342 G. H. KNIBBS. 
the dotted line a b Fig. 4, and the vertical through I or J in the 
same figure. 
Xu Ssin [x (1-E)}---) 
0 ...(46) 
¥ = Scos[x(1 — =)]---. 
X, the correction in altitude, is of very doubtful value ; it would 
be idle to insert the corre¢tion m in such a case, as it rarely amounts 
to more than a few tenths of a second, and observations of this 
type are not of sufficient precision to justify it. 
17. Measurement of the angles between the diaphragm wires.— 
This measurement must be accurately made in order to obtain the 
best possible results from the preceding formule; it can be effected 
in the following manner: Let a large sheet of Bristol board, 
ruled with a set of close very fine and accurately parallel lines, be 
placed at right angles to, and with its centre in, the prolongation 
of the vertical or azimuthal axis of the theodolite and sufficiently 
far away to secure a distinct image by focussing. This will require 
the telescope to be placed in the direction of the axis, i.e. to be 
set at x = 0°, and the diagonal eye-piece to be used. Setting the 
azimuthal circle at zero, rotate the outer azimuthal axis until on 
moving the telesecope vertically, i.e. in zenith distance, the inter- 
section of the wires moves in a direction perfectly parallel to the 
ruled lines; and clamp in this position. Reset to the reading 2 = 0° 
and rotate the inner azimuthal axis, until each wire becomes 
successively parallel to the ruled system of lines. ‘The successive 
readings give the angles between the wires and the vertical. The 
mean of a series of observations would of course be taken. 
A second but less exact method is the following :—Set up the 
instrument in the ordinary manner with the telescope pointing 
horizontally, and at right angles to a vertical surface on which a 
Bristol board hus been suitably placed. With a fine pricking 
point mark on this the intersection point or points, and the outer 
edges of the diaphragm wires. Rotating the azimuthal axis so that 
the telescope rotates azimuthally only, mark also the trace of the 
intersection point on each side of the mark first made. This last 
