374 A. D. Bache on a Magnetic Survey of Pennsylvania, etc. 
Table continued. 
: x x. 
Stations. observed. |, A oN 4 
Utica, 3°55 3°49 +0°06 
Schenectady,* 3-51 3-51 0-00 
roy, 3-58 3:52 -0'06 
West Point, 4-04 3-85 +0'19 
New York,* 4-01 4-01 0:00 
Milford,* 3-77 3-88 -O'FL 
Bushkill, 3-86 3-92 -0:06 
Princeton, 4:23 4°07 +0°16 
tion and dip +0°062. For the previous hypothesis, these quan- 
tities are respectively +0:030 and +0:059, showing but } ttle 
gain in the representation of the observations by the additional 
term dLdM cos L. : 
For the general representation, the probable errors are 0 050 
and +0°051. 
Representation of the total force, 
From the expressions : 
X=3'900 —0°1934 dL-++-0°0134 dM cos L, 
I=73°25-+-0-912 dL— 0-0690 dM cos L, 
we have to deduce the total force p=X sec I. 
In the expression for X, 
dL—=lat.—41°34 and dM=long. —77°°45, 
in the expression for J, 
dL=lat.—41°-32 and dM=long.—77°'39. 
We have in 
Long. 81°00 X= 4200 we 
Lat. 39 971 =71°828 } agirendins 
Long. 77°50 X= 3°600 oe 
Lat. 42 89 I —74°-676 t — 
Long. 74°00 X= 4200 ) 4a. 
Lat. 39 -60 I —71°-861 | gas1s-49. 
Assuming in the expression for the-total force, 
9=9 o +/+2dL+ ydM cos L, 
dL and dM as in the expression for X, we find: 
7 ~=13'55+0-0451 dL—0-00682 dM cos L. 
_ The lines of equal total force of 13°45, 13°5, 13:55 
. 
. pass through the following positions: 
and 188 
