vigation common? What is navigation proper? Upon what principles and sciences does navigation depend? Upon what combining causes does a ship's movement depend? To what errors is the science of navigation exposed? What are the methods of finding the latitude of a ship? What are the methods of finding a ship's longitude? What is plane sailing? What is traverse sailing? What is parallel sailing? What is middle latitude sailing? What is mercator's sailing? What is current sailing? What is oblique sailing? What is windward sailing?

CHAP. XXX.

INSTRUMENTS USED FOR ACCOMPLISHING THE PURPOSES OF NAVIGATION.

Of these, the principal are,

The compass, by which pilots ascertain and direct the course of ships. This consists of a circular brass box, which contains a paper card, marked with the thirty-two points into which the various directions of the winds are usually divided, fixed on a magnetic needle that always turns towards the north pole, excepting a small declination variable at different places.

The needle, together with the card, turns on an upright pin fixed in the centre of the box. In the centre of the needle is fixed a brass conical socket or cap, whereby the card, hanging on the pin, turns freely round the centre.

The top of the box is covered with glass, that the motion of the card may not be disturbed by the wind. The whole of this apparatus is in

closed in another box of wood, where it is suspended by brass hoops, to preserve the card in a horizontal posture.

The compass box must be so placed in the ship, that the middle section of the box, parallel to it's sides, may be parallel to the middle section of the vessel along its keel.

The invention of the compass is usually ascribed to Flavio da Melfi, or Flavio Gioia, a Neapolitan, about the year 1302. Others assert that the knowledge of this most useful instrument, was brought from China by the famous Venetian traveller, Marco Paulo, in 1260. Certain it is, that at first, the compass was fixed on a small piece of cork, which floated in water instead of being suspended on a pivot; the very practice which the Chinese still retain. This instrument has been greatly improved by Dr. Knight and Mr. Smeaton.

THE LOG.

The log is a small piece of wood, of a triangular form generally, about a quarter of an inch thick, and five or six inches from the angular point to the circumference. It is balanced by a thin plate of lead, so as to float perpendicularly in the water. To this is fastened a cord, or line, about one hundred and fifty fathoms long, one end of which is affixed to a reel in the gallery of the ship. This line, from the distance of about ten, twelve or fifteen fathoms off the log, has certain knots or divisions, which ought to be fifty feet from each other. The length of each knot is the same part of a sea mile, as half a mi

nute is of an hour. A nautical mile is computed at 6120 feet.

The use of the log and line is to keep account, and make an estimate of the ship's way, or distance run. This is done by observing the length of line unwound in half a minute of time, told by a half minute glass; for so many knots as run out in that time, so many miles the ship sails in an hour. Thus, if there be four knots veered out in half a minute, the vessel is computed to sail at the rate of four miles an hour. To whom the honour of this invention is due, is unknown. To heave the log, as it is called, it is thrown into the water on the lee side of the ship, and let run till it comes without the eddy of the ship's wake. Then one person, holding a half minute glass, turns it up just as the first knot, or the mark from which the knots begin to be reckoned, turns off the reel, or passes over the stern. As soon as the glass is out, the reel is stopped, and the knots which are run off are reckoned, and the parts estimated.

CHARTS.

The charts usually employed in the practice of navigation, are of two kinds; plane charts, and Mercator's charts. The plane chart is a representation of some part of the surface of the earth, in which the meridians are supposed parallel to each other; the parallels of latitude at equal distances, and, consequently, the degrees of longitude and latitude every where equal to each other.

Mercator's chart has the meridians straight lines,

parallel to each other, and equidistant. The parallels are also straight lines, and parallel to each other. But the distance between them increases from the equator towards either pole, in the ratio of a secant of the latitude to the radius.

HADLEY'S QUADRANT.

This is the chief instrument used at present, for observing altitudes at sea. The form of this instrument is an octagonal sector of a circle, and, consequently, contains forty-five degrees. It consists of the frame; an arch, or limb; an index and subdividing scale; an index glass; a fore horizon glass; a back horizon glass; coloured, or dark glasses; and vanes, or sights. This machine is very useful in navigation. By it the most accurate observations may be taken. It is easy to manage, at the mast head as well as on deck; by which its sphere of observation is greatly extended. It is formed upon a principle which originated with the celebrated Dr. Hooke, and was completed by Sir Isaac Newton; though it has received its name from Mr. Hadley, who first published an account of it.

THE SEXTANT.

The sextant is an instrument for the purpose of measuring, with accuracy, the angular distance between the sun and the moon, or between the moon and a fixed star, in order to ascertain the longitude by lunar observations. It is, therefore, constructed more carefully than the quadrant, and is more complicated. It has an arch, divided into 120 degrees; each degree is divided into three parts; each of those parts, therefore,

contains twenty minutes, which are subdivided into half minutes; an horizon glass; an index glass; and several coloured glasses. The sextant is furnished, likewise, with a plain tube without glasses; two telescopes; mirrors, and a magnifying glass.

THE CIRCULAR INSTRUMENT OF REFLECTION. This instrument was made to correct the errors to which the sextant is liable; especially an error which arises from the inaccuracy of the division on the limb. It consists of a circular ring or limb of metal, connected with a perforated central plate by six radii, and divided into 720 degrees; two moveable indices; two mirrors; a telescope; and coloured glasses.

THE SHIP'S JOURNAL.

A journal is a regular and exact register of all the various transactions that take place on board a ship, whether at sea or land; and more particularly an account of the ship's way; whence her place at noon, or at any other time, may be justly ascertained. That part of the account which is kept at sea, is called sea-work; and the remarks inserted in the journal during the time the ship is in port, is called harbour-work.

At sea, the day begins at noon, and ends at the noon of the following day. The first twelve hours, namely those contained between noon and midnight, are denoted by the letters P.M. or post meridiem, after midday, and the other twelve, that is the hours which elapse between midnight and noon, are denoted by A. M. ante meridiem, before midday. So that the work of