The deviation, if any, is ascertained by what is termed swinging the vessel. The correct magnetic bearing of some distant object is first obtained. The vessel is then slowly swung round—this can often be done by carrying the mooring line along one side of the deck aft, and then along the other side to the bows again—and as the vessel’s head comes to each point of the compass, a compass bearing is taken of the object, and the difference between that bearing and the true magnetic one is noted.
A table can then be drawn out showing the deviation to be applied for each point of the compass towards which a vessel is steering.
When steering by compass, do not keep the eyes constantly fixed on the compass card, as this is extremely wearying, especially by night. When you have got the vessel on her course, get some object ahead—a distant vessel, a cloud, a star, if no land is in sight—in line with the mast, or one of the shrouds, and steer for that; referring only occasionally to the compass, so as to correct the direction and get a new mark to steer for when necessary.
The Log Ship and Line are employed to calculate the speed at which a vessel is travelling through the water. The log ship ([Fig. 46]) is a quadrant-shaped piece of wood, loaded with lead at the bottom, so that it may preserve an upright position in the water. It is attached to the log line by means of a bridle.
Fig. 46.
The ordinary log line is 120 fathoms long. The log glass generally used is a common sand-glass, running out in 28 seconds. The line is divided into equal distances by knots, each distance being the same fraction of a nautical mile as 28 seconds is of one hour; the distance therefore between two knots is about 47 feet.
A bit of bunting is attached to the line about 12 fathoms from the log ship. These 12 fathoms are called the stray line.
The process known as “heaving the log” is performed as follows: One hand stands by, with the glass in hand, full end downwards; another hand heaves the log ship overboard, which floats away astern; the line is then steadily paid out from the reel, in such a way that no strain is put on the log ship.