OjST solae physics. 
i 
fig. 4 that they give us the position-angles and the error of ver- 
tically with great precision. For let I., II., III., IV. represent 
the position of the wire-ends, and A, B, C, D the extremities of two 
lines, drawn parallel to the wires, but passing through the centre 
C of the sun-picture. If N represent the true north point of our 
picture, obviously the angle NCA is the angle A-f-/, which 
has been determined previously. But the position of the point 
A is given by the quantity 
— 90°= a ! = reading corresponding to A. 
Fig. 4. 
Similarly, the position of D by 
/3 + S 
and obviously 
— — 90°= /3'= reading corresponding to B; 
/3'— a'=90°, 
if the imaginary lines through the centre, or the wires which are parallel to them, are 
perpendicular to each other. In general we shall find 
/3'-«'=90 °±0; 
and consequently the quantity ip- will have to be applied as a correction to A+i on 
account of error of perpendicularity. 
Finally, since the true reading of the point A, reckoning from the true north line, 
ought to be exactly 
* _J 
A+/-P2’ 
while it was found to be 
it is necessary to apply the quantity 
(a + ,+|M^±X_90°) • 
with its proper sign, as a correction to every circle-reading, in order to deduce the true 
angle of position. 
The following is an example of this part of the measurement of a picture, the numbers 
I., II., III., IV. corresponding to the readings a, (3 , y, S : — 
May 26th, 1863. 
I. 0-089 43 20-2 (a) II. 0-096 134 45-3 (/3) 
III. 0-096 223 20-9 (y) IV. 0-091 312 19-6 (S) 
2)266 41-1 2)447 4-9 
133 20-5 s 
223 32-4 5 
