Fig. 50.
An azimuth compass is one specially adapted for taking bearings. Its card is more carefully divided than that of the steering compass, and it is fitted with sight vanes.
However, bearings sufficiently accurate for practical purposes can be taken with the ordinary compass. Hold a piece of string across the centre of the compass, and, looking along it, direct it towards the object whose bearing is required, as if taking an aim with a gun. The direction of the string will then indicate the bearing on the compass card beneath.
In taking cross bearings endeavour to obtain a difference between them of as near 90° as possible; for if the difference be small, as, for example, 10°, or large, as 150°, a small error in the bearing will cause a great error in the calculation of the vessel’s position.
If, when directing one’s course out of sight of land, as, for instance, from Yarmouth to the mouth of the Elbe, head winds are met with, and it becomes necessary to tack, it is an advantage as a general rule to sail on that tack on which the vessel looks up best for her port, and not to go about until she has brought herself to a position on which the other tack is the most favourable, and so on. The ship thus constantly keeps her port in the wind’s eye, and any change in the direction of the wind can be taken advantage of. But if a vessel stands on, till the tack be a losing one in order that she may fetch her destination on the next tack, a change in the wind may put her dead to leeward of her port and she will have lost ground by the ill-judged tactics.
When tacking out of sight of land, the direction and length of each successive board can be pricked out on the chart by using the dividers and parallel rule in the manner already described, and the position of the vessel at any time will thus be known.
Before land is lost sight of, what is termed a departure is taken from the last well-known object on the shore. Its bearing is taken by compass, its distance by log is estimated, and the time is noted. A departure can also be taken by cross bearings.