140 



compass, or magnetic needle, at intervals of thirty minutes of dis- 

 tance. These hnes are approximate, yet as nearly correct as the 

 materials from which they are drawn would permit. 



Innumerable difficulties at once present themselves in an under- 

 taking of this kind. Returns of variations are too often made by 

 those styling themselves surveyors, who are not sufficiently conver- 

 sant with the mode of ascertaining them. Others, who are com- 

 petent, are often too intent on gain to spend sufficient time to insure 

 accuracy in their observations. Defects in the instruments used, 

 and occasional local attractions, have their influence in producing 

 inaccuracies. 



The expression, " as true as the needle to the pole," has obtained 

 credence from the very common belief that the magnetic needle 

 invariably points to the polar star ; and further, that if any varia- 

 tion does exist — when ascertained-^it will be found to be constant. 



From observations, it is ascertained that at the south line of the 

 State the magnetic needle, instead of pointing to the true north, 

 varies to the east about 12'^ 40', while at the north line of the 

 State it exceeds 18°. 



It will be perceived, in tracing the lines of equal variation on the 

 map, that they are not in a direct line, but in many places very 

 much curved ; and .that these curves are also very irregular. In 

 several of them there appears a decided " sag," or rapid depres- 

 sion to the south soon after crossing the crest of the coast range 

 2;oing east. 



At a few points in the State, extraordinary local attraction is 

 experienced. This is particularly the case a few miles westerly of 

 the tow^n of Sonora, in townships one and two south, ranges twelve, 

 thirteen and fourteen east, where the variation is from fourteen 

 to eighteen degrees. The district of country bordering on the 

 " Buttes," west of the city of Marysvilie, in townships fourteen, 

 fifteen and sixteen north, ranges one, two, three and four east, indi- 

 cate changes of variation from fourteen and one-half to seventeen 

 degrees. West of " Lassen's Peak," in townships thirty and thirty- 

 one north, ranges, one, two -and three east, the variation is from 

 fourteen to seventeen degrees. In the vicinity of " Shasta Butte," 

 in townships forty-one and forty-two north, ranges one, two, three 

 and four west, the variation is from fourteen to eighteen degrees. 

 About the Humboldt, Mount Diabolo and San Bernardino very little 

 if any local attractiqn is experienced. This is especially the case 

 on what is styled the " Desert." 



xVdded to these difficulties in tracing lines by the magnetic needle 

 may be mentioned the constant but irregular changes termed " di- 

 urnal " and " annual." 



From observations continued during a series of years in different 



