The speed of a ship is ascertained by the " patent log " or by a " log ship " and sand glass, the latter still being preferred by most sailing ship masters. The "log ship," two forms of which are given in Figs. 1 and 2, is hove over the weather quarter attached to the log line divided into "knots," a "knot" bearing the same proportion to a mile as the sand glass running in seconds does to an hour. Sand glasses, or " log glasses," are made to run 14 seconds and 28 seconds (the former being for use when fast travelling, and the knots by line have of course to be doubled if spaced for 28 seconds). It must be remembered a nautical mile is 2,027 yd., usually called 6,080ft. It corresponds with the minutes of arc; thus there are 300 x 60 = 21,600 of arc, or nautical miles, on a "great circle" (or the equator). The number of yards therefore in a " great circle " divided by 21,600 will give the number of yards in a nautical mile. In calculating the length of a knot in feet, the rule adopted is this. To the seconds run by the glass affix a cypher and divide by 0. The remainder when doubled gives the inches.

Thus for a 28-second glass 280.

280/6 = 46 + 4, or the distance between adjacent knots =46 ft. 8in.

This is not correct, but the error is for safety, as the ship, unaffected by favourable currents, will be behind her position by log or " dead reckoning," as it is called. To calculate the exact length between adjacent knots on the line, multiply 2,027 by 14 or 28 and divide by 3,600. Fig. 1 is the wooden log ship; it is a wooden quadrant about 1/2in. thick and 10in. diameter, the arc being weighted with lead to make the log float vertically. The end of the log line L passes through a hole and is secured by knotting at the back, while a wooden peg P is attached to a span B from the line L. When the log line is suddenly checked in its running out, this wooden peg withdraws its hold in the quadrant, and the log ship is hauled in with ease. The canvas log is shown in Fig. 2. The log line is attached at D to the canvas bag C, at the mouth of which is a span seized to the peg at E, which is pushed into a wooden ferrule W seized to the log line; when the line is checked the peg withdraws as in the former case, and the bag closes, being hauled in bottom foremost. The " patent log," by which name the several revolving logs go at sea, is self-registering, and not hove at intervals as the former kind. A meter is clamped to the taffrail, showing on its face by three hands the units, tens, and hundreds of nautical miles run since the last setting, which is done at noon. This meter is a simple train of wheels to which motion is imparted by a threaded pin. At the back of the meter and attached to the pin is a brass universal joint J (Fig. 3),to which is secured the end of a line sufficiently long to clear the eddies and backwash of propellers, etc. At the other end is the spinner S, a three-bladed brass fan, pitched to revolve at such a speed that after being towed one mile the unit hand on the meter shall have made one revolution from 0 to 0, the intermediates being marked 1/4, 1/2, 3/4. Sometimes a light fly-wheel is attached to the line just abaft the taffrail, but this is not really necessary. Ball bearings between a cone collar on the shaft and a dished bearing in the back of the meter would be an improvement. Owing to the length of flexible line through which the revolutions are transmitted, the motion is a succession of spurts, but this, however, does not affect the correctness of distance registered in the twenty-four hours.

Working a Ship's Log.