THE

MODERN PRECEPTOR.

CHAPTER IX.

NAVIGATION.

NAVIGATION is the art of conducting a ship from one port to another across the ocean, and of ascertaining her position at any given point of her course.

In navigation two things are supposed to be given, the direction in which the ship sails or her course, and the distance she runs over in that direction. The direction or course is ascertained by means of an instrument, called from its usefulness to seamen, the mariner's compass, consisting of a circle of strong paper or card, having its circumference divided into 32 equal parts, each division being termed a point of the compass; and as the circumference of every circle is divided into 360 degrees (vol. i. p. 383), the interval between any two points must be the 32d part of that circumference, or 11 deg. 15 min.

When in any part of the northern hemisphere, we turn our face towards the sun on the meridian, or at noon, he is then due south from us, and the pole star immediately opposite, behind us, is due north; our left hand is turned to the east, and our right to the west. The horizon is therefore divided

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divided into 4 equal parts or quadrants, each containing 90°, by radii drawn from the place where we stand to the north, south, east, and west points. If we divide the quadrant formed by the radius pointing north and that running east into two equal parts, we obtain a point partaking equally of these two directions, and therefore styled north-east again bisecting the arch between N and NE we have the NNE point, and bisecting the arch between NE and E we have the point ENE. If between N and NNE we assume another equidistant division, it is called N by E, while the intermediate point between NNE and NE is called NE by N: the point midway between NE and ENE is on the other hand termed NE by E, while that equidistant between ENE and E is styled E by N. In this manner each quadrant of the compass and of the horizon is divided into eight points, and the whole circumference consequently into thirty-two, which begininning at N and running round by W S and E, are named as in the following table:

The points are subdivided into halves, quarters, and eighths. The circumference of some compasses is also divided

Every thing ought to be carefully avoided which may in any way tend ✯ mislead the navigator: it is proper therefore to observe that certaia nations of

divided into degrees, minutes, &c. for determining the direction of the ship's course with greater accuracy; because this course may not exactly coincide with any point or even eighth part of a point, and the bearing is expressed thus, N. 35° 17 E. S. 73° 29′ W.

To the lower side of the compass card is fixed what is called the needle, which ought to be a straight bar of hard steel, with square ends: this needle being rendered magnetic, and nicely balanced on the top of a slender pin so as to move freely round this central point, will place itself in the direction of the meridian, one end pointing to the N. and the other to the S. This magnetic meridian, however, coincides with the true meridian of the earth only in very few places: the angle of deviation is termed the variation of the needle; and this variation itself is not fixed at any one place, but undergoes a gradual change. When the needle points in a direction to the eastward of the true north, the variation is said to be easterly, and westerly when it points to the westward of north. The following table contains a statement of the variation (or declination from the true meridian) of the magnetic needle at London in different years, by which it appears that 250 years ago the needle pointed considerably to the eastward of N.; that 150 years ago it pointed due N.; and that a few years ago, it pointed 24 or upwards of two points to the westward of N.

of Europe, viz. the French, the Portuguese, the Spaniards, and the Italians, have a peculiar mode of denominating the divisions of the compass, in which the expressions un quart, un quarto, signifying not a quarter of a point in the English sense, but a whole point itself, as being the quarter of the space containing 4 points comprehended between any of the cardinal points, N. W. S. and E, and the intermediate points NW, SW, SE, NE. In this way N E. in a Spanish work on navigation means in English N. by E.; SWIW, represents our SW. by W., &c.

Years.