CHARTS are constructed upon the True Meridian, or Mercator's Scale, the degrees noted on the margins being proportioned to the position charted — north or south of the equator, on Which 60 nautical miles are represented by each degree.

When "Admiralty Charts" are issued by the London Chart Agent, they are supposed to have received all necessary corrections up to date.

When they are once out of the agent's hands there is no guarantee that further corrections will be made before they are sold by local firms at the different ports, and purchasers should obtain some assurance that the charts are correct to date.

All small corrections of any importance that can be made by hand are notified by Notices to Mariners.

Extensive corrections that cannot be conveniently made in this way are put upon the plates, after which fresh copies are issued.

The Track Chart of that part of the ocean which is being traversed at the time is publicly posted, together with the observations noted at noon each day, on all regular passenger ships. The course sailed is marked as a line with figures denoting the exact latitude and longitude and the number of miles covered since noon of the previous day, together with remarks as to the state of the weather, wind and sea, according to the Beaufort notation formula, which will be found below on page 54.

Austria, Belgium, Chile, Denmark, Germany, Italy, Japan, Norway, Russia, Sweden, the United States of America, and Holland (for all charts published at Batavia and for some published at the Hague), adopt the Meridian of Greenwich.

Holland, for most charts published at the Hague, adopts the Meridian of Amsterdam (W. Tower of Wester Kerk), which is assumed to be in longitude 4° 53' 4" E. of Greenwich.

France adopts the Meridian of Paris Observatory, assumed to be in longitude 2° 20' 15" E. of Greenwich.

Spain adopts the Meridian of San Fernando Observa­tory, Cadiz, assumed to be in longitude 6° 12' 24" W. of Greenwich.

Portugal adopts the Meridian of the Marine Observa­tory (Lisbon Castle), assumed to be in longitude 9° 8' 24" W. of Greenwich.

The Pulkowa Observatory of St. Petersburg (sometimes referred to in Russian Charts) is assumed to be in longitude 30° 19' 40" E. of Greenwich.

The Royal Observatory of Naples (sometimes referred to in Italian Charts) is assumed to be in longitude 14° 14' 43" E. of Greenwich.

Sea or Nautical mile = one-sixtieth of a degree of latitude, and varies from 6,046 ft. on the Equator to 6,092 ft. in lat. 60°.

Cable's length = the tenth of a nautical mile; or approximately, 100 fathoms or 200 yards.

A Knot = a nautical mile an hour, is a measure of speed, but is not infrequently, though erroneously, used as synonymous with a nautical mile.

The Sea Mile=6080 feet.

The Statute Land Mile=5280 feet.

The earth being round, its convexity limits the vision even on a level expanse like the sea. The line of vision on the seashore, of a man of ordinary height (say six feet), would be intercepted by the horizon at 3.24 miles. If he were looking at an object 44 feet in height, say a flag on a masthead, the flag would seem to be on the horizon if it were 9.35 miles distant. In this case add the height of the object, 44 feet, to the height of the eye from the ground, 6 feet equal 50 feet, and find the corresponding distance in the list below. A carrier pigeon at a mile above the earth would only command a field of 96 miles in radius.

This table shows the distance from sea-level at which objects are visible at different elevations: —