296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
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 subse- 
quent time, roughly estimate the distance already run and approxi- 
mately 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. 
Yor 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 build- 
ings (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 recog- 
nize 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 are 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 
