﻿BETWEEN THE UNITED STATES AND MEXICO. 



221 



Isoclinal Lines. 

 Geographical Positions of Points. 



Horizontal Intensity. — Combining the eight stations of 1855 among themselves, we 

 obtain as a mean result, in latitude 31° 27', longitude 108° 45', X = 6.21, with a prob- 

 able error of +0.01; also an increase of -)- 0.10 in going south 30'.7 or east 506', and 

 the direction of tlie line of equal horizontal intensity N. SIF E. 



It may be observed, that this rate of change in going south is evidently too large, for 

 according to the decrease of dip it would require a southing of 42' to cause an increase 

 of the horizontal force of -)- 0.10, if the total force was constant ; but as the latter also 

 decreases in going south, the actual rate of change of horizontal force is probably only 

 half of that above obtained. Moreover, when Ave consider that the same value of X is 

 found ou the coast of Mississippi, between New Orleans and Mobile, we see that the 

 small increase in going east should be decrease of similar amount, and that the direc- 

 tion of the line should be about N. 94° E. The distribution of stations is such that 

 these discrepancies in the rate of change are quite within the probable errors of obser- 



VOL. YI. NEW SERIES. 29 



