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TH

THE NEW NAVIGATION

By L. B. STEWART

HE method used for determining the position of a ship at sea, popularly known as the New Navigation, is not really very new, as it was proposed about forty years ago by Marcq St. Hilaire, a French admiral. It was soon introduced into the navy, where its advantages were at once recognized, but it is only now coming into use in the merchant service. Like Sumner's method, of which it is strictly speaking a modification, it makes use of "lines of position", or Sumner lines, to determine by their intersection the position of a point of observation; and only differs from that method in the means by which those lines of position are found.

It is proposed here to give a brief account of the methods that have been used for finding a ship's position by taking two observations of a heavenly body, allowing a considerable interval of time. to elapse between them, or of two heavenly bodies at as nearly as possible the same time; and then plotting the resulting lines of

position on a chart. The result of some observations at Toronto will also be given, which were taken in order to test the precision of the graphical process, and the applicability of the method to the use of the explorer on land.

Sumner appears to have stumbled upon the method that bears his name purely by accident. Sailing from Charleston, S.C., in 1837, bound for Greenock, Scotland, he found himself after a stormy voyage approaching the coast of Ireland in a southerly gale, having had no opportunity for some days for taking an observation to determine his position, and with his dead-reckoning very unreliable. About 10 o'clock in the forenoon of a certain day the sun came out, and he at once took an observation, noting the Greenwich time. The best he could do was to use his dead-reckoning latitude in reducing this observation, which he did, finding the corresponding longitude. Not being satisfied with this result he then assumed two other latitudes in turn, one 10', and the other 20', north of his dead-reckoning position, and deduced the corresponding longitudes. He then plotted the three positions thus found on his chart, and found that they all lay on the same straight line, which bore E.N.E. This line when produced passed through. the Small's Light; and he put his ship on that course, and in about an hour sighted the light almost dead ahead. He concluded that the same observation might have been taken at any one of the three assumed positions, at the actual position of the ship, and at the Small's Light.

The line which Captain Sumner thus drew on his chart was a small portion of the circle of position,-so small, relatively to the whole, that its curvature was not apparent-whose centre or pole was the point on the earth's surface vertically under the sun, and whose angular radius was the sun's zenith distance as given by the observation. The point vertically under the sun, or the sub-solar point, of such a circle evidently lies on the meridian whose longitude is equal to the sun's hour angle at Greenwich, and its latitude is equal to the sun's declination. At any point on such a circle the same observation may be taken.

In Fig. 1 is shown a stereographic projection of the circles of position determined by the observations of the sun taken at Toronto on February 18th, 1919, the results of which are given below.

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Stereographic projection circles of position, observations of the sun, Feb. 18, 1919

Sumner's next step was to take a second observation of the sun about two hours after the first, and to reduce it in the same manner, using in turn two assumed latitudes, and computing the two corres

ponding longitudes. Thus two points were found on a second line of position, which was also plotted on the chart.

The intersection of the two lines of position would give the position of the ship if it had been at anchor between the two observations. In the interval, however, it had sailed 50 miles on a course E. by N., and the method of allowing for the run of the ship is best shewn by the aid of Fig. 2 (not drawn to any scale).

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In that figure AA is the first line of position, and BB the second; and it is necessary to find two points, one on each of these lines, at a distance of 50 miles apart on a course E. by N. This is best done by drawing a line on the given course through S, the dead-reckoning position at the first observation-or through any point near the intersection of the two lines of position. Then, to find the length of this line on the chart, compute, or take from a table, its departure, and multiply that quantity by the secant of the latitude, thus finding the difference of longitude covered between the observations. The meridian given by that difference of longitude will intersect SS' in the required point S'. If the course of the ship between the observations were to lie more nearly north and south than east and west, then the length SS' is best cut off by means of a parallel of latitude found by reducing the run of the ship to meridional difference of latitude.

Having plotted on the chart the course sailed over between the observations a line is then drawn through S' parallel to AA; its intersection S2 with BB is the position of the ship at the second observation. A line through S2 parallel to S'S determines S1, the position at the first observation, if this is required.

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This was the original Sumner method of determining the ship's position on a chart, and it entailed a considerable amount of labor, as it was necessary to compute four longitudes from the observations. As soon, however, as the fact was realized that these position lines are portions of circular arcs whose centres lie vertically under the sun-or other body observed-it was perceived that the direction of such a line is perpendicular to that of the sun. It was then only necessary to determine the azimuth of the sun in order to find the direction of a line of position, and this may be done either by the aid of tables or diagrams, or by means of a compass bearing. One point on such a line would then determine it completely, and that point was given by the observation, from which the longitude was computed, using in the reduction the latitude found by deadreckoning.

This method is illustrated in Fig. 3, in which S and S" are the first and second dead-reckoning positions, AA and BB the two lines of position, SS' the run of the ship between the observations, CC a line through S' parallel to AA and intersecting BB in the point S2, the position of the ship at the second observation. S2 S, drawn parallel to S'S gives S1, the position of the ship at the first observa

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