MEAN DENSITY OF THE EARTH. 
393 
mean of the preceding value's we get a specific gravity of 
2.90 for the specimens on the Island of Hawaii. 
Mauna Kea has the form of a truncated cone, so that its 
effect on the force of gravity would be intermediate between 
that of a cone and an infinite plain. As the difference be¬ 
tween the ratios of ^ for the two forms is only about of 
the value of either, a direct mean between them will in 
all probability approach closely to the actual effect of the 
mountain. The attraction of an infinite plain on a point 
above it is entirely independent of the distance of the point 
from the plain. In this case where the point is in the upper 
surface of the plain and h is its thickness, we have for the 
plain effect the expression 
2 7r d h or 15.54 d. 
The attraction of the earth at the upper station (Waiau) 
and the lower station (Kawaihae) are respectively 
16566.8 A and 16587.6 J, 
the linear unit being one mile throughout. The forces of 
gravity at the two places, are when corrected for their differ¬ 
ence of latitude, 
g (at Waiau) = 978.0723 dynes. 
g (at Kawaihae) = 978.8035 dynes. 
The foregoing values lead to the equation 
- 0.0007470 = - J 
from which ^ = 0.541. 
If we consider the mountain as a cone with an altitude 
of 2.473 miles and radius of base (a) of 30 miles, the attrac¬ 
tion becomes 
2 - 5 h [ ] - vw+t] = 14 - 26 5 
and the resulting value of ^ is 0.589. 
51-Bull. Phil. Soc., Wash., Vol. 12. 
