334 Notes on Map Projections. 
its area unchanged, its two diagonals become unequal ; one in- 
creasing and the other diminishing in receding towards the cor- 
ners of the map, the greatest inequality being towards the east 
and west polar corners. Though great circles between stations 
on the earth are generally projected into curves, the amount of 
deviation for moderate limits is very slight on a Bonne projection. 
The scale is néarly uniform over the entire projection, being ac- 
curate along the parallels and along their radii, but being, too 
great along one diagonal of the quadrilaterals, and too small along 
the other. In an area of 120° longitude and 70° latitude, a dis- 
tance of 7,000 miles is in error but th. This projection has 
thus many excellent qualities for topographical maps; and its 
defects of oblique intersections, of unequal diagonals, and of 
scales varying with the point of the compass, are not very serious 
in a limited area, as in the map of France, or that of England 
and Wales. A special set of tables for each central parallel is 
required in this method ; and the extent of these is so vast as to 
make impracticable the conception of a universal set of tables. 
The French tables of Plessis are based on the parallel of 502". ot 
45°, and are available for any area centered on this line, except ° 
t 
THE POLYCONIC PROJECTION, ITS PROPERTIES AND VARIETIES: 
The operations of the coast survey being limited to a narrow 
belt along the seaboard, and not being intended to furnish a Map 
of the country in regular uniform sheets, it is preferred to make 
a re 
gular series projected on Bonne’s method. In fact, each sheet 8 
projected strictly as a local map, and its connexion with t 
adjoining sheets is established solely by the points of triangula- 
tion. In reductions, including several sheets, the plotting ° 
points is the first step, and the change of scale is then made y 
corresponding squares. By the aid of the coast-survey said 
rectangular polyconic projection can at one made for & d 
‘locality or subdivision of the United States, or for the Unit 
