BETWEEN THE UNITED STATES AND MEXICO. 



257 



Isoclinal Lines, 



GEOGRAPHICAIi POSITIONS OF POINTS, 



Long, 



1 



1 



1 



54" 



55" 



56° 



57° 



58° 



60° 



00° 



— ■ ■ ■ ■ > 



61° 



6a° 



1 



03° 



Pip. 



95 



/ 



25 30 

 25 38 

 25 46 



25 34 



26 02 

 26 09 



1 



2C 09 

 26 18 

 26 26 

 26 35 

 26 43 

 26 51 



26 59 



26 48 



26 57 



27 06 

 27 15 

 27 24 

 27 33 

 27 41 

 27 49 



27 27 

 27 37 

 27 47 



27 56 



28 05 

 28 14 

 28 22 

 28 31 

 23 40 



28 07 

 28 17 

 28 27 

 28 37 

 28 47 



28 56 



29 05 

 29 14 

 29 23 

 29 32 

 29 40 

 29 47 



29 54 



30 00 



29 05 



30 10 



28 47 



28 57 



29 08 

 29 18 

 29 28 

 29 38 

 29 48 



29 57 



30 06 

 30 16 

 30 24 

 30 32 

 30 39 

 30 46 

 30 53 



30 59 



31 05 

 31 11 

 31 17 

 31 22 

 31 26 

 31 31 

 31 35 

 31 39 



1 



• 9 



29 26 

 29 37 



29 49 



■ 30 00 



■ 30 11 



30 21 

 30 22 

 30 43 



30 52 



31 02 

 31 U 

 31 20 

 31 23 

 31 36 



1 



31 43 

 31 50 



31 57 

 ^ 04 

 ^ 11 



32 16 

 ^ 21 

 ^ 26 

 32 31 

 32 26 



• / 



• / 



• / 



• t 



• 1 





<7u 



96 



fl7 







^ ^ ^ ^ A A A b ^ ^ 













A A A A A A A B ■ V 









98 

 99 







v v v s v ■ v ^ v v 



Abb a m a a ^ m a 



1 









1 











100 

 101 



1 



102 

 103 



104 

 105 



106 

 107 

 108 

 109 

 110 

 111 

 112 

 113 

 114 

 115 



116 

 117 



118 

 119 

 190 

 121 

 122 

 123 





A K K A A ^ A a ^ ^ 



^ V ^ ^ W ^ ^ H V ■ 

 1 



1 

 1 







1 

 \ 

 1 



1 









^\I vv 













1 

 1 









fti«»i««*at 







1 























31 59 



32 09 



33 18 

 32 27 

 32 35 

 32 43 

 32 50 



32 57 



33 04 

 33 10 

 33 16 

 33 23 

 33 28 

 33 33 



32 49 



32 58 



33 07 

 33 16 

 33 25 

 33 33 

 33 49 

 33 49 



33 57 



34 04 

 34 U 

 34 18 

 3-4 24 

 34 30 



























, , ,, 







B ^k^ ^fc ^k^ L A ^ " A A 







































\ Latitude 









1 

 1 















1 























i 













1 







1 







■ 



























35 07 



















35 15 

 35 22 

 35 29 

 35 ^ 













I":::::::: 





36 19 

 36 27 

 30 35 





















'. .. . 









37 35 











1 

 1 











36 42 I 37 45 













1 

 1 







■ 





36 49 



37 54 



38 02 

 38 11 









1 













1 















1 





*«■■■« m m ^ m 



1 















1 











Hor 



Combining tlie eight stations of 1855 among fhemselves, we obtain as 

 ie 31° 27', longitude 108° 45', X= 6,21, with a probable error of 

 \ of + 0.10 in going soiitli 30'. 7 or east 506', and the direction of the 



line of cc[ual horizontal intensity N. 87|° E. 



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

 ing to the decrease of dip it would recjuire a southing of 42' to cause an increase of the hori- 

 Eontal 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 we consider that the same value of X is found on 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 direction 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 



: nor is the value of the mean result 



the probable errors of observation and station errors ; 



affected thereby, which forms a valuable datum in absolute measure, in an almost inaccessible 



region of the globe. 



In order to compare this measure of force with that previously obtained on the northern line 

 by observations with a Fox apparatus, and expressed in the arbitrary scale, we may have 

 recourse to the observations of Captain Lefroy at Toronto, which gives the total force in British 



Vol. I 



33 



