172 
HINTS TO TRAVELLERS. 
minute, which may be taken as the limit of accuracy to which the hour 
angle can be plotted, and consequently need be calculated. The accuracy, 
however, decreases as the divisions become smaller near the circumference 
and in high latitudes. 
In the south-west quadrant, the radius of the circle and the radii of 
all the declination circles up to 32°, the limit of the moon’s declination, 
are divided into scales of one hundred parts. 
Parallax in Declination. 
Plot on the diagram the position of the place of observation from its 
known latitude and the hour angle, counting the hour angles from right 
to left —that is, from the circumference towards the centre. Call this 
point A. Draw a straight line through the centre of the circle and that 
division of the circumference representing the moon’s declination, above 
or below the line 00 according as the declination is north or south, 
and in the same side of the circle as that from which the hour angles 
commence to count. Denote this line by CB. 
The length of the perpendicular drawn from the point A to the straight 
line CB, produced if necessary, is a measure of the parallax in declina¬ 
tion. With a pair of compasses, find what proportion the length of this 
line bears to the radius of the circle, which is divided into a hundred 
parts on the diagram; multiply this proportion by the horizontal parallax 
of the moon, and the product is the parallax in declination. 
Let us take an example— 
Latitude, 10° 30' N.; moon’s declination, 20° 50' 30" N.; moon’s 
horizontal parallax, 59' 16"; hour angle, lh. 40m. 
On the diagram the point A is plotted at lat. 10° 30' N., and hour 
angle lh. 40m., counting the hour angles from the circumference towards 
the centre as numbered in the low*er line of figures. CB is drawn through 
the centre C and the division on the circumference representing the 
declination 21° N. approximately. 
If the diagram represents an orthographic projection of the Earth on a 
vertical plane passing through the centres of the Earth and moon, the 
point A and the line CB are the projections of the place of the observer 
and of a line joining the centres of those two bodies. 
AD, being the perpend’cutar dropped from A on to BC, is a measure 
