To ascertain the Variation of the Compass.
The rule for ascertaining this by the bearing of the rising or setting sun is given in all epitomes of navigation, and need not be repeated here; but this is only practicable in a level country, where the sun can be seen within at most a few minutes of the proper times, and cannot be carried out among mountains or thick forests. In such case we were in the habit of setting up two sticks (A A), stretching a thread (B) across them, and from the middle of this letting fall a plumb line (C); then, taking a flat board, with a sheet of paper (D) stretched on it and projecting a little over the edge, make a hole in the projecting part to let the plumb line hang through, taking care not to vitiate its correctness; then select an object (E), the small upright stem of a tree, or set up a stick at a good distance, 100yds. or 200yds., or more, as nearly compass north of the plumb line as possible—if it is not exactly so, note its deviation on either side with the utmost correctness, and draw on the paper the line F D, which, if prolonged, would reach the tree at E. Then, between ten and eleven in the forenoon, set out the artificial horizon, and take an altitude of the sun; call “stop” sharply at the moment of observation, and let an assistant, with a straight-edged rule and pencil, draw the line of shadow cast by the plumb line at that moment; read off, and let your assistant record your observed altitude beside the line he has drawn; rest a minute or two at least; then take another observation, and so on, till three or five, or any odd number of lines have been drawn, forming a pencil of rays from the plumb line. Leave the sextant clamped at the last observation, and in the afternoon, before the sun comes down at that altitude, be ready with the horizon, and call “stop” at the moment, or even just before, the sun comes into contact, your assistant being ready, as before, to draw the shadow of the plumb line. You will now find that the short interval of rest taken between the morning observation will give you time to set your sextant to the next altitude, and to wait quietly till the sun comes down to it; do the same with the rest; and, when all are drawn, take the mean of each pencil of lines, and between them draw an exact central line, radiating like the rest from the plumb line; it will be evident that this must represent the shadow when the sun’s centre was exactly on the meridian, and therefore it must be due north and south. Measure the angle between this and the magnetic north line already drawn, and the result in degrees and minutes will be the variation of your compass; or, take the bearing of the line by compass, and the result in points or in degrees will be the variation, which call east or west, as the compass north is to the east or west of the true meridian. Take the bearing again of the tree or mark set up in the morning to ascertain that your paper has not been shifted during the observations. If your magnetic line has not been drawn exactly north and south it is not of great importance; but remember that the exact error must be ascertained and allowed for by addition or subtraction in calculating the angle between it and the true north. An unknown error, however small, renders an observation useless; but a known error, however great, involves only the trouble of correction.