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CHAPTER IV.

INSTRUMENTS USED IN NAVIGATION.

THE LOG-SHIP AND GLASSES.

159. The necessary instruments of Navigation are the LOG, which, with the help of the sand-glasses for measuring small intervals of time, or a watch shewing seconds, gives the velocity or rate of the ship; and the COMPASS, which gives the direction of the ship's track.

160. The LOG consists of the log-ship and line. The log-ship is a thin wooden sector, of about five inches radius, the circular edge being loaded with lead to make it float upright, and present its broad face to the water, so as not to slip through it. The

Sand-glass.

The Log.

inner end of the log-line is fastened to a reel which is kept on board, while the outer end is fixed to the log-ship. At

ten or twelve fathoms from the log-ship, on the log-line a bit of bunting rag is placed, to mark off a sufficient quantity of line, called stray-line, to let the log-ship be clear of the ship's eddy before the time is counted.

161. The log-line is divided into equal portions, called knots, at each of which a bit of string, with the number of knots upon it, is put through the strands. The length of a knot depends on the number of seconds that the glass runs, and may be thus determined-The feet in a knot: the feet in a mile the seconds the glass runs: the seconds in an hour.

=

Or,

the feet in a knot

the feet in a mile

the seconds the glass runs the seconds in an hour (3600)*

Reel.

The nautical mile is about 6083 feet, from which we obtain, by the proportion above, for the 30 seconds' glass

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In the same manner may the length of a knot be found for a glass that runs any other number of seconds.

162. It is plain from the above that the ship sails as many miles per hour as there are knots run off the line during the seconds that the glass runs. Both the length of a knot and the seconds that the glass runs are liable to become faulty; and it is necessary to know how to correct the distance that has been measured by a faulty line or a faulty glass, or when both the line and glass are faulty.

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the true distance when both the glass and line are faulty (C).

Hence, if the line only is faulty, use (A); if the glass only is faulty, use (B); if both glass and line are faulty, use (C). 163. Since we have seen above (Art. 161) that when the glass runs 30 seconds, the true length of a knot may be taken at 50 feet; that is, when

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and substituting these values in (C), we get

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164. To correct the distance given by the log and half-minute glass, when the line, the glass, or both are faulty.

RULE.

Multiply three times the given distance by the measured length of a knot, and divide the product by five times the seconds which the glass takes in running out, and the quotient will in any case be the true distance.

Example.

A ship sails 143 miles, but it is found on examination that

the length of the knot on the log-line is 51 feet, and that the glass runs out in 28 seconds; what is the true distance?

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In the following exercises the true distance is required:

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165. Called after the patentee, Edward Massey, is an instrument for registering the distance a ship has actually gone through the water. It consists of a rotator or fly, which communicates motion to wheels, which register on three dials the distance sailed since it was immersed. It is drawn behind the ship by means of a line varying from 20 to 50 fathoms, according to the size of the vessel, its use being to let the log be so

far astern that its action will not be affected by eddy or the ship's wake. By means of this instrument, 100 miles may be registered without once taking in the log. Every time, however, the course is changed the log must be taken in, and a fresh commencement of the register made.

OF THE COMPASS.

166. The Compass is an instrument which utilises the directive power of the magnet; it consists of a circular card attached to

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peculiar property points

always in a definite direction at any given place; the circum

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