be turned to the north. When the instrument is inverted;
if the observer turn himself round at the same time, so as
to face the south, then the telescope will be pointed as
before to the west; but since the index glass is now undermost,
the inferior arc will now be measured precisely as if
his face were to the north, but with the advantage of the
same lights seen in the erect position of the instrument.
In using this instrument a t sea for the first time, considerable
difficulty arises from the constant change in the
plane of the instrument from the perpendicular position, in
which it is absolutely necessary that it should be held, in
order to obtain a correct observation. What at first appears
to be a defect, however, is a real advantage, namely, that
whenever it is held in the least degree out of the vertical
plane, the two horizons (that seen direct, and the reflected
one) cross each other, and it is only when the plane is ver*
tical that the horizons can appear parallel.
The object is to get the two horizons to coincide exactly,
and for this purpose it will often be necessary to have them
of different shades. This is managed, as in the sextant, by
means of the screw, which raises or lowers the telescope.
When the telescope is brought nearer to the plane of the
instrument, the reflected horizon becomes dark and distinct,
but when screwed off it becomes fainter, and is not so well
defined. Practice alone can teach the degree of intensity
which is most favourable. In general it is best to have one
horizon dark, and the other light; then bring them very
nearly to coincide, and wait till the ship is steady, at which
moment a slight, touch of the tangent screw brings them
exactly to cover one another. I t will happen, of course,
that, when the coincidence is perfect, there is only one horizon
to be seen, and a doubt remains whether all is right,
but a slight motion of the instrument, by making the
horizons cross each other, defines them a t once.
I t is advisable to take several observations, and the
safest way is to take otie first with the index glass uppermost,
arid then with the instrumént inverted, after which to
return to the first, and so on for two or three times each
way;
In the pages which follow, there is given a table containing
the result of all the observations made during this
voyage, preceded by several sets óf obsérvations in the
fullest detail. From the table it will be observed how
seldom the dip, actually measured, agrees with that inferred
from the mean refraction. Some of these' experiments shew
very remarkable differences, and point out the great utility
o f this instrument.
The practical navigator, particularly if he has been in
hot climates, will recollect how discordant his observations
for latitude always were, and how few even of the best observers
agree in their determination of the latitude of the
same place, simple as the observation is thought to be.
The cause is quite clear; and though it equally affects altitudes
taken for absolute time, the disagreement is less
obvious, and it will often happen that a chronometer going
extremely well appears to vary every day from inaccuracy
in the observations. Thus it is, I think, generally admitted,
that it is almost impossible to rate a chronometer from altitudes
observed with the sea horizon. Nor is this difficulty
removed by taking equal altitudes, because the refraction