338 Notes on Map Projections. 
well-tested right-angled ruler, with twenty-four inch legs, and a 
hard pencil; or first. carefully construct a single accurate perpen- 
which lay off the meridian distance from the perpendicular, and 
draw the parallel lines through the three equidistant points thus 
obtained for each. Take from the tables for each required point 
of intersection between parallels and meridians, its appropriate 
length of arc of parallel, from which subtract the corresponding 2. 
Lay off this difference from the central meridian each way on its 
proper perpendicular, and erect, towards the pole, at the point so 
formed, a perpendicular equal to the corresponding value of y in 
the tables—its extremity is the point of intersection required. 
Through all the corresponding points of intersection trace the 
parallels and the meridians. Erase the auxiliary lines, and write 
on the margin the numerical latitude and longitude. 
The following mode is more rapid and better checked: Lay 
off first the longest arcs of parallel, and then take the length of 
a single subdivision of the parallel in a pair of hair-spring divi- 
ders, and step it off on the perpendicular to the right and left 
of the central meridian; being careful net to prick the paper. 
Having adjusted the dividers so as to bring the extreme points 
Equidistant polyconic method—{ Inadinissible in projections cov- 
ering more than one square degree.) 
Proceed as before to graduate the central meridian, and to con- 
struct a central parallel. Construct the points of meridian inter 
section with the top and bottom parallels, and as many interme- 
diate parallels as are requisite closely to determine the meridians. 
Through these points then draw the meridians. Starting now 
from the central parallel, lay off on each meridian the distance to 
the required parallel equal to that on the central meridian, and 
trace the parallels through these points. Proceed in like mannef 
to construct the others, using always the central parallel as 2 base, 
ant the totals measured from it along the central meridian 12 
aying 
