360 G. B. Airy on Pendulum experiments 
volving round them, can easily be found—(the proportion for 
those which have no satellites is found by a very indirect pro- 
cess, and with far less accuracy); and that if the absolute mass 
of the earth be known, the absolute mass of each of the others 
can be found. As their dimensions are known, their densities 
can then be found. ‘hus it rests upon such inquiries as those 
on which this discourse is to treat, to determine (for instance) 
whether the planet Jupiter is composed of materials as light as 
water, or as light as cork. 
The obvious importance of these determinations had induced 
philosophers long since to attempt determinations of the earth’s 
density : and two classes of experiments had been devised for it. 
The first class (of which there was only one instance) is the 
attraction of a mountain, in the noble Schehallien experiment. 
It rests, in the first place, upon the use of the zenith sector; and, 
in the next place, upon our very approximate knowledge of the 
dimensions of the earth. [The construction of the zenith sector 
was illustrated by a model: and it was shown, that if the same 
star were observed at two places, the telescope would necessarily 
be pointed in the same direction at the two places, and the differ- 
ence of direction of the plumb line, as shown by the different 
points of the graduated arc which it crossed at the two places, 
would show how much the direction of gravity at one place 1s 
inclined to the direction of gravity at the other place.] Now, 
from our knowledge of the form and dimensions of the earth, 
tions inclined to these natural directions. In order to infer from 
this the density of the earth, a calculation was made (founded 
upon a very accurate measure of the mountain) of what would 
have been the disturbing effect of the mountain if the mountain 
had been as dense as the interior of the earth. It was foun 
that the disturbance wonld have been about 27 seconds. But 
the disturbance was really found to be only 12 seconds. Conse- 
quently the proportion of the density of the mountain to the 
earth’s density was that of 12 to 27, or 4 to 9 nearly. At 
from this, and the ascertained density of the mountain, it followed 
