To obtain Levels or low Altitudes.
In mapping a country, it is desirable to give as nearly as possible the comparative heights of the hills; or it may be necessary to take the altitude of a low star, or the height of a tree or other object. For this purpose take a wide-mouthed jar or glass bottle, such as is used for the collection of specimens; round the neck of it splice or tie a cord or thong of leather to carry it by; then tie or fix by sealing-wax, gum, or resin, two hairs or threads crossing each other at right angles; and then, standing over the jar with the face towards the object to be measured, take a pocket, or full-sized sextant, and bring the top of the object to the spot where the threads cross each other, taking care to keep the eye so perpendicularly above their junction that the crossed lines and their reflected image may appear in one; then read off the observed angle—for instance 110° 42′ 30″. Apply the index error, which suppose to be 30″ subtracted, and subtract 90° from the corrected altitude, the remainder will be the true altitude; thus:
| ° | ′ | ″ | |
|---|---|---|---|
| Observed [delta] | 110 | 42 | 30 |
| Index error | 0 | 0 | 30 |
| 110 | 42 | 0 | |
| 90 | 0 | 0 | |
| Altitude required | 20 | 42 | 0 |
It will conduce very greatly to steadiness if a rest is used; the stand of a theodolite, with the instrument removed, or of a photographic camera, or three forked sticks set up so that the sight could be directed downwards through a central aperture, would answer the purpose. If the sextant is set to 90°, allowing for index error, and the horizon is swept round with it, the eye being kept steadily perpendicular to the cross lines, it will at once show what objects are above or below the level of the observer’s eye. A looking-glass may be used in the same manner, but it must be very carefully tested in every direction with a spirit level and even then it is but an inefficient substitute for the mercury, which levels itself, and which cannot possibly give a false result, because nothing but agitation can destroy the perfect level of its surface, and unless it is at rest it cannot reflect an unbroken image, and no observation can be made.
A looking-glass may also be suspended vertically, its surface being tested by two plumb lines, one on each side its centre. A horizontal line may be stretched at a foot or two from its front; then, if the observer, retiring a few paces, raises or lowers himself till the line and its reflection appear in one—his eye and the line must be in the same horizontal plane—and he may observe the altitude of the sun, or other celestial body, when in the zenith or too high to be observed in the mercurial artificial horizon; this plan, however, is only an approximation to correctness, and should only be used when the mercurial horizon is not available.