Ex. True course S. E., with variation 1 points westerly, gives compass course S. b W. W.

Ex. True course S.W., with variation point easterly, gives compass

Ex. True course E., with variation 1 points easterly, gives course to

Ex. True course E., with variation 24 points westerly, gives course to be steered S.E. 6 E. & E.

In British waters, to find the true course made good, apply the variation to the left of the point steered by compass; on the contrary, to find the magnetic or compass course to be steered, apply the variation to the right; because the Variation is Westerly.

DISTANCE AND THE NAUTICAL MILE.

The distance that a ship makes good is reckoned in nautical miles, and no other measure of distance is recognised in Navigation. This mile is the 21,600th part of the earth's circumference (360°=21,600'), and is estimated to be about 6,087 feet, or 2,029 yards. It is roughly taken to be 1,000 fathoms, and hence a cable's length (which is the tenth part of a nautical mile) is 100 fathoms. A nautical mile=1-1528 statute mile.

The knot is the equivalent of the nautical mile, and is the term generally in use in connection with a vessel's rate of sailing. When the rate is not determined by a Patent Log, the log-line and sand-glass are used now, the length of a knot on this line must be in the same proportion to the nautical mile that the seconds of the glass are to the hour; 3,600 sec. 1 hour, and take 6,000 feet 1 mile; but as 3 to 5 is in the ratio of 3,600 to 6,000, hence, for the 14 seconds-glass the length of the knot will be 23-3 ft., being the result of 14 x 53. This determination is the safe one of having the reckoning ahead of the ship.

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LATITUDE AND LONGITUDE.

Latitude without longitude, or longitude without latitude, is of no use in fixing the position of a place on the land or at sea; both must be known for that purpose, and I must here offer a few words of explanation on the subject.

The Latitude of a place is its angular distance, North or South, from the equator, an imaginary circle round the globe, equidistant from the poles; and it is reckoned from 0° at the equator to 90° at the pole. We in the northern hemisphere are in N. Lat.

Now, you know we may sail either North or South. If, being in Lat. N., we sail N., we increase our latitude, or distance from the equator; but by sailing S. we decrease our latitude. And note that the northing or southing we make is called difference of latitude, generally written Biff. Lat.

The latitude left (written Lat. left) may be a point of land, an island, or a lighthouse; or it may be a spot on the ocean where, at any hour, an astronomical observation had been taken to determine the position, and from which we had departed.

The latitude arrived at (written Lat. in), if not determined by an observation, is obtained by applying the northing or southing made good to the latitude left. Thus Lat. left ± Diff. Lat. Lat. in.

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The middle latitude (written Mid. Lat.) between two places is the latitude of the parallel passing midway between them; hence half-way between the Lat. left and Lat. in.

The Longitude of a place is the angular distance of the meridian passing over the place from the first meridian, reckoned on the equator. With us the first

*

meridian is that of Greenwich Observatory, which is Long. 0°; and thence longitude is reckoned East or West to 180°.

The difference of longitude is written Diff. Long. ; but you must note that a ship, when she sails eastward or westward, though she makes difference of longitude, does not make it under that name; in fact, the east or west distance by which she has departed from any meridian is called Departure, which has to be converted into difference of longitude by calculation, by inspection (as will presently be shown), or by projection, as on p. 33; and you will further note, that it is only on the equator that the minutes (') of longitude coincide with the miles of departure; everywhere else a given number of miles of departure makes a greater number of minutes of longitude; thus, on the 60th parallel, 30 miles of departure make 60' or 1° of longitude.

Let us suppose our departure (written Dep.) converted into Diff. Long.; then, being in Long. E., if our Diff. Long. be E., we increase our longitude, or distance from the meridian of Greenwich, but if our Diff. Long. is W., we decrease it. Similarly, being in Long. W., Diff. Long. W. will increase it, but Diff. Long. E. will decrease it.

Much that has been said of latitude now also applies to longitude.

Longitude left (written Long, left) implies the meridian whence the ship departed; and longitude in (written Long, in) is the meridian at which the ship has arrived. And Long. left Diff. Long.=Long. in.

You can readily see that when a vessel sails true South or North, she does not change her longitude,

* Meridians are great circles passing through both poles, and perpendicular to the equator.

though, of course, she has changed her latitude. And, conversely, if a vessel sails true West or East, she alters her longitude, though she has not changed her latitude. When she sails on any other course than true West or East, or true North or South, she alters both her latitude and longitude.

THE USE OF CHARTS.

The learner should supply himself with a small chart of the World, and one of the English Channel, which we will now suppose to be before him.

Charts are representations of parts of the surface of the globe, constructed in such a manner as shall be most serviceable to the navigator. The projection used is that known as Mercator's, on which all the meridians are equidistant straight lines, and the distance between successive parallels increases polarwise (see Chart, p. 20).

I will explain this projection. You know that on the globe the parallels of latitude are small circles parallel with the equator, and that the distance between any two near the equator is practically the same as that between any two nearer the pole. On the other hand, the meridians converge towards, and meet at, the poles; they are widest apart at the equator, and the distance between any two meridians has already decreased, in latitude 60°, to half what it is at the equator. For the purposes of Navigation a map on the globular projection is useless. But what would be the consequence if the parallels and meridians were projected into equidistant straight lines, as on the old plane charts ?—great distortion of such parts of the earth's surface as lie in the higher latitudes; and it is wonderful how the old navigators did so much with

them.

breadth as

The plane surface of the chart, to be useful in Navigation, must preserve the relations of length and they exist on the spherical surface of the globe; and herein consists the excellence of Mercator's Chart. The meridians are parallel straight lines, and the degrees of longitude are all equal and equidistant; hence, contrary to what is actually the case on the surface of the globe, the meridians are as wide apart in the higher latitudes as on the equator. To compensate for this distortion the parallels of latitude, which are parallel with the equator and at right angles to the meridians, have the degrees of latitude unequal, being extended in length in the same proportion as the degrees of longitude on the globe diminish; and, consequently, the extension is the greater the greater the distance from the equator. The table used in the construction of such a chart is that called Meridional Parts (Tab. VI.); it gives the increase in length of small portions of the meridian, expressed in minutes of the equator. The most important advantage possessed by this kind of projection is the representation of the bearing and course between any two places as a straight line, which corresponds with the most important apparent fact in the mind of the seaman.

Mercator's Projection has been compared to a cylinder unrolled the cylinder, of indefinite length, being such an one as we might suppose to circumscribe the globe at the equator.

The outermost parallels (see Chart on p. 20), at the North and South parts (top and bottom) of the chart, are the graduated parallels on which longitude is measured —nothing else; distance never.

The outermost meridians, on the East and West (right and left) sides of the chart, are the graduated meridians on which distance and latitude are measured.