m __ _ 



Magnetism in the United States. 3 



states. {Am, Phil. Trans. IS-iG, and Smithsonian Contrib. 1852.) 

 The values of horizontal intensitj in this series are originallj 

 expressed in terms of the force at Cincinnati, and have been 

 converted into British units tlirough the observations at Toronto, 

 which is one of the stations. 



b. Observations in various parts of the middle and Avcstern 

 states, by Prof E. Loomis, Am. Phil. Trans.^ vols. vii. and viii. 



6. Observations made on the Mexican boundary surveys, un- 

 der the direction of Major AV. H. Emorj^, U. S. A., recently 

 presented by him to the American Academy of Sciences. 



7. Observations made by Caj)tain Whipple's party in the Pa- 

 cific railroad explorations, near the 35"^ parallel of latitude. This 

 series, not heretofore published, was kindly furnished by Cap- 

 tain A. W. Whipple, U. S. Topographical Engineers, and is 

 given in full in Table II. It will be 'seen that a large number 

 of the stations are at a great elevation above the sea level- The 

 effect of elevation on the action of the earth's magnetism has 

 generally been found insensible, and in the absence of any 

 known correction the observations have necessarily been used 

 without regard to height. The observations have been made 

 with a Fox dip-circle — Cambridge, Massachusetts, being used as 

 a reference station for the intensitv- The numbers in the table 

 denoting the total intensity may be considered as referring to 

 the arbitrary scale in which the total force at London is 1'372, 

 with the usual uncertainty on account of secular variation. 



For use on the map they have been multiplied by 7*41 to give 

 the total intensity in British units, and by the cosme of the dip 

 for the horizontal force. The factor 7-41 is the ratio of 13'32, the 

 total intensity in British units at Cambridge, Massachusetts, from 

 observations by Professor Bond, and the Coast Survey observa- 

 tions in the vicinity, to 1*798, the reference number in the table. 



8. Table III gives some observations of declination not before 

 published, collected from various sources for this discussion. 



Collections for secular variation. — The observations in the dis- 

 cussion have been reduced to the common date of Januarj^, 1850, 

 by the best values for the annual change that could be arrived at. 



The annual change for the declination and dip has been used, 

 as found in the discussions by Mr. C, A Schott, printed in the 

 U. S- Coast Survey report for 1856. 



For the northwestern states we .deduce from scanty data, and 

 have applied an annual change of irom V'Q to 2' decrease of 

 easterly declination. 



Determinations of intensity in absolute measure are of so re- 

 cent a^date that but little is known of its secular variation. Ob- 

 servations of the horizontal force at Toronto, Boston, New York, 

 Philadelphia, and Pascagoula, made during the interval between 

 1843 and 1855^ concur in showing an annual decrease of nearly 

 TflV^-p^^t ^f t^^ force. If we suppose the total force to remain 



