XVI. 



Discussion of Observations for the Isodynamic, Isotonic, and Isoclinal Curves of Terres- 

 p-ial Mafinetis7n on and near the Line of the Boundary Survey between the United 

 States and Mexico, made in 1849, 1850, 1851, and 1852, xmder the Orders of 



W. H. ElMORY, 



ASTKOKOMER OF THE BOUNDAKT COMMISSION, 



And combined ivith Observations at San Francisco {California), and Dollar Point 

 (East Base), and Jupiter {Texas), furnished by 



A. D. BACHE, 



SUPERINTENDENT OF THE UNITED STATES COAST StJRVET. 



With a Map. 



( Communicated to the Academy, Fehruary 13<7j, 1855, hy W. C. Bond, Director of the Observatory of 



Harvard College.) 



The magnetic elements contained in this paper have been obtained with a Fox Dip- 

 Circle, — the same which was used by Major Emory for the observations published in 

 Volume V. of the Memoirs of the Academy. 



The values of total intensity are expressed in units of the intensity at Falmouth, 

 England. To reduce them to the arbitrary standard commonly adopted, they should 

 be multiplied by the coefficient 1.374. As observations of this element with Mr. 

 Fox's apparatus are liable to be affected by changes in the magnetic condition of the 

 needles, it would be desirable to know to what extent such changes may have taken 

 place. The successive comparisons, between 1844 and 1854, which have been made at 

 Cambridge, with the instrument used by Major Emory, furnish the data for estimating 

 their amount. If it be allowable to assume that there has been no sensible secular 

 change of total intensity at Cambridge since 1842, at which time its value by direct 



