8 RESEARCH IN CHINA. 
night. We were therefore obliged to accept sights to more or less sharply 
defined mountain summits. The uncertainty of our direction and the 
possibility that one or another sighted peak might not be accessible from 
our line of march made it desirable to have a number of possible triangula- 
tion stations located in advance. It was not, however, always possible 
to have as many intersections as we might like on the point that we were 
ultimately obliged to occupy, and the method of locating the triangula- 
tion points was by resection. By this method at least one sight from an 
established point to the point to be occupied was necessary. ‘This sight 
was drawn on the plane-table sheet while at a previous station, and when 
the table was set up at the indeterminate station the alidade was placed 
precisely along the pencil line and sighted back to the station from which 
the line was drawn. ‘The table being thus very nearly oriented, sights 
were taken to a number of other established points, and the orientation 
was adjusted until a sufficient number of lines passed precisely through a 
point which must lie on the initial line of foresight. It was then accepted 
as the location of a new station. So far as possible, only stations which 
had been occupied were used to determine the position of a new station, 
but conspicuous peaks on either side of the belt of survey were carefully 
located, and when their position had been fixed by the intersection of 
four or more lines they were used as checks, and in one or two instances 
to locate a station. ‘The difficulties in the way of precise work were 
increased by the condition of the atmosphere, which was often unfavorable 
on account of dust. Clearness was exceptional, and it was never safe to 
occupy a station far in advance, lest a sufficient number of control points 
in the rear should not be visible. In some instances it became necessary 
to remain on the mountain summits until toward sunset, to take advantage 
of the clearer atmosphere, which then sometimes follows upon an unfavor- 
able condition during the day. 
In the course of graphic triangulation, any errors of orientation which 
may creep in become speedily apparent in the triangle of error that is 
formed by the lines which should intersect in a point. The work from 
T’ang-hién to T’ai-ytian-fu checked out satisfactorily in this respect. As 
a final check a second base line was measured near ‘T’ai-yiian-fu, at the 
close of the survey. Its position is shown on sheet B III. It is the dis- 
tance between two conspicuous pagodas, which could not be measured 
directly as the line lay across the city, but which was determined indepen- 
dently by measuring, with care, a base line half a mile long, from which 
the distance between the pagodas was determined by triangulation with 
the theodolite. 
