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 instance of this fact is afforded by the case of the North American Continent, 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 elements indicated on Sir E. Sabine’s maps, viz., a determination of decimation 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. G. M. Whipple, the Superintendent of the Kew Observatory, in the course of some conversation respecting a journey to Western America which I 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 magnetical 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 accordingly 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 Committee 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 I time Professor Balfour Stewart generously undertook to use his influence with the Council of the Owens College to obtain for me the 3 admirable magnetic equipment belonging to that institution. After some experience with both sets of instruments, I decided to accept those belonging to the Owens College, partly because they were of more modern construction than the others, being furnished with certain 5 contrivances and improved modes of adjustment, suggested by Dr. J Stewart’s extensive experience, which made them more convenient to use, whilst they materially added to the trustworthiness of their indications; and partly (which was of great importance) because the instrumental constants had been comparatively recently determined and verified by Mr. Whipple. The Owens College instruments consisted of a dip-circle by Dover, No. 6; a unifilar magnetometer by Elliott Brothers, No. 27; and a marine chronometer by Frodsham, No. 4066. I would here record my sense of obligation to Professor Stewart and the Council of the Owens College for the readiness with which they placed their instruments at my disposal. My thanks are also due to the Kew authorities for the loan of a couple of tripods. I am specially indebted to Mr. Whipple and to Mr. Figg, the Magnetic Assistant at the Kew Observatory, for instruction in the use of the instruments, for advice in the conduct of the observations, and for assistance in their subsequent reduction.
I. A magnetic survey of the fortieth parallel in North America between the Atlantic Ocean and the Great Salt Lake, Utah