370 
PRESTON. 
have an equation in which the densities of the mountain and 
of the earth are the only unknown quantities. The substitu¬ 
tion of the former of these quantities given by a study of 
the rocks composing the mountain leads to a complete de¬ 
termination of the only remaining quantity—the mean 
density of the earth. 
The first case treated is on the island of Maui. The ex¬ 
tinct volcano Haleakala occupies practically the whole 
island, as far as attractions on its south flank are concerned, 
West Maui being too distant and too far to the west to exert 
much influence on the plumb line at Kaupo, in the direction 
of the meridian. The principal part of the island is of some¬ 
what regular outline, about 30 miles from east to west, 25 
miles from north to south, and slightly over 10,000 feet high. 
The triangulation connecting Haiku and Kaupo is so well 
expanded that it is possible to derive the geodetic position 
of the latter point through two separate chains of triangles 
which greatly strengthens the result. Moreover, the mean 
density of the mountain was independently determined by 
measuring the force of gravity near the sea level and at the 
summit. The rock specimens gathered by our party and 
those previously studied by Professor E. 8. Dana are amply 
sufficient to give a close approximation to the mean density 
of the mountain, but a check was had on the result by as¬ 
suming the value of 5.576 given by Harkness for the earth’s 
mean density and deriving a value of the density of the 
mountain as a whole from the pendulum observations. 
The determination of gravity also threw light on a ques¬ 
tion of geological interest. Various opinions were held as 
to the way in which Haleakala was formed. The adoption 
or rejection of certain theories depended entirely on whether 
the mountain was solid or hollow—whether it was as heavy 
as would appear from its volume and supposed density. The 
pendulum work indicated that there was no defect of gravity 
below the mountain, as has frequently been found beneath 
similar formations, and that, as a whole, its average density 
was approximately equal to that of the surface rocks. 
