20 
MINTS TO TftAVMLLEftS. 
These errors are generally included in the term <f centering error. 
The original error included in [1] and [2] can be determined at the 
National Physical Laboratory, where apparatus for the purpose is estab¬ 
lished. Those under [3] and [4] are manifestly variable. 
In a good sextant the original error should be small, amounting only 
to a few seconds, but instruments are made which have much larger 
errors, and as these are enormously multiplied in their effect in some 
observations, such as lunars, a traveller should always have this error 
determined before leaving England. 
The Box or Pocket Sextant . 
The box sextant is constructed on the same principle as the larger 
sextant; it is enclosed in a brass box, varying in size from 3 to 4 inches 
in diameter, and from an inch and a half to two inches deep. 
This instrument is very portable, light, and easily adjusted. It is 
more correct than the compass for measuring horizontal angles, as an 
angle can be read to within 1' by means of the vernier on the graduated 
arc. It can also be used on horseback, and in all sorts of weather, 
and, when not required for use, can either be carried in the pocket, or 
slung in a leather case over the shoulder. 
The instrument, as shown in the drawing, is ready for use: the cover, A, 
is screwed on to the lower part of the instrument, and serves as a handle 
when taking an angle; B is a graduated arc, divided into degrees and 
half degrees; C is the index bar, having a vernier at the end, divided to 
read the angle to 1'; D is a milled screiv by which the index bar is 
moved; attached to the end of the index bar, on the inside of the box, 
is the index glass, E; the horizon glass , E, one half of which is silvered, 
is also inside the box; Gr is a small magnifying glass attached to the 
top, to enable the observer to read the angle more clearly; there are 
dark glasses, to be used when observing the sun, not shown in the 
drawing. H is the adjusting screw , which is screwed into the top for 
safety; it is made with a square, like a watch-key, and when required 
for use has to be removed from the position shown in the drawing; I is 
the telescope, which should be fitted at the eye-end with a revolving disc N, 
which is provided with shades of different intensity, to be used with the 
artificial horizon; in taking angles the instrument can be used without 
