78 Review of the Results of the U. 8. Coast Survey. 
The base which serves as the commencement of the primary _ 
triangulation for the eastern and middle states lies within the 
state of Massachusetts, a little to the north of Rhode Island. 
Its length is over ten miles and its direction nearly northeast 
and southwest: it was measured in 1845. Surveys for a verifi- 
cation base have been made in the northeastern part of the state 
of Maine, on Epping Plains. Since the commencement of the 
survey not less than nine primary and thirty-five secondary 
bases have been measured, making a total length of about 180 
iles. i 
As the different stations of the primary triangulation are at 
different heights, it is necessary to measure vertical as well as 
horizontal angles, and finally, in consequence of the spheroidal 
figure of the earth, to reduce the plane triangles, which are the 
direct results of the measurements, to spheroidal triangles, at the 
level of the sea. In this manner after immense labor, both of — 
observation and of calculation, the positions of the primary sta- _ 
tions are at length fixed, and these now serve as starting points 
for the secondary triangulation which determines the general 
outline of the coast in detail, and the positions of rocks, reefs, _ 
and islands. The triangles observed are now smaller but very — 
much more numerous, and the labor of observation and reduc- 
tion even greater than before. Then comes the topography of — 
the coast, the work at every successive step running more and 
more into detail. The coast line is now traced and laid down in © 
charts of elaborate minuteness and finish. Harbors are surveyed — 
and mapped out by innumerable soundings; the exact character — 
and value of each being determined. The nature of the bottom 
with reference to anchorage, the depth and direction of channels, 
currents, tides and prevalent winds, the proper position of light 
houses, buoys and fog-bells, all form subjects of special and mi 
nute attention. In this manner the entire coast from Mt. Desert 
island to Cape Fear has been almost completely surveyed, and — 
that portion of the sea-coast may be regarded as nearly finished. 
But beside the bays, harbors, and sounds of the coast, the rivers 
receive their share of attention, small triangulations being cat 
ried up to the head of tide waters, based upon one of the sides 
of the larger work. In this manner the river shores are accU- 
rately mapped, while careful soundings determine the bars and 
channels. The mouths of the larger rivers offer special subject’ 
of examination of the highest interest and importance. We 
refer to the changes in the depth and position of the channels — 
produced by the effects of currents. The characteristics of the — 
delta of the Mississippi, and the enormous quantities of matter 
annually brought down by the current are too familiar to require 
notice in this place, but the changes in the entrance to the ha 
bor of New York have not until very recently attracted attenti 
to the same degree. | 
