216 
PRESTON. 
the correction is one of frequent application. It is, more¬ 
over, a quantity that cannot be ignored, when we consider 
that the uncertainty of the latitude results is probably not 
more than one-tenth part of the influence of elevation on 
our higher stations. 
Considered as a contribution to science, as well as in its 
map-making usefulness, the transcontinental arc must be 
studied from several points of view. A simple chain of 
mathematical figures from the Atlantic to the Pacific will 
measure the distance and give the basis for a correct delinea¬ 
tion of the country; but by stopping at this point we should 
fall far short of our full duty. In the progress of the work 
golden opportunities have presented themselves for the de¬ 
velopment of the coordinate parts of every well-ordered trigo¬ 
nometrical survey, and these occasions have not been allowed 
to pass by unheeded. Incident to the work in question and 
growing out of its results has come knowledge of the laws of 
refraction, the aberration of light, the deflections of the plumb- 
line, the force of gravity, and the mean density of the earth. 
Twenty stations have been occupied on the arc for gravity 
measures, and the results have already been presented to the 
public. No one can now deny the value or scientific interest 
in work of this kind, since it has been conclusively shown in 
Europe that a measurable connection exists between the de¬ 
flection of the plumb-line and the variations of gravity. On 
a line from Kolberg to Schnee-Koppe the plumb-line was 
drawn almost invariably in the direction of an excess of 
gravity, as revealed by the pendulum, and it has even been 
laid down as a rule that the variations of gravity can be re¬ 
ferred to the attraction of matter of a density of 2.4, a mil¬ 
lionth of a meter in the former corresponding to a meter of 
thickness in the latter. 
The pendulum-work may be considered as still in its in¬ 
fancy. Nevertheless both the method of making the obser¬ 
vations and the interpretation of the results have passed 
through several distinct stages. First we had a ball of 
metal suspended by a fine wire. Then came the reversible 
