FORM AND CONSTITUTION OF EARTH—STEWART. Se 
briefly. Foremost among these is the method by pendulum experi- 
ments. In 1743 Clairaut published his work on the figure of the 
earth, which contains a remarkable theorem showing a connection 
between the force of gravity at a point on the earth’s surface in given 
latitude and the compression of the earth. The part played by the 
pendulum in the application of this method is the determination of 
the force of gravity, as the time of oscillation of a pendulum varies 
directly as the square root of its length, and inversely as the square 
root of the force of the earth’s attraction. If then the time of vibra- 
tion'of a pendulum of a known length be observed, the value of the 
force of gravity follows. 
Since 1808 pendulum experiments have been made in various 
parts of the world ranging from the Southern Hemisphere to Green- 
land and Spitzbergen. In 1901, by a discussion of about 1,400 
observations of g made during the nineteenth century, Helmert ob- 
tained the value of the compression 1 :298.3. 
Other methods of finding this quantity, which are purely astrono- 
mical, are by lunar perturbations and lunar parallax. There are 
certain disturbances of the moon’s motion that are caused by the 
earth’s spheroidal figure, and their expressions are therefore in 
terms of the compression. If then the amount of the lunar pertur- 
bations is found by observation the compression of the earth can be 
found from it. The value found in this way is 1:297.8. 
Observations of lunar parallax may also be employed for this pur- 
pose, whether made by the meridian method or the diurnal method. 
Sir David Gill has lately advocated the use of this latter method. 
He considered that if all the observatories situated not too far from 
the Equator were to cooperate in taking systematic observations of 
the moon a very precise value of the compression could be found. 
To sum up: It is probable that the final values of the major semi- 
axis and the compression of the elliptic meridian will be found to 
differ but little from the quantities: 
a—6,378,200 
c=1:298 
To return to the term “isostasy’’—this condition may be defined 
as follows: Imagine a spheroid concentric with the terrestrial spheroid 
and whose surface is everywhere about 76 miles within that of the 
latter, then the pressure on all parts of the surface of this inner 
spheroid, due to the weight of the superincumbent crust, is the same. 
In other words, all prismatic columns of the earth’s crust having the 
same cross section and extending from the surface down to the 
isostatic surface—as it may be termed—have the same mass. Hay- 
ford’s investigations show that geodesy furnishes positive proof of 
the existence of isostasy, and places the isostatic surface at a depth 
