TRIANGULATION AND COMPUTING. 65 
supervise such office computations. Since the creation in 1903 of the 
separate section bearing* the above title, all office work connected with 
the reduction and publication of the results of spirit leveling has been 
in charge of the chief of that section. 
The methods and instruments employed in the astronomic and trigo- 
nometric work, as well as in the spirit leveling, of the Geological Sur- 
vey have developed as the work has progressed. This development 
has been in the direction of economy and efficiency as well as accuracy, 
so that at the present time, with a diminished expenditure for both 
field and office work, a much higher degree of accuracy is obtained 
than was possible at the outset. 
Tricing illation. — In early years the instruments used in triangulation 
were vernier theodolites reading to 10 seconds, with circles 6, 7, 8, 10, 
and 11 inches in diameter. In 1889 there were substituted for these, 
8-inch theodolites reading by microscope to 2 seconds, and these have 
since been exclusively employed in the primary triangulation. It is 
believed that the instruments at present used are of as high a grade as 
those in use in any other part of the world, and that, by employing 
sharper signals, by exercising more care in selecting the times for 
observing, and by taking a larger number of measurements of angles 
than heretofore, results can be obtained with them equal to the best 
secured in geodetic work. 
The signals used differ with the differing facilities afforded by the 
various localities. The commonest form — that generally used where 
sawed timber can be obtained — is the ordinary quadripod and pole. 
In the Rocky Mountain region a monument of stones is generally used. 
Each station is marked with a bronze tablet cemented in solid rock or 
in a dressed-stone post set in the ground, and sometimes by an iron 
bench-mark post. One or more witness marks are always left. 
Base measurement. — In the early years of the work base lines were 
measured with secondary base bars. These were used until 1887, 
when steel tapes 300 feet in length, under constant tension, were sub- 
stituted and have since been employed. They have been found more 
advantageous for the following reasons: The base can be measured 
more rapidly, and, owing to the diminished number of contacts, with 
quite as great accuracy. By making the measurements on cloudy days 
or at night, the correction for temperature can be determined nearly 
or quite as accurately. Longer bases are measured, thus simplifying 
the expansion; and because of the cheapness and speed of this method 
bases are measured more frequently, thus affording a greater number 
of checks on the triangulation. After the base line has been fully 
prepared for measurement and the men handling the tape have had a 
little training, a speed of a mile an hour can be attained, so that under 
favorable conditions a base 5 miles in length can be measured twice in 
one day. 
Bull. 227—04 5 
