cted is explained in
detail in the Arts. in Bowditch you were given to read last night. You
do not have to actually construct such a chart, as the Government has
for sale blank Mercator charts for every parallel of latitude in which
they can be used. It is well to remember, however, that since a mile or
minute of latitude has a different value in every latitude, there is an
appearance of distortion in every Mercator chart which covers any large
extent of surface. For instance, an island near the pole, will be
represented as being much larger than one of the same size near the
equator, due to the different scale used to preserve the accurate
character of the projection.

_The Polyconic Projection_

The theory of the Polyconic Projection is based upon conceiving the
earth's surface as a series of cones, each one having the parallel as
its base and its vertex in the point where a tangent to the earth at
that latitude intersects the earth's axis. The degrees of latitude and
longitude on this chart are projected in their true length and the
general distortion of the earth's surface is less than in any other
method of projection.

[Illustration]

A straight line on the polyconic chart represents a near approach to a
great circle, making a slightly different angle with each meridian of
longitude as they converge toward the poles. The parallels of latitude
are also shown as curved lines, this being apparent on all but large
scale charts. The Polyconic Projection is especially adapted to
surveying, but is also employed to some extent in charts of the U. S.
Coast & Geodetic Survey.

_Gnomonic Projection_

The theory of this projection is to make a curved line appear and be a
straight line on the chart, i.e., as though you were at the center of
the earth and looking out toward the circumference. The Gnomonic
Projection is of particular value in sailing long distance courses where
following a curved line over the earth's surface is the shortest
distance between two points that are widely separated. This is called
Great Circle Sailing and will be talked about in more detail later on.
The point to remember here is that the Hydrographic Office prints Great
Circle Sailing Charts covering all the navigable waters of the globe.
Since all these charts are constructed on the Gnomonic Projection, it is
only necessary to join any two points by a straight line to get the
_curved_ line or great circle track which your ship is to fo