and parts of adjacent States. 369 
Gen. Sabine’s resulting isoclinal lines in his seventh contribu- 
tion to terrestrial magnetism (Phil. Trans. Roy. art III, 
1846, p. 237) refer to.an average period between 1840 and 1842, 
and correspond i in their position very closely to those now pre- 
sented;—they are deduced from independent data 
Distribution of the Magnetic Horizontal ae and construction of 
isodynamic lines for 1842, 
If we group the observed intensities in the same manner as 
the dip, the mean epoch 1842°0 may Lbaeias be assumed, and 
all observed intensities be reduced to that 
Correction to epoch—We have the tllbwing direct compari- 
sons with Mr. Schott’s observation of 1862. 
Locality. Date. Hts +. ee | Feces 
Jan. 1843 | 4:320 | Aug. 1862 | 4-255 | 0-065 00033 
July 1840 | 4078 | July “ 4012 | 0066 | 0-0030 
Aug. * 4°207 « « | 4138 | 069 | 00031 
c”-1841 | 3°792 Aug. “ 3728 | 0-064 | 00030 
1841 | 3677 “ ‘ 3:639 | 0°038 | o-0018 
July 1841 | 3-983 ue 3-924 | 0-059 | 0°0028 
Jan. 1842 | 4:166 eee 4088 | 0°078 | 0:0039 
Mean, | 0°0030 
The average annual decrease in the value of X, between 1840 
and i is, therefore, 0°00380, or when expressed i in parts of X, 
eq 000076. This result agrees tolerably well with that 
dednaed te Mr. Schott in the Coast Survey Report of 18 
where 0- 00110 was found. 
Se or 0°00105 when aides in winch 
Formation of pede for ~ — tical expression of the distribution of 
the magnetic horizontal force, referred to the epoch 1842°0, 
At se ued thus *, the horizontal force was dete: 
mined by vibrations; at those not “esa by Lloyd’s statioal 
method. 
* From Coast poe R. ne 1861 (yet in manuscri 
fei2s, Os token xX=4- ae 
—— erhen (mean of three results), — 
a tuES, =4-320 
: In J uly and Noveaber, a X=4-160 
4/166 } Mean, 4166, for 1842-0 
“ July, 1248, 4172 
\ 
