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 2 M 2 divided

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 to mislead the navigator: it is proper therefore to observe that certain 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 230 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 anv 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.

Besides the variation or declination of the magnetic needle from the direction of the earth's axis, it is subject to another irregularity, by which in the northern hemisphere that end pointing to the north is drawn a little downwards from the horizontal line, and the reverse in the southern : this is called the dip of the needle. At the equator the needle being equally acted upon by both poles of the earth, maintains a horizontal position; but if it be moved towards either pole, the end next to the pole towards which it is removed will gradually dip more and more below the horizontal line; and would probably, if it could be carried quite to the pole, there stand perpendicular to the surface of the earth. At London in the year 1786, the north end of the magnetic needle dipped 17° 52′ below the horizon, and in 1805, the dip was found to be 19° 39'.

The instruments usually employed to ascertain the distance run in a given time by a ship at sea, are the log, and the half-minute glass. The log is commonly a thin piece of timber, shaped like the quadrant of a circle of 5 or 6 inches radius, having a plate of lead attached to the circular part, so as to make the log swim perpendicularly in the To this piece of timber is fastened a small cord or line from 100 to 150 fathoms in length, divided into equal

water.

parts

parts by pieces of twine, with as many knots on them as there are spaces between each and the first mark from the log: from this the distances themselves are called knots, and they ought to bear the same proportion to the nautical mile that half a minute bears to an hour, or the knots ought to be ' of a nautical mile, which being equal to a minute of a degree of a great circle on the earth, or to 6,100 English feet, each knot should contain nearly 51 feet: as a precaution however against mistakes and accidents, it is customary to make the distance between any two knots only from 48 to 50 feet. The glass employed at sea is a small sand-glass of the ordinary construction, but containing only so much sand as will run from the one end to the other in half a minute. When the log is thrown from the ship into the water, the number of spaces or knots on the line run out while the glass runs half a minute is observed and on the supposition that the ship's motion for a certain time, as an hour, is tolerably equable, it is ascertained by the following proportion: as the half minute run by the glass to the number of knots shown by the log, so is one hour to the number of miles sailed in that time.

In king's ships and East Indiamen it is usual to heave the log every hour, but in other vessels once in every two hours is considered to be sufficient: but with every precaution and allowance that can be applied, this mode of measuring a ship's rate of motion, notwithstanding the many improvements suggested by ingenuity and experience, must be subject to great uncertainty, from the unequal force of the winds during even the short space of an hour, from the motion of the water by currents and waves, and from other contingencies which do not admit of any accurate computation.

Were it possible at sea to determine a ship's position by celestial observation with sufficient accuracy, much of the uncertainty of the ordinary method of measuring her course

and