A Magnetic Survey of the Fortieth Parallel. 133 



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 

 contrivances and improved modes of adjustment, suggested by Dr. 

 Stewart's extensive experience, which made them more convenient to 

 use, whilst they materially added to the trustworthiness of their indi- 

 cations ; and partly {which was of great importance) because the in- 

 strumental 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 Prodsham, 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 sub- 

 sequent reduction. 



In addition to the ordinary magnetic instruments, I provided my- 

 self with an excellent 7-inch sextant by Casella, a couple of artificial 

 horizons, some spare thermometers, an aneroid for altitudes up to 

 5,000 feet, and a hypsometric apparatus for greater elevations. The 

 fortunate circumstance that I was accompanied by Dr. Schuster has 

 added, in no small degree, to the completeness of the work ; for that 

 gentleman, being provided with one of Pistor and Martin's small 

 prismatic circles, and being very expert in its use, kindly offered to 

 take charge of the observations for time and latitude throughout the 

 journey. It is in great measure due to this assistance that I have 

 been able to develope the original intention into what may be con- 

 sidered as practically a magnetic survey of the fortieth parallel across 

 the American Continent, that is to say, of the district which lies mid- 

 way between the line of observations already run along the northern 

 and southern boundaries of the United States territory, 



The observations were begun at Kew, which was adopted as a base 

 station, and were resumed on the other side of the Atlantic, at New 

 Haven, Connecticut. Determinations were next made at La Junta in 

 South Colorado, the eclipse station selected by Professor Asaph Hall, 

 of the United States Naval Observatory ; afterwards at Manitou, at 

 the base of Pike's Peak ; at Denver, and at Cheyenne in Wyoming. 

 No further observations were made until we arrived at Salt Lake City, 

 where, thanks to the kindness of President Taylor, we were permitted 

 to make a complete series in an excellent spot in a garden to the side 

 of the late President Young's house. The transit instrument, by 

 Wiirdermann, of Washington, belonging to the Community, which is 



