132 



Dr. T. E. Thorpe. 



" A Magnetic Survey of the Fortietli Parallel in North America 

 between the Atlantic Ocean and the Great Salt Lake, 

 Utah." By T. E. Thorpe, Ph.D., F.E.S. Received 

 March 25 : Read May 1, 1879. 



A glance at the maps which accompany Sir Edward Sabine's 

 " Contributions 'to Terrestrial Magnetism," published at various times 

 in the " Philosophical Transactions of the Royal Society," is sufficient 

 to show that considerable as is our knowledge of the magnetic history 

 of our globe, there are still very large areas of the earth's surface 

 about which we have practically no information. A remarkable in- 

 stance of this fact is afforded by the case of the North American Con- 

 tinent, more especially by that portion belonging to the United States. 

 Magnetic observations have been made, with more or less assiduity, at 

 different places in the Eastern States, for many years past ; but over 

 the immense tract of country lying between the Mississippi and the 

 Pacific Ocean — a tract comprising upwards of 200,000 square miles, 

 there is only a single determination of one of the three magnetic ele- 

 ments indicated on Sir E. Sabine's maps, viz., a determination of 

 declination at Salt Lake City. A series of observations was made 

 some years since by United States officers along the Mexican frontier, 

 and a similar series was carried out by the English and American 

 officers employed on the North American Boundary Commission. 

 But the latter observations, I believe, have not yet been published. 



My attention was first drawn to this matter by my friend Mr. Gr. M. 

 Whipple, the Superintendent of the Kew Observatory, in the course 

 of some conversation respecting a journey to Western America which 

 1 contemplated making in order to observe the total solar eclipse of 

 last July. As my station was to be in the State of Colorado, which 

 is practically midway between the northern and southern boundaries 

 along which the observations above referred to had been made, it 

 seemed to Mr. Whipple advisable to seize the opportunity to add to 

 our znagnetical knowledge of the intervening area to the extent even 

 of securing observations on a single spot only. Further consideration 

 showed that the project might be extended with advantage, and ac- 

 cordingly I offered to make a complete series of observations of the 

 three elements — dip, intensity, and declination, at such times and 

 places as the circumstances of travel would allow. The Kew Com- 

 mittee kindly offered me the loan of the instruments which the Rev. 

 S. J. Perry had employed in Kerguelen's Land and in other places on 

 the occasion of the recent Transit of Venus Expedition ; at the same 

 time Professor Balfour Stewart generously undertook to use his in- 

 fluence with the Council of the Owens College to obtain for me the 

 admirable magnetic equipment belonging to that institution. After 



