Every school child that has reached the third standard knows that the globe is represented as criss-crossed by a large number of lines running from pole to pole, that is from north to south, and others right round the globe in the opposite direction, or from east to west. These lines cross each other at right angles. The up and down ones, from pole to pole, are meridians of longitude; the East-West ones are parallels of latitude. Now, since these are all numbered as degrees, the space between them being 1°, the latitudes from the Equator to the poles on either side of it as 1° to 90°, and the meridians from Greenwich to its opposite point on the other side of the world 1° to 180°, it follows that if a seafarer can ascertain at the same time what particular degree of both latitude and longitude he is in, a glance at his chart or sea-map shows him the position of his ship. This operation (finding the latitude and longitude) is performed in a variety of ways, but the simplest, and consequently the most universally used at sea, is by measuring the sun's height above the horizon at noon for the latitude, and about three hours before or after noon for the longitude. This is done by means of a pretty instrument called a sextant with the greatest ease and speed. At noon, the moment the sun reaches his highest point for the day, it is twelve o'clock, and a calculation, made in one minute, shows exactly how far the ship is north or south of the Equator. The observations for longitude take a little longer. From the sun's height, at the moment of observation, is calculated the exact time at the ship. And as a chronometer, which every ship carries, shows the exact time at Greenwich, the difference between the two expresses in hours and minutes (easily convertible into degrees and miles) the distance east or west of Greenwich, the first meridian of longitude; for every degree (60 miles) is equal to four minutes of time. Having found the latitude and longitude, the master makes a little dot upon the chart at the exact point where the lines of latitude and longitude which he is on cross one another, and sees as plainly as if he were standing at a well-known street-crossing where he is.