577 
figure of the earth. 
stations. We cannot imagine a situation in which there is a 
greater probability that the difference of latitudes would be 
made too large, than when the latitudes are observed at two 
stations at the level of the sea, with high land between them. 
But this would depend much on the nature of the rocks. In 
Peru, for instance, though far more mountainous, it is pro- 
bable that the disturbance was not great. If then, as Captain 
Sabine proposes, any preliminary survey be made, it would 
perhaps be proper to examine not only the circumstances 
which would affect the practical facility of the operation, but 
also those which might have an influence on the determina- 
tion of the latitudes ; and should these be found not very un- 
favourable, a meridian arc of four degrees measured with 
great care would be highly valuable. 
ADDITIONS. 
The equations of Art. 15, it will easily be seen, are equally 
true, whether the interior of the mass be supposed to be 
fluid, or to consist of solid shells of different density, in which 
the radius of each separating surface is expressed by the 
formula a j 1 + * . 1 — p/ 2 — ( — [- A j . [// 8 — ^ 4 j . 
And the expression for gravity in Art. 19, and all its con- 
sequences, hold equally on either supposition. 
A theorem similar to Clairaut’s may be shown to be 
true, to whatever order the investigations be extended. For 
upon using the value of R found in the investigation above, 
and upon carrying all the operations one step farther, it 
would be found that the equation of equilibrium could not 
