CURVES OF MAGNETIC ELEMENTS ON THE LINE OF THE MEXICAN BOUNDARY SURVEY. 373 



determination was 1.777 in units of the common scale, there would be a correction 

 of + 0.002 X 1.374 to be applied yearly, for the interval elapsed since 1844, to the in- 

 tensity derived from this instrument by using the weights, and of — 0.005 X 1.374, 

 when the deflectors are employed. 



In the manuscript copy of the observations received from Major Emory, the 

 particulars respecting the mannner of making the observations have not been given. 

 It can only be assumed that the mean of the above corrections, = — 0.0015 X 1.374 

 X (1851-1844) = — 0.014, is to be applied to all the total intensities, (reduced to 

 the common scale,) contained in the present paper. 



In projecting the lines of equal variation, dip, and total intensity, as represented 

 upon the accompanying map, no attempt has been made to consider the curvature of 

 the lines. A complete determination of this element will best be made by combining 

 the results here presented with others from points lying at some distance from the line 

 of the survey. 



The accompanying tables contain the principal steps of the reductions furnished by 

 Major Emory, agreeably to the following formula. In constructing the lines upon the 

 map, however, a difierent combination has been employed. 



Formula 



for computing, tJieoreticalhj, the Variations in the Magnetic Declination, LiicnsitTj, or Dip, due to Changes 



in the Latitude and Longitude. 



[From the Fifth Report of the British Association for the Adrancement of Science, 1835.] 



xsd U + y2d L.d M=2d L.dlV,Ior D] 

 xs d L.dM-{-y^dM' = t d M . d [V, I or D}, 



in which 



X = variation of the magnetic clement in Latitude. 



3/ = " " " " in Longitude. 



d L = difference of Latitude, from the origin. 

 dM= " Longitude, " 



d V = " Declination, " 



d I = " Intensity, " 



d D = " Dip, " 



- =: tang. Z ; Z being the angle made with the meridian by the line passing through all the points of 



y 



equal declination, intensity, or dip. 



