There are two general types of compasses in use: one known as the pocket compass or movable-needle compass, the other as the mariner’s compass or floating-card compass. The former is generally used on land and has a fixed card with the various points marked upon it and a movable needle which points to the north, while the mariner’s compass has a card with the points which revolves and there is a notch or lubber’s mark on one side of the case which should be so placed that when facing north the north mark on the card is exactly in line with the lubber’s mark.
Use of Compass in Boat
A—Mariner’s compass. B-C—Pocket compass.
In a boat the floating-card or mariner’s compass is almost a necessity, for with it the boat’s bow may be headed in the direction or course desired, whereas with a pocket compass the dial remains stationary and the needle moves about and as a result some mental calculation is necessary in order to steer a course correctly.
This will be better understood by studying the accompanying illustration. In this you will see that at A a boat with a mariner’s compass is headed northeast, and that if the course is to be altered to any given point of the compass it is merely necessary to turn until the desired mark is in line with the lubber’s mark.
In the diagram B, however, the boat is apparently headed north although the same course is being steered as in A. This is because the compass used is a fixed-card compass with a movable needle and the needle moves as the boat’s course is changed, while the card remains stationary, and although the boat is really headed northeast the needle points to the northwest. In other words, when using such a compass it is necessary to read it backwards and if you wish to steer northeast swing the boat until the needle points northwest, and so on, for every direction. This, of course, is very confusing and it can be avoided only by shifting the position of the compass so as to bring the needle directly over “north” each time the boat’s course is altered as shown at C. By doing this the boat’s bow will correspond to the direction being steered, as indicated on the compass card, but it is often very inconvenient, if not impossible, to move a compass constantly while bobbing about in a sea or tacking, although on land it is no trouble to turn the compass until the needle and “north” are in line and then proceed in the desired direction.
Moreover, a pivoted needle is often very erratic and swings wildly when in a boat and for these reasons a floating-dial compass should always be used. Many pocket compasses are made with moving dials, or cards, and these will serve very well for small boats, but they are not to be compared to the true boat compasses for steadiness, accuracy and convenience.
Sometimes one may find oneself without a compass and may wish to obtain a general idea of direction and in such a case it is of great value to know that an ordinary watch or clock may be made to serve as a compass.
To use a watch as a compass, place it horizontally, with the hour hand pointing directly towards the sun, or until the shadow of the hour hand falls directly beneath the hand itself. When this position is attained south will be exactly halfway between the point of the hour hand and the figure 12; counting from left to right, or southward, if before noon and from right to left if after noon.