Latitude is reckoned in degrees (°), minutes ('), and seconds ("). Sixty seconds make one minute ('), and sixty minutes make one degree (°). A minute of latitude and a mile of latitude are one and the same.

The lines, or parallels, of latitude are drawn across the surface of the globe, running east and west, being parallel (hence their name) to the equator.

Circles perpendicular to the equator, and extending from pole to pole, are called meridians. All meridiaus meet at the poles, and there the longitude is nothing, for, although the latitude of the poles is 90°, there can be no longitude, because the poles are the extremeties of the earth's axis, and at those points there is neither east nor west, and a ship at the north pole can only sail a south course until she has gone far enough away from the pole to find meridians to cross. In a like manner a ship at the south pole can only steer a north course until she, too, finds meridians to cross.

One of these great circles, or meridians, is selected by different countries, and called the first, or prime meridian, and longitude is counted from this point, both east and west, one hundred and eighty degrees each way, until the meridian on the directly opposite side of the earth is reached. Thus longitude runs twice the amount of latitude in figures, for where the latter only counts as high as 90°, the former extends to 180°; but the four counts of latitude, from equator to pole, of 90° each, make the 360°—the circumference of the globe (the same as the circumference of any circle), and the two counts of longitude of 180° each, from the first meridian to the meridian directly opposite it on the other side of the world, gives the

same value, 360°. It will be seen that the longitude of any place on the earth's surface must be contained somewhere within the limits of 180°, either east or west of the first meridian.

The meridian which passes over the national observatory of a country is selected as the first meridian of that nation, and the charts issued are laid. out accordingly. Thus the French adopt the meridian of Paris, the Spaniards that of San Fernando, and the English that of Greenwich.

The United States of America has also a national observatory at Washington, but for convenience the Americans also consider the Greenwich meridian as a first meridian in calculating longitude. Thus it will be seen that the establishment of a first meridian is dictated by will only, differing from the great equatorial line which is governed by the measurement of equi-distances between the extremities of the earth's axis.

It is plain that as longitude is counted 180° each way from a certain meridian, one half the globe will be east of this line, and the other half west; and any place which lies on the eastern half will be in east longitude, and any place on the western half will be in west longitude.

When the sun is opposite a meridian, it is midday (noon) at that place, and midnight on the side of the globe directly opposite.

From the above remarks it will be evident that the longitude of a place will be its distance east or west from a certain meridian; but as a ship can sail from pole to pole without departing from a given meridian, we must also know the ship's latitude so as to locate its position on the meridian. With the latitude and longitude both given, the lines will cross one another, and definitely fix the ship's place. So it is that latitude and longitude are dependent upon one another to accurately determine a position on a chart. Longitude is said to be the distance of a place east or west from a given meridian, but this means the distance in degrees, minutes, and seconds of longitude, not in nautical miles, for it is only on the equator that a mile of longitude and a nautical mile (or sailing mile) are equal. The meridians are of normal size on the equator, but leaving the equator, going either north or south, the meridians converge, growing closer and closer to one another as the poles are approached; and on reaching the latter, as has been before noted, we find no longitude, all the meridians meeting at a point.

So it is that a ship sailing sixty nautical miles either east or west on a parallel removed from the equator, will run over more than a meridian of longitude, for although the space between the meridians continually shortens, the so-called degrees of longitude are "divided into sixty spaces called

longitudinal miles, and a nautical mile will run over a greater or less amount of longitude according to the length of the longitudinal miles, which are governed by their distance from the equator.

The following table gives the value of a degree of longitude in nautical miles for every parallel of latitude, north or south of the equator.

THE MARINER'S COMPASS.

The mariner's compass is a circular card on which are marked the letters N., S., E., W., standing for the four cardinal points. The space between each of these points is equal to one-quarter of the card, and these

quarters are called "quadrants." Half-way between the cardinals there are found the letters N.E., S.E., S.W., and N.W., showing that they are the intermediate or intercardinal points, representing a direction halfway between the cardinals. Thus N.E. is half-way between north and east; S. E., halfway between south and east; S. W., half-way between south and west; and N. W., halfway between north and west. Between these cardinals and inter-cardinals are other points, the names of which will be seen by reference to the compass diagram on page 19. The card has affixed to its under side a magnetized steal bar, riveted on the north and south line of the card. This card is balanced on a pivot rising from the bottom of the compass bowl, and on which it traverses freely; so that, although the compass case may be turned entirely around, the card will not change its position, the magnetized steel bar holding the north of the compass card toward the north point of the heavens, while the case revolves around the card.

BINNACLE.

The compass bowl is hung in gimbals so as to allow the card to retain a horizontal position in spite of the rolling and pitching of the vessel. What is known as a liquid compass has the distinctive peculiarities of a buoyant card in a liquid resisting medium, the mean density of the card being so adjusted to the density of the liquid as to produce a small downward pressure upon the pivot. The compass bowl is provided with a self-adjusting expansion chamber, by means of which the bowl is kept constantly full,

without the show of air bubbles on the one hand, or the development of undue pressure on the other from changes of temperature.

The cards of some liquid compasses float about in oil, and others in a mixture of alcohol and water-a sufficient quantity of spirits being used to prevent freezing.

The vertical black line marked on the interior surface of the bowl is called the "lubber's point," and the compass must be so placed that a line drawn through the pivot and lubber's point will be parallel to the direction of the keel.

Calling the names of the different points in rotation, beginning at north is called "boxing" the compass. The following shows the order: