projection is quite simple, and can be mastered by any one that has a knowledge of drawing and simple arithmetic. It

is extremely useful to a navigator, in case charts are lost

or destroyed, to be able to construct a substitute for temporary use.

From the the nature of their construction, however, the inexactness of the Mercator's chart fully reveals itself in high latitudes, and for this reason is quite impracticable for polar navigation.

EXAMPLE. Draw the frame of a Mercator's chart extending from latitude 40° to 43° N, and from longitude 105° to 108° E, on a scale of 2 inches to a degree of longitude.

SOLUTION.-Draw a line at the bottom margin of the paper to represent the parallel of 40°. On this line lay off three lengths of 2 inches each, and divide each length into 60 equal parts, representing nautical miles; at the extremities of the line, carefully erect perpendiculars to represent meridians. Then consult the table of meridional parts and take out the values corresponding to each degree of latitude, and obtain the differences as indicated below.

This complied with, take, with the dividers, 1° 18.6′ from the scale of longitude, and lay it off on each perpendicular from the base line, and through the points thus obtained draw the parallel of 41°. In like manner, from the parallel of 41°, lay off 1° 19.8′ and draw the parallel of 42°. Proceed similarly and get the parallel of 43°. Divide this last parallel into degrees and minutes the same as the parallel of 40° at the bottom margin, and draw the meridians of 106° and 107° east longitude. The result will be as shown in Fig. 13, representing a chart on Mercator's projection, according to the limits required.

EXAMPLES FOR PRACTICE

1. Construct a Mercator's chart extending from 179° E to 178° W and from 49° S to 52° S on a scale of 1 inch to each degree of longitude.

2. Construct a Mercator's chart on the scale of inch to a degree of longitude extending from 60° N to 63° N and from 56 W to 60 W.

HOW TO USE CHARTS

49. When a chart is properly spread out, the top of it is north, the bottom south, the side to the right east, and the side to the left west; if, for any reason, it is otherwise, north will be indicated by the direction of the north point of the compass diagram situated somewhere on the chart. The

latitude scale is found on the right and the left side of the chart, and the longitude scale at the upper and the lower margin of the chart. On a general chart, such as that of the North Atlantic Ocean, the compass indicates true north, and the variation, its direction and amount, is marked by means of letters and dotted lines; in the majority of smaller charts, the north point of the compass rose indicates magnetic north. 50. The instruments used in connection with charts are the parallel ruler, the dividers, and the course protractor.

51. The Parallel ruler, Fig. 14, is usually made of ebony or gutta percha. The two parts are connected by cross-pieces of brass

working on pivots in such a manner that they may be spread apart or pushed together and still

remain parallel to

FIG. 14

each other. They are used for the purpose of transferring the direction of a bearing or course to the nearest compass diagram, or vice versa. For instance, in Fig. 15, if it be required to find the bearing between a and b, the edge of a closed parallel ruler is laid between the two places, as

in the figure, and then the upper part of the ruler is pushed forwards to the center c of the nearest compass diagram, and the bearing read off at once.

In some cases it may be necessary to take several " steps" with the ruler in order to reach the diagram, but the operation in itself is so simple as to need no further explanation.

The main point, however, is to preserve the parallelism of the last step with the first.

52. The dividers, shown in Fig. 16, are used to lay off and measure distances. They should be made of well

tempered steel; their points should

be sharp, but not too fine. When using the dividers to lay off a distance into a number of equal parts, hold them at the top between the thumb and forefinger and step off the spaces, turning the instrument alternately to the right and left. When laying off a distance in this manner, great care must be exercised not to press the divider points into the chart; it should be turned lightly, and then the final point is indicated by a pencil mark, lightly applied.

53. The course protractor is shown in Fig. 17. The outer edge is a semicircle with a center at O, and is divided into 360 parts. Each division is one-half of 1°, and, for convenience, the degrees are numbered from 0° to 180° from both A and B. This instrument is used for laying off or measuring courses or bearings, and should be made of transparent horn or celluloid and with a radius of at least 3 inches. Protractors are often made of metal, in which case the central part is cut away to make the chart under it visible.

FIG. 16

54. When using the protractor, it must be placed so that the line OB, Fig. 17, will coincide with the line forming one side of the angle to be laid off, or measured, and the center O must be at the vertex of the angle. For example, let it be required to find the bearing between the two places a and b, Fig. 18. Proceed then as follows: Lay any straight edge,

for instance, that of a ruler, between and over the two places; then, place the protractor along the ruler with its center con one of the meridians, as shown in figure, and read off the

bearing as indicated by the number of degrees of the arc de, reckoned from the meridian-in this case N 75° W.

55. To Lay Down a Position on the Chart.—When the latitude and longitude of a ship or a place is given, its