220 
PRESTON. 
discordant may always be reconciled by carefully gathering 
facts and seeking the cause. 
The supreme test of a result is its verification by independ¬ 
ent methods or its agreement with known laws, and in con¬ 
clusion I shall call attention to a few tests that have been 
applied to the work in question. The simple operation of 
closing a triangle is of daily occurrence. In the best work 
of our arc the accuracy attained is such that the lines of 
sight are within a fifth of a second of the truth, which 
amounts to saying that the linear value of the uncertainty 
is only towot of an inch on the limb of the instrument used. 
Independent determination of points in latitude and lon¬ 
gitude by astronomical observation will be within 10 feet of 
the astronomical position. In order to fully appreciate this 
degree of precision, we need only reflect that a change of 10 
feet in the position of a latitude instrument on the earth’s 
surface corresponds to a change of height of one end of the 
level by utoVtto part of an inch—that is to say, we would ob¬ 
serve the same effect in our bubble by moving the instru¬ 
ment 10 feet as we would by lifting one end of the level the 
amount stated. This shows that 10 feet on the surface of 
the earth determined by astronomical means is a very small 
and almost inappreciable quantity. 
The longitude work has an accuracy equal to that just 
cited, the uncertainty being about of a second of time in 
primary positions. 
The quality of the triangulation is best shown by a com¬ 
parison of bases. The Fire Island one, nearly 9 miles long, 
was determined in five different ways through 1,800 miles 
of triangulation, and the extreme range of the results is only 
two-tenths of a meter. The value from Kent Island base, 5 
miles long and 263 miles away, only differed from that given 
by the Atlanta base, nearly 6 miles long and 868 miles away, 
by one centimeter. 
Another striking example is found in the agreement of 
the American bottom base, measured by the Coast and Geo¬ 
detic Survey, and the Olney base, measured by the United 
