AOJDR£SS BY PR0FKS80R DAUBHNV. XXV 



in making observations on the line of no variation, or, as he prefers to 

 call it, the line of convergence which passes through Siberia ; and, by 

 a fortunate concurrence of circumstances, the north-western expedition 

 lately undertaken by British navigators, has afforded the means of ob- 

 taining, at the very same time, corresponding ones on the similar line, 

 which extends from Hudson's Bay through the United States of America. 

 Thus the position of these lines in the above two most interesting lo- 

 calities, has been almost simultaneously determined with an exactness 

 before unequalled. 



In conjunction with Captain Sabine, Professor Lloyd, of Dublin, has 

 contributed, in another way, at the instance of the Association, to ex- 

 tend our acquaintance with the empirical laws of this interesting de- 

 partment of science. This they have effected by determining the dip 

 and variation of the magnetic needle in different parts of Ireland, which 

 it was considered the more important to ascertain, fi-om the situation of 

 that island in the most westerly point of Europe, at which observations 

 could be instituted. 



The distribution of the earth's magnetism through this country was 

 determined by the above-named observers ; first by a separate series of 

 observations relating to the force of that portion of the magnetic influ- 

 ence which operates horizontally ; secondly, by a similar series on the 

 dip of the needle ; thirdly, by means of observations both on the dip 

 and intensity of the magnetic force made at the same time and with the 

 same instruments. 



It would occupy too much of the time of the Association, were I to 

 attempt to point out, however briefly, the precautions adopted, and the 

 corrections applied, in order to arrive at accurate results. I shall tliere- 

 fore only remark, that the method by which the intensity of the mag- 

 netic force was ascertained, resembles in principle that by which philo- 

 sophers determine the force of gravity. For as a pendulum when set in 

 motion oscillates on either side of the vertical line by the force of gravity, 

 so the needle, when drawn out of its natural position, will oscillate on 

 either side of the magnetic meridian by the earth's magnetic force, and 

 hence, in either case, the force may be inferred to vary, inversely, as the 

 square of the time in which a certain number of vibrations are performed. 

 In order, however, to arrive at trustworthy results, many precautions 

 must be adopted, which are pointed out in detail in Professor Lloyd's 

 memoir, and in particular one relating to temperature ; it being found 

 that the same needle will vary in force about 1 -4000th part for every 

 degree of Fahrenheit. Having, however, arrived at a determination of 

 the intensity of the magnetic force at the two extremities of the Island 



