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marked off on the map, and, in the case of aerial travel, determined by a prominent object on the horizon, such as a clock tower, an isolated tree, etc. Knowing his speed, the pilot may, at any subsequent time, roughly estimate the distance already run and approximately deduce his proper position.

This position will, of course, be very frequently rectified, according to the indications of the map compared with the features of the terrain. The aviator will endeavor to recognize characteristic points, such as a high building, for instance, or the crossing of a canal with a railway line, or the junction of two rivers, etc. Now, the compass will not always suffice. Unknown to the pilot, a water current may cause the ship to drift; similarly, an unseasonable change of wind may pull the dirigible or aeroplane out of its course, when traveling in log or above the clouds. Both the seaman and the aerial pilot, when opportunity occurs, by the lifting of the fog or daylight reappearing, must recognize their position and, if necessary, rectify their route.

For this the seaman must make use of the sun or stars and deduce from their observation the geographical coordinates, that is, the lon-· gitude and latitude of his position. Owing to the rather precarious conditions of the arrangements on board it would be pretty difficult for airmen to use this method. Fortunately for them traveling over sea is rather an exception. They would be saved any reckoning by placing all over the country, at convenient intervals, aeronautical marks, that is, by writing, on the ground itself or on the roofs of buildings (by means of conventional signs or very conspicuous numerals) the longitude and latitude of the corresponding site. Having read these, the pilot with the help of a map, would be enabled to recognize his proper position and to reckon the distance of his destination, as well as its new orientation, that is, the inclination, to the meridian, of the new direction to be followed.

Notwithstanding every artifice, however, this method is still long and difficult enough, and would only be useful were it possible to exactly follow the arc of a great circle cutting the pilot's destination which, as a matter of fact, is rendered impossible by side winds and terrestrial obstacles. Airmen, like most seamen, will prefer to mark off the point on a map, or more simply to trace it on a simplified sketch, such as the index diagram (fig. 1, showing the fitting together of the sheets of the detailed map), upon which can be read the approximation of both required elements-distance and orientation. For instance, an aviator flying over Bourges (the town marked by a black point at the bottom of sheet 72 of the index diagram) toward Pau (black dot in the lower half of sheet 39) would at once see that the distance remaining to be covered is nearly equal to 4.25° of latitude measured on the diagram. Each degree being 67 miles long, the required distance would be roughly 285 miles. Moreover, the

direction to be followed is 26° to 27° W. of the meridian to the south, say, magnetic S. 40° W. (the magnetic declination at Bourges being 13.5° W.). The compass should therefore show the complement, say, N. 140° W.

If necessary, this diagram could even suffice for traveling. A straight line having been drawn joining starting point and destination, the pilot can see that his way will run successively right across the upper left angle of sheet 62, then cross the lower half of sheet 61, thereafter cut the upper edge of sheet 51 about the center, pass through the right-hand corner of the lower half of sheet 50, cut diagonally across sheet 40, graze the upper left corner of sheet 30, and finally end about the middle of the lower half of sheet 39.

Let us suppose that at any moment the pilot observes on the ground below a mark similar to that of figure 2, with the two figures 5 and 0 on either side, respectively, and with a large dot occupying the proper position of the town of Angoulême in the upper part of sheet 50 of the index map. He would at once conclude that he had deviated to the right of his route, and after a rough estimation he would incline 25° to 30° to the left. But, in every case, a detailed map would be necessary for landing.

Having examined many systems, the permanent committee for aerial navigation of the public works department of the French Government recently proposed for this map and for aeronautical signs the following solutions that seem to be the simplest ones and also the most likely to be adopted by other countries:

2. AIR MAP.

In order to attain the required object and to keep to the necessary clearness, an air map must show only the details required by airmen, either for finding their way or for landing.

To the first category belong the characteristic geographical features of the terrain, such as railways, main roads, channels, streams and rivers, lakes, forests, bushes, clumps of isolated trees, large areas of a similar cultivation, large boroughs with their outline and principal streets, chimneys of factories, clock towers, high towers, and, briefly, all objects liable to attract the attention of the pilot from a distance, either by means of their shape, dimensions, color, or situation.

To the second category belong turf pits, thick hedges, irrigation or drainage canals, electrical power lines traversing fields, and generally all objects liable to impede landing; and, in addition, gasometers, aerodromes and sheds where, if necessary, refuge or help could be obtained.

In the opinion of all competent authorities, the scale of 1 to 200,000 seems to be the most convenient one. On a smaller scale the map would be less clear; on a larger scale unwieldy without

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Longitudes East from the Antiméridian of Greenwich.
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Distances from South Pole

5° 4° 3° 2° 1° 0°
Longitude West

Sector 30

23° 4° 5° 6° 7° 8°

Longitude East of Greenwich.

| Sector 31

FIG. 1.-Index diagram showing for France the sheet lines of the Air Map.

Limits of the sheets of the Air Map.

Limits of the sheets of the World Map.

37....72 Numbers of the sheets PAU.... BOURGES, etc.

30, 31, 32 Numbers of the sectors of the World Map.

K, L, M Letters of Zones of the World Map.

Chief towns of Departments.

Itinerary from Bourges to Pau.

1320

FIG. 2.-Aeronautical mark.

9

Erected on the roof of a building in the environs of Pau, which occupies, in the lower half of sheet 39, the relative position represented by the large dot, both on the above frame and on the index diagram (fig. 1). The coordinates of the SW. corner of the sheet 39 are: 133° south polar distance and 179° new longitude E. reckoned from meridian 180° E. or W. of Greenwich (that is, 43° lat. N. and 1° long. W. of Greenwich).

appreciable advantage. In many countries maps on this scale already exist, but having been produced with special objects, either economical or strategical, they only imperfectly satisfy the wants of airmen. A new special map is therefore necessary. A provisional model, submitted by the Aero Club of France, showing specially the typical buildings by their profile in black, was adopted by the committee.

At my suggestion it was decided that the new map should be a subdivision of the International Map of the World, on a scale of 1 to 1,000,000,1 for the production of which, on the happy initiative of England, a common agreement was recently arrived at between the principal States of the civilized world.

The "world map" would furnish index diagrams for the fitting together of the sheets of the "air map," It would also be useful for drawing up schemes for long journeys, or for measuring the distance between two widely separated points and obtaining the orientation of the line adjoining them.

The "world map" is to be designed with the meter as unit of lengths, and the meridian of Greenwich as origin of the longitudes. The sheets are limited by meridians drawn out at successive intervals ́of 6°, extending from Greenwich, and by parallels traced out at successive intervals of 4°, reckoning from the Equator.

The meridian sectors, from longitude 180° east or west of Greenwich, are given numbers from 1 to 60, increasing in an easterly direction. The zones, extending from the Equator on each side to 88° latitude, are given letters from A to V preceded by the words "north or south.” The polar areas are lettered Z. Each sheet shall bear the name of the locality or most important geographical feature on the territory represented, and in addition the number of the sector and the letter of the zone crossing each other on the sheet in question. For example, the sheet of Paris will be named "North M. 31."

For each sheet the corresponding part of the terrestrial ellipsoid is represented by a modified polyconic projection constructed on its central meridian. The meridians are straight lines and the parallels arcs of circles the centers of which lie on the prolongation of the central meridian, so that the radius of each one is equal to the generatrix of the cone tangent to the ellipsoid along the corresponding parallel. The alterations of angles, distances, and areas are practically small enough to be neglected.

On the other hand, the sheets of the air map will be limited by meridians and parallels at successive intervals of 1°, reckoned from the same origins as for the world map. Each sheet should cover an area of 1° of longitude and 1° of latitude. Twenty-four sheets of this map will therefore cover the same area as the corresponding sheet of

1 That is not to say that the air map will be an exact amplification of the world map, but nearly so.

the world map on the scale of 1: 1,000,000. As the scale is five times as large, each of these sheets will be of approximately the same dimensions as the corresponding sheet of the 1: 1,000,000 map.

It is necessary, however, to introduce a more simple method of notation. It is customary to distinguish between longitudes east and west of the initial meridian and between latitudes north and south of the Equator. The nomenclature of the latitude and longitude therefore changes as one passes across either of these lines of origin. In other words, degrees of longitude increase to the right cr left from the initial meridian and degrees of latitude increase both north and south from the Equator. This notation is apt to cause trouble.

In order to overcome this disadvantage, the committee at my suggestion has decided to number the longitudes from 0° to 360° in an easterly direction, commencing from longitude 180° east or west of Greenwich, and to substitude for latitudes polar distances measured from the South Pole and reading from 0° to 180°. This is done in order that in the Northern Hemisphere, in which are situated most of the inhabited countries, the number may increase as usual from the Equator northward. Moreover, in the Northern Hemisphere the units of degrees would be the same for a south polar distance as for the corresponding latitude. The units of the longitude thus adopted will be the same, between 180° and 360°, as they are now for longitudes east of Greenwich. The tens of degrees, in the case of south polar distances, will be increased by 9 units, and in the case of longitudes east of Greenwich by 18 units.

In the Southern Hemisphere the polar distances will be the complements of the corresponding latitudes, and in the hemisphere extending from 0° to 180° of longitude the new longitudes of places will be the supplements of the present longitudes west of Greenwich. In addition to the usual notations, these new ones should be printed on the world map and on the index diagram, figure 1.

Each sheet of the air map will bear the name of the most important locality on the area covered by it and shall be numbered by the coordinates of its southwestern corner. This number shows the number of degrees of longitude and south polar distances in the coordinates of any point in the sheet, and in order to obtain the complete coordinates of any given point it will be sufficient to add to these figures the tenths of degrees or minutes obtained from the marginal scales.

The air map will be constructed on the same modified polyconic projection as that used for the world map. Each sheet shall be 56 centimeters, or 22 inches, high and from 41 centimeters to 34 centimeters broad (in France). The corresponding breadth near London would be 12 inches. Each sheet in France would therefore cover an

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