19 o Lieut. Foster’s account of the 
of the plate a tangent to the sphere at each ; that is, at E and 
Q the plate would revolve about the line E Q, and at S and N 
about the line S N, &c. But with the plane of the plate in 
the plane of the secondary, he was most desirous that obser- 
vations should be made ; and at the points s, w, N and e , fig. 4. 
I also made a set of observations at the points e, w, E,Q,%. 5 , 
the plane of the plate being in the plane of the equator. 
In consequence of the extent of the changes in the daily 
variation, I was under the necessity of making the observa- 
tions in a different manner from that adopted by Mr. Christie ; 
they were in general made as follows. 
The circular iron plate before mentioned, being divided 
into six equal parts, marked o, 60, 120, 180, 240, 3 00, and 
the instrument above described so adjusted, that when the 
plate was placed on the copper pin T or V, its centre would 
occupy the position required ; the plate was then placed on 
the pin, with the point o, coinciding with the fixed mark or 
index, and the direction of the north end of the needle noted ; 
after which the plate was made to revolve three times for 
instance, gently by the hand, its upper edge moving from 
east to west, and o, being again brought to coincide with the 
index, the direction of the north end of the needle was again 
noted ; and the same was done, after making the plate re- 
volve in like manner from west to east: the difference 
between the first and second reading, gave the deviation due 
to rotation from east to west ; and the difference between the 
first and third, that due to rotation from west to east. The 
plate was then moved to a distance, in order that an allowance, 
if necessary, might be made for the change in the direction of 
the needle caused by the daily variation. After this, the plate 
