Hence it is evidently unnecessary that the index error should be previously known, and even preferable that its amount should be such as to avoid the needless introduction of negative quantities by positions on different sides of zero.
In the preceding description, it is supposed that the eye is looking directly through the unsilvered glass at the horizon, and that it also perceives the opposite horizon after two reflections; but an inspection of the figure will shew that the observer's head would necessarily intercept the rays from the horizon behind him. To obviate this, both the direct and the reflected rays are received in coming from the unsilvered glass, (and after passing through the field-glass of the telescope) on a mirror placed at an angle of 45º, which reflects them to the eye. By this ingenious contrivance, the obstruction is removed, and the opposite points of the horizon may be both seen at one moment.
In practice, it is most convenient to direct the telescope to the same part of the horizon in both cases. Thus, if the east and west parts of the horizon be observed, and that the index glass be uppermost, and telescope pointing to the west, the observer is on the south side, and his face must 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 at 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 vertical 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. It 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 at once.
It is advisable to take several observations, and the safest way is to take one first with the index glass uppermost, and then with the instrument 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 of observations 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 of 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 in all probability will be different at the two observations. With an artificial horizon, indeed, the changes in refraction are not felt, because, at a considerable elevation above the horizon, the changes are very trifling. But it often happens in practice, that the artificial horizon cannot be used, and we are then reduced to the sea horizon, where the changes of refraction are always the greatest. In the Yellow Sea, for instance, we had no opportunity of landing during all the time that the squadron was at anchor, till the day before we sailed. So that during nearly a fortnight that the ships were at anchor, the sea horizon was necessarily used. I need only to refer to the observations taken off the Pei-ho, viz. from No. 37 to 62, to shew how extremely fallacious the results must have been.
It is much to be wished that this excellent instrument should be brought into general use in navigation.