp being withdrawn.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 which he 

 explains. He then details a set of experiments, by which he 

 was convinced, that a uniformity of action obtained to whatever 

 part of the circumference the heat was applied. He next instituted 

 a series of observations for determining the laws which govern 

 the magnetic phenomena, resulting from the application of heat 

 as above described ; the results of which are stated in the form 

 of tables. 



Four poles appear to be produced, two north and two south, 

 the two north both lying in one semicircle, and the south in the 

 other, but not in alternate quadrants, and all of them lying 

 rather nearer to the center than the line of junction of the two 

 metals. The experiments were pursued in a variety of positions 

 of the plate, with respect to the meridian and horizon, and with 

 a similar general result. 



From these experiments the author concludes, that uniformity 

 of junction of the two surfaces of a thermo-magnetic combination, 

 is no obstacle to the developement of transient polarity. Re- 



