0 deg.) to the
South Pole (90 deg.). The difference of Latitude between any two places is
the arc of the meridian intercepted between the parallels of Latitude of
the places and is marked N or S according to the direction in which you
steam (T n').

The Longitude of a place on the surface of the earth is the arc of the
equator intercepted between the meridian of the place and the meridian
at Greenwich, England, called the Prime Meridian. Longitude is reckoned
East or West through 180 deg. from the Meridian at Greenwich. Difference of
Longitude between any two places is the arc of the equator intercepted
between their meridians, and is called East or West according to
direction. Example: Diff. Lo. T and T' = E' M, and E or W according as
to which way you go.

Departure is the actual linear distance measured on a parallel of
Latitude between two meridians. Difference of Latitude is reckoned in
minutes because miles and minutes of Latitude are always the same.
Departure, however, is only reckoned in _miles_, because while a mile is
equal to 1' of longitude on the equator, it is equal to more than 1' as
the latitude increases; the reason being, of course, that the meridians
of Lo. converge toward the pole, and the distance between the same two
meridians grows less and less as you leave the equator and go toward
either pole. Example: TN, N'n'. 10 mi. departure on the equator = 10'
difference in Lo. 10 mi. departure in Lat. 55 deg. equals something like 18'
difference in Lo.

The curved line which joins any two places on the earth's surface,
cutting all the meridians at the same angle, is called the Rhumb Line.
The angle which this line makes with the meridian of Lo. intersecting
any point in question is the Course, and the length of the line between
any two places is called the distance between them. Example: T or T'.

_Chart Projections_

The earth is projected, so to speak, upon a chart in three different
ways--the Mercator Projection, the Polyconic Projection and the Gnomonic
Projection.

_The Mercator Projection_

You already know something about the Mercator Projection and a Mercator
chart. As explained before, it is constructed on the theory that the
earth is a cylinder instead of a sphere. The meridians of longitude,
therefore, run parallel instead of converging, and the parallels of
latitude are lengthened out to correspond to the widening out of the Lo.
meridians. Just how this Mercator chart is constru