A Pocket Mapping Instrument. — Zatie. 241 
erect, the < E G F should equal the latitude. Adjustment for 
different latitudes could be made both by a series of notches 
in the arm E and by having the base F adjustable so as to 
slide to and from G. 
When not in use the arm E folds down, tlie cord contracts 
out of the way and nothing dangles as in the dial compass. 
But if F or E be changed the graduation of the dial must also 
be changed. If H is the number of hours before or after noon, 
P the point where at that time the shadow of the cord crosses 
the edge of the frame, m the projection of P on F G, we find 
mG — Pm is fixed, — by the formula ^ == ■^^\^, 
If it might be necessary to change the adjustment on a jour- 
ney when tables are not at hand these two thumb-nail tables 
might be written on the back of the frame : 
To find latitude ;— if latitude=\°=c° + S° 
where S is small, we have sin X=sin (c°+8°) 
=sin c+4-Icos c-''^ ; if we let c=36° 50 ' sinc=.6 
bO 7200 ' -^— •' 
cos c^.8 ; and for the United States 8 will be. 
small and this formula nearly true. 
&m lai— --I ~ — I 12000 -I 
Only the first two terms need be used. 
The frame is further marked with heavy lines in front to 
indicate the courses which are moat used by woodsmen and 
are easily platted with coordinate paper, as given in the table 
below. 
The figures show graduation only on one side of the frame, 
but it is best to have it run all around. 
On the back of the "geograph" is a simple clinometer, — a 
weighted string fastened on the pencil side. There is a pro- 
tractor scale on the back about the point of fixture, and the 
pointed weight is shoved into a slot when not in use. A 
couple of small spirit levels set at right angles into the frame 
would improve it. 
The geograph may be used in two ways. First with coor- 
dinate paper in the usual fashion of woodsmen. The side of 
the geograph being brought to point north, the rulings of the 
sheet of coordinate paper cut to fit the frame run the same 
way. The course which we wish to go being picked out from 
those marked on the frame, a sight is taken over and along it 
and pursued. Having gone that way as far as we want, prefer- 
ably some number of paces which stands in a simple ratio to 
r.M. 
A.M. 
cot.H. 
12 
12 
oo 
1 
11 
3.732 
2 
10 
1.732 
3 
9 
1.000 
4 
8 
0.577 
5 
7 
0.268 
6 
6 
0.000 
