236 



heat up to the verge of its fusion. This increase of resistance 

 is not caused by the increased density of the current, by the 

 increased distance of molecules, or the employment of mole- 

 cular force in generating heat; but is exactly proportional to 

 the temperature. The same is the case with copper wire, and 

 the amount of change bears in both the same ratio to the ori- 

 ginal resistance. This change should be attended to in all 

 measures of resistance. 



The heat generated by a current is as the product of its 

 square into the actual resistance, but that attained by a wire 

 ignited in air as the square root of this product. 



The cooling power of air is, in these experiments^ as the 

 temperature ; that of radiation as its square. 



A wire thus ignited is dark at the two extremities, but the 

 temperature rapidly rises as the distance from them increases, 

 and soon becomes uniform over a large extent of the wire. 



Its thermic equation shows that this uniform temperature 

 exceeds the mean by an amount varying from a seventh to a 

 tenth. 



The Rev. Dr. Robinson next proceeded to notice a fact of 

 some interest which he lately observed with the Rosse tele- 

 scope. It related to a remarkable planetary nebula, Herschel's 

 figure 44. This looks, in smaller instruments, like an oval 

 disc, reminding one of the planet Jupiter ; but it appears to 

 be a combination of the two systems which he had formerly 

 described. In both these the centre consists of a cluster of 

 tolerably large stars : in the first, surrounded by a vast globe of 

 much smaller ones ; in the other by a flat disc of very small 

 stars, which, when seen edgeways, has the appearance of a 

 ray. Now this nebula, which he had recently observed through 

 Lord Rosse's telescope, has the central cluster, the narrow 

 ray, and the surrounding globe. He would also add, as a re- 

 markable proof of the defining power of this vast instrument, 

 that he saw with it, for the first time, the blue companion of 



