ARTIFICIAL HORIZONS. 
23 
by a bubble. This form answers fairly well in the clay-time, when the 
sun is the object observed, but at night there is so much loss of light 
with the black plate that it becomes extremely difficult to use in star 
observations. In order to overcome this difficulty, artificial horizons of 
this class have been constructed with a brass frame containing a black 
plate on one side, for day observations, and a silvered mirror on the 
other, for night. To the frame are attached fixed levels, by which it 
can be brought to a true horizontal position. This is a very portable 
instrument, but its use can only be recommended in the absence of 
a mercurial horizon, and when the glass used in its composition has 
been ground into a true plane, and tested at the National Physical 
Laboratory in the same manner as a sextant index-glass. Every care 
must be taken to level this instrument accurately, or all observations 
taken by means of it will be of little value. Any form of artificial 
horizon that is used should be kept clean and free from dust. 
Should the artificial horizon be broken or lost, a substitute may be 
formed by treacle or other viscous liquid, or even, in calm weather, by 
water, in a tray or basin. 
Sextant-Stand. 
Though sextant-stands vary considerably in the manner in which they 
are constructed, the object in all cases is the same, viz.:—to provide a 
means by which the sextant can be fixed in any position convenient 
to the observer, and also to give that steadiness, so important in sextant 
observations, which is often wanting in the traveller’s hand after a hard 
day’s journey, or an attack of fever. Cary, 7, Pall Mall, has succeeded 
in making a very convenient form of this instrument, and one that is in 
many respects superior to the old form. The only adjustments are to 
place the stand as level as possible, and in such a position that the plane 
of the sextant shall be in the plane of observation. 
Transit Theodolite . 
The following are the names of the various parts of this instrument to 
which reference is made in the remarks on its adjustments. 
A is the Vernier-plate ; it is furnished with two verniers 3 a 3 180° apart 
