138 
HINTS TO TRAVELLERS. 
sary, but the traveller who wishes to obtain a good determination of 
an astronomical position must pay regard to them. 
To observe the altitude of the sun , using an artificial horizon.— ’Flu, the 
trough of the horizon with quicksilver, and put on the roof. Put down the 
suitable shades before the index and horizon glasses, set the index of the 
sextant to zero (0°); then with the artificial horizon between yourself and 
the sun, retire, looking into the horizon, until you see the sun’s reflected 
image in it; look through the telescope collar, or plain tube, and horizon 
glass of the sextant at the sun itself; unclamp the index, and move it 
forward. This will bring the reflected image down, follow it with the 
eye until it slightly overlaps that in the horizon; clamp the index, and 
screw the inverting telescope into the collar (no time should be lost in doing 
this, or the sun’s image may pass out of the field); then with the tangent 
screw make the contact perfect. It is always better to bring the object 
down into the horizon without the telescope; by so doing time is saved, 
and the unpractised observer is less likely to be mistaken as to which limb 
he is observing. The following rule will, however, prevent any such 
mistake :■—In the forenoon, or when the sun is rising, if the lower limb 
is observed, the images are continually separating; if the upper limb 
is observed, they are continually overlapping; and the contrary in the 
afternoon, or when the sun is falling. When the telescope is fitted with 
a dark shade to screiv on to the eye end , it should always be used instead of the 
moveable shades. If a roofed artificial horizon is used, the sides should 
be plainly marked, and it should be reversed at each set of three altitudes, 
except when equal altitudes are observed to find the error of the watch, in 
■\yhich case the observations must be taken with the same side of the roof 
towards the observer.* In placing the horizon on the ground it should 
have one of the glazed sides of the roof in a direct line with the sun, so 
that its sides cast no shadow. Any object seen in the mercury appears 
to be just as much below the horizontal plane as it really is above it; 
all angles, therefore, observed in an artificial horizon must be halved, 
after the index correction has been applied. 
The foregoing remarks apply equally to stellar observations, the only 
difference being that no dark shades are required. 
* This is by way of precaution against irregularities in the glass plates; and, 
with a roof of known excellence, is hardly necessary. 
