'Lithological Survey of Schehallieno of 5 
the mountain. P is the north observatory ; the part of the 
section coloured with a reddish brown represents the granular 
quartz, supposed here to constitute the interior as well as the 
summit of the mountain. The dark colour represents the 
schistus ; the two belts of grey are the limestone strata on 
the north and south sides. OR or the elevation of the south 
observatory above the lowest part of the base of the moun- 
tain is 1440 feet. 
Fig. 3. is a section of the mountain in the direction perpen- 
dicular to the meridian of the south observatory. This section, 
though not referred to in any of the computations, is useful 
for enabling one to form an idea of the structure and figure 
of the mountain. 
Draw OL (Fig. 2.) parallel to the horizon. With OR as an 
axis, and with a radius of 13333 feet, suppose a cylinder to 
be described, and let it be cut into two semi-cylinders by a 
plane passing through O R perpendicular to RQ the me- 
ridian. Then the whole of the mountain on the north side of 
this plane disturbs the direction of the plummet by draw- 
ing it toward the north. But the part of the mountain to 
the north of this plane, and between the levels O and R is 
equal to one of the semi-cylinders above mentioned, minus 
the empty space between the surface of the ground and the 
horizontal plane passing through O. If therefore S denote the 
attraction of the semi-cylinder, and V that which the void 
space would have were each pillar in it to consist of matter 
of the same density with the part of the same pillar which is 
under the surface, S —V will represent the attraction or dis- 
turbing force of all that part of the mountain which is north 
of OR and under the level of O. 
