THE TRANSCONTINENTAL ARC. 
209 
The ultimate necessity of such a step had been apparent 
for some time. The intricate’and ever-growing network of 
trigonometric lines along the coast and down the Appalachian 
chain demanded a greater extension in order to fully sub¬ 
serve its original purpose. The secondary and tertiary work 
along the low coast line of the south required control. Eigh¬ 
teen thousand miles of triangulation exist from the mouth 
of the Chesapeake to Mobile bay. The various conditions 
under which it was executed, as well as the different methods 
of measurement, made some check operation imperative; for 
we must remember that around the shores of Florida water 
signals are employed, and direct measurements along the 
beach are resorted to in order that heavy cutting through 
the mangroves may be avoided; so that a primary triangu¬ 
lation in a direct line from Washington to Mobile becomes 
a necessity to test the work done along the Atlantic and Gulf 
coasts. 
In the same sense that a line of control is necessary for the 
work along the eastern and southern shores, there exists a 
still greater obligation to bind into one harmonious whole 
the geodetic operations on the Atlantic and Pacific coasts. 
It was to supply this link that the “ transcontinental arc ” 
was begun, and we now have the satisfaction of knowing 
that it is practically completed, and that our country can 
point with pride to the largest arc of longitude ever meas¬ 
ured by any single government. 
The author disclaims all pretension toward having con¬ 
tributed to any very great extent to the accomplishment of 
this work. It is, however, not possible, even were it desir¬ 
able, to name all the persons to whom credit is due. Among 
the many officers whose names appear with more or less fre¬ 
quency two occupy an unique position. One has carried the 
triangulation over the Rocky mountains, and for twenty 
years has practically made all his observations above the 
clouds. The other has had charge of the laborious calcula¬ 
tions, and for a quarter of a century has given the subject 
special attention. The former is Mr. William Eimbeck, to 
