254 
PRESTON. 
Bessel, who in 1841 collected all available data and deduced 
results which were universally adopted until 1866, when 
Clarke by the same process gave values which are now re¬ 
garded as an improvement on those of Bessel. This brings 
us to the strictly recent work, and leads to an examination 
of the measurement of the 52d parallel of north latitude in 
Europe and the 39th parallel in America. To this might 
be added a short measurement on the 56th parallel, also, in 
Europe. The results all point in the same direction, namely, 
that the curvature is greater than would be required on an 
oblate spheroid of the dimensions of our earth. The fact 
being established that all three of the parallel arcs indicate 
a bulge in the continental profile, the way is open for the 
application of theories that would bring about such a result, 
and that theory which seems to provide most consistently 
for such phenomena is precisely the tetrahedral one of which 
mention has already been made. First, as to the facts. 
Clarke’s spheroid is larger and flatter than Bessel’s. It rep¬ 
resents that regular figure most nearly coinciding with the 
actual earth. Measurements on the 56th parallel show a 
greater curvature than would be required for the mean fig¬ 
ure. It is true that the arc is a short one, but the evidence 
is unmistakable as far as it goes. Then comes the great arc 
of 70° on the 52d parallel, which has a radius of curvature 
486 meters shorter than would be required on the Clarke 
ellipsoid, and finally in our own county the transconti¬ 
nental arc from Cape May to San Francisco indicates a 
greater curvature than is required to fit the adopted shape 
of the earth resulting from the best modern data on the 
subject. The principal arcs measured and in progress in 
North America are shown in Plate 12. 
Now as to the tetrahedral theory. To begin with a simple 
example, it is known that rubber spheres immersed in water 
tend towards a tetrahedron. The reason is evident. The 
sphere of all geometrical bodies has the least surface for a 
given capacity. The tetrahedron, on the contrary, has the 
greatest surface for the same condition. When pressure is 
