397 
iron arising from its rotation. 
that the centre of the plate being to the west of the needle 
or in longitude 180°, the deviation of the north end of the 
horizontal needle ought to be towards the easty which agrees 
with the observations. 
Having determined from observation the value of x when 
the rotation of the plate produces no effect, and the corre- 
sponding value of we may determine the value of , in- 
dependently of/p ; and the value of may then be deter- 
mined from the observed values of d' and 9 ^ when x = o or 90°. 
Substituting these numerical values for 
equations (5) and (6), we may deduce the values of 9 ' and 9 ^ 
corresponding to different values of x, and compare them 
with those actually observed. 
From Tables A, B, and also Table VII, it appears that the 
deviation due to the rotation of the plate vanishes when the 
latitude of its centre is very nearly ’54° 45', or as nearly as 
can be determined when 3 cos. 2 x + i=o, in which case 
x = 54° 44' 08". We shall therefore have from Table A, 
Tan. 9° 09' 10" = 
3 sin. 109° 28' 16" ^ 
cos. 70° 15' 2 m 
F r 
2 m 
Whence = 49-9504 = 50 very nearly. 
2/p 
When X = o, tan. 9 ' = — ; and since 9 -. 
’ cos A 2ctR3 ’ 
4/55" 
F r 
— 2 
and <y= 70° 15', we obtain ^ = .056524. 
r T 
— z.lll 
Also when x = 90°, tan. 9 ' = - 1 ^.. _ Fr 
cos. J 2mR* 
Fr + + 
= — 22'25"; SO that, ^ = .059494. 
r r 
where 
