DIVIDERS

The dividers are nothing but an instrument for measuring distances, etc., on the chart.

THE LOG

There are two kinds of logs--the chip log, used for measuring the speed of the ship, and the patent log, used for measuring distance run.

The chip log consists of a reel, line, toggle and chip. Usually a second gla.s.s is used for measuring time. The chip is the triangular piece of wood ballasted with lead to ride point up. The toggle is a little wooden case into which a peg, joining the ends of the two lower lines of the bridle, is set in such a way that a jerk on the line will free it, causing the log to lie flat so that it can be hauled in. The first 10 or 15 fathoms of line from the log-chip are called "stray line," and the end of this is distinguished by a mark of red bunting. Its purpose is to let the chip get clear of the vessel's wake. The marks on the line (called knots) are pieces of fish line running through the strands of the reel line to the number of two, three, four, etc. A piece of white bunting marks every two-tenths of a knot. This is because the run of the ship is recorded in knots and tenths. The knots of fish line are 47 feet 4 inches from each other.

The log gla.s.s measures 28 seconds in time. For high rates of speed, a 14 second gla.s.s is used. Then the number of knots shown by the log line must be doubled. The principle of the chip log is that each division of the log line bears the same ratio to a nautical mile that the log gla.s.s does to the hour. In other words, if 10 knots or divisions of the log line run out while the 28 second gla.s.s empties itself, the ship's speed is 10 knots per hour. If ten knots or divisions run out while the 14 second gla.s.s empties itself, the ship's speed is 20 knots per hour.

The patent or towing log consists of a dial, line and rotator. The large circle of the dial records the knots and the small circle tenths of knots. When changing course, read the log and enter it in the log book.

When changing course again, read the log again. The difference between the two readings will be the distance run.

Both logs are liable to error. A following sea makes them under-rate, a head sea over-rate. With both logs you must allow for currents. If a current is against you--and you know its rate--you must deduct its rate from that recorded in the log and vice versa. The reason for this is that your log measures your speed through the water. What you must find is your actual distance made good over the earth's surface.

Put in your Note-Book:

Between Sandy Hook and Fort Hamilton, bound due North, speed by chip-log was 10 knots, tidal current setting North 2 knots per hour; what did the ship make per hour? Answer: 12 knots.

At sea in North Sea ship heading S x W, patent log bet. 8 A.M. and 12 M.

registered 32 miles, current running N x E 2 knots per hour; what was the actual distance made good? Answer: 24 miles.

Directions for allowing for a current setting diagonally across a ship's course will be given in the proper place.

a.s.sign for Night Work the following articles in Bowditch: Arts.

161-162-163-164-165.

THURSDAY LECTURE

THE CHART

_Aids to Navigation_

A chart is a map of an ocean, bay, sound or other navigable water. It shows the character of the coast, heights of mountains, depths at low water, direction and velocity of tidal currents, location, character, height and radius of visibility of all beacon lights, location of rocks, shoals, buoys, and nature of the bottom wherever soundings can be obtained.

The top of the chart is North unless otherwise noted. When in doubt as to where North is, consult the compa.s.s card printed somewhere on the chart. On sea charts, such as those of the North Atlantic, only the true compa.s.s is printed, with the amount and direction of Variation indicated by lines on the chart.

Parallels of lat.i.tude are shown by straight lines running parallel to each other across the chart. The degrees and minutes of these parallels are given on the perpendicular border of the chart. Meridians of longitude are shown by straight lines running up and down, perpendicular to the parallels of lat.i.tude, and the degrees and minutes of these meridians are given on the horizontal border of the chart.

Put in your Note-Book:

A minute of lat.i.tude is always a mile, because parallels of lat.i.tude are equidistant at all places. A minute of longitude is a mile only on the equator, for the meridians are coming closer to each other as they converge toward either pole. They come together at the North and South poles, and here there is no longitude.

I can explain this very easily by reference to the following ill.u.s.tration:

[Ill.u.s.tration]

As every parallel of lat.i.tude is a circle of 360 the distance from A to B will be the same number of degrees, minutes and seconds whether measured upon parallel AA' or EE', but it will not be the same number of miles as the meridians of longitude are gradually converging toward the poles. On the other hand, the distances from A to C, C to D, D to E, etc., must be the same because the lines AA', CC', DD', EE' are all parallel. That is why the distance is always measured on the lat.i.tude scale (i.e. on the vertical border of the chart), and a minute of lat.i.tude is always a mile on the chart, no matter in what locality your ship happens to be.

You should be able to understand any kind of information given you on a chart. For instance, what are the various kinds of buoys and how are they marked?

Put in your Note-Book:

1. In coming from seaward, red buoys mark the starboard side of the channel, and black buoys the port side.

2. Dangers and obstructions which may be pa.s.sed on either hand are marked by buoys with red and black horizontal stripes.

3. Buoys indicating the fairway are marked with black and white vertical stripes and should be pa.s.sed close to.

4. Sunken wrecks are marked by red and black striped buoys described in No. 2. In foreign countries green buoys are frequently used to mark sunken wrecks.

5. Quarantine buoys are yellow.

6. As white buoys have no especial significance, they are frequently used for special purposes not connected with Navigation.

7. Starboard and port buoys are numbered from the seaward end of the channel, the black bearing the odd and red bearing the even numbers.

8. Perches with b.a.l.l.s, cages, etc., will, when placed on buoys, be at turning points, the color and number indicating on which side they shall be pa.s.sed.

9. Soundings in plain white are in fathoms; those on shaded parts are in feet. On large ocean charts fathom curves, showing the range of soundings of 10, 20, 30, 40, etc., fathoms are shown.

10. A light is indicated by a red and yellow spot. F. means fixed, Fl., flashing; Int., intermittent; Rev., revolving, etc.

11. An arrow indicates a current and its direction. The speed is always given.

12. Rocks just under water are shown by a cross surrounded by a dotted circle; rocks above water, by a dotted circle with dots inside it.

Practically all charts you will use will be called Mercator charts. Just how they are constructed is a difficult mathematical affair but, roughly, the idea of their construction is based upon the earth being a cylinder, instead of a sphere. Hence, the meridians of longitude, instead of converging at the poles, are parallel lines. This compels the parallels of lat.i.tude to be adjusted correspondingly. Although such a chart in any one locality is out of proportion compared with some distant part of the earth's surface, it is nevertheless in proportion for the distance you can travel in a day or possibly a week--and that is all you desire. The Hydrographic Office publishes blank Mercator charts for all lat.i.tudes in which they can be used for plotting your position.

It makes no difference what longitude you are in for, on a Mercator chart, meridians of longitude are all marked parallel. It makes a great difference, however, what lat.i.tude you are in, as in each a mile is of different length on the chart. Hence, it will be impossible for you to correctly plot your course and distance sailed unless you have a chart which shows on it the degrees of lat.i.tude in which you are. For instance, if your Mercator chart shows parallels of lat.i.tude from 30 to 40 that chart must be used when you are in one of those lat.i.tudes. When you move into 41 or 29, you must be sure to change your plotting chart accordingly. In very high lat.i.tudes and near the North pole, the Mercator chart is worthless. How can you steer for the North pole when the meridians of your chart never come together at any pole? For the same reason, bearings of distant objects may be slightly off when laid down on this chart in a straight line. On the whole, however, the Mercator chart answers the mariner's needs so far as all practical purposes are concerned.

The instruments used in consulting a chart, i.e., parallel rulers, dividers, etc. have already been described. The only way to lay down a course and read it is by practice.

The one important thing to remember in laying down a course, is that what you lay down is a true course. To steam this course yourself, you must make the proper correction for your compa.s.s error.

a.s.sign for Night Work in Bowditch, Arts.

9-239-240-241-243-244-245-246-247-248-249-251-252-253-254-255-256-257-258.

If any time in cla.s.s room is left, spend it in laying down courses on the chart and reading them; also in answering such questions as these: