22 
HINTS TO TRAVELLERS, 
star may be used in preference to the sun, in which case no shade will be 
required. 
The adjustment by a terrestrial object is here given to meet the case of 
an instrument having to be adjusted in the day-time when the sun is not 
visible. Care should be taken when purchasing a box sextant to see that 
the maker has made the box wide enough to admit a finger to wipe the 
glasses, as dull reflectors much increase the difficulty of observation. 
The Artificial Horizon . 
The artificial horizon is "a reflector, the surface of which is perfectly 
horizontal; it is used in combination with the sextant for observing 
altitudes. Though the principle of all is the same, there are several 
forms of this instrument, the most common, as well as the best, being 
a small shallow trough, containing pure, clean mercury,* which 
reflects the image of a celestial body. This is protected from the 
disturbing effects of the air by a roof, the two sloping sides of which 
are made of glass plates accurately ground to true planes: these must 
be carefully examined to see that they are of uniform thickness and 
density. Should the traveller have the misfortune to break one of his 
glasses, and replace it by one not tested, he must be careful to reverse 
the roof between two observations, or once in a set. Captain George’s 
horizon, in which a glass plate floats on the surface of the mercury, is 
in some respects more convenient; but it is more liable to errors arising 
from any disturbance communicated to the mercury by wind. 
Another form of artificial horizon is the black plate. It generally 
consists of a plane of black plate-glass set in a metal frame, and levelled 
* The best method of cleaning the mercury is to pass it several times through 
a funnel of rough paper, the aperture through which it runs being very small, 
but if the mercury is not pure it gives an imperfect reflection, and its level is 
apt to be untrue. The quicksilver of commerce is generally mixed with lead, 
bismuth and zinc, which have to be dissolved out of it by nitric acid; it may, 
however, in case of emergency, be rendered serviceable by shaking it for some 
considerable time in a bottle with a little powdered sugar, or even sand, and 
afterwards straining it through a piece of fine linen or chamois leather, hut it is 
a troublesome and not very satisfactory process. 
