176 REPORT—1896. 
Now if H be the hour angle, D the reduced declination, and M the 
meridian zenith distance, log versin H + Leos lat. + Lcos D—20=log n, 
where » is a natural number and m+ vers M=covers alt. 
Again, to find the azimuth, vers sup. (lat.+alt.)—vers polar dist.=m, 
where m is another natural number, and log m +L sec lat.+ L see alt. —20 
=log vers azim. reckoned from south. Thus, on June 12 (local time 
12 hrs. 11 mins.) :— 
Log versin H=3-061310 
L cos lat. =9-801356 
L cos D =9:963379 
Log n=2°826045 ., n= 670 
Vers M =113258 
Covers alt. =113928 
.. Alt.=62° 23' 
Vers sup. (lat. + alt.) =607395 
Vers N. polar dist. 606058 
ji SMepy 
Log m = 3126131 
L sec lat. =10:°198644 
L see alt. =10°333900 
3658675 
Vers azim.=001556 
.. Azim.=5° 28’ west. 
Two lines are then drawn on the negative, one vertical and one hori- 
zontal, intersecting in the centre of the sun’s image. Two corresponding 
points in the cloud are selected, and their respective linear distances from 
the vertical and horizontal lines measured as accurately as possible. In 
some hazy cases this cannot be done with greater accuracy than about 
the =J,th of an inch ; but these are exceptions, and as a rule some small 
speck or sharp angle of cloud can be found, the position of which may be 
fixed with certainty to the ;},th part of an inch. From these linear dis- 
tances the angular displacement is easily found, either by direct calcula- 
tion of the tangents or by reference to a previously constructed scale. By 
adding or subtracting from the sun’s azimuth, as the case may be, the 
position of the cloud point in azimuth from the two stations is determined, 
and thence the horizontal distance of the point vertically beneath the 
cloud from either station. 
Similarly the altitude of the cloud point from the same station is ob- 
tained from the corresponding plate, and the height above the horizontal 
plane then computed. 
Now if a and 6 be the angles from the stations A and B respectively, 
the difference of their sum from 180° gives the angle subtended by the 
base line at X, the point vertically beneath the cloud. ‘Then the distance 
AX is given by the equation : 
Log AX=L sin }—L sin AXB+log AB 
