214 
PRESTON. 
over 100 miles in length, each of which has been observed 
from both directions. 
This statement calls to mind some special devices employed 
on the work. On the western coast a tree was cut off at a 
distance of 100 feet from the ground and an observing scaf¬ 
fold mounted on the standing shaft. This ingenious method 
overcame intervening obstacles, and at the same time secured 
great economy in construction. In Maryland, where the 
level nature of the ground demands signals of extraordinary 
height, the plan was adopted of first building the ordinary 
scaffold 175 feet high and then putting a pole 83 feet long 
on top of it. This furnished a signal for observation 258 
feet from the ground at half the expense necessary for com¬ 
plete tripods. In another instance a pole 120 feet long was 
erected on a scaffold of equal height. In the Sierra Nevadas 
heliotropes are brought into use, and the signals from dis¬ 
tant stations 100 miles away are visible only as a beam of 
light shining with the brilliancy of a star of the second mag¬ 
nitude. The great length of line precludes the possibility of 
recognizing any natural object, and the low clouds cut off 
the mountain peak itself; yet through the mist and vapor 
comes the reflected ray of sunlight, and upon this the ob¬ 
server directs his telescope. 
The full value of the transcontinental arc is not at first 
apparent. Not only is it national in its use, but every State 
through which it passes comes into possession of accurate 
geographical positions; and not alone that, but every State 
lying near the thirty-ninth parallel can, at slight expense, 
fix its own boundaries with an accuracy not inferior to that 
given by the best government surveys in the world. No less 
than sixteen States are directly benefited by the triangula¬ 
tion across the continent, and as many more have the possi¬ 
bility of benefit. 
With the advance of refined methods and the ability to 
execute work of a magnitude unknown in earlier days, we 
are now obliged to carry the computations to a degree of 
precision commensurate with the newly imposed conditions. 
