588 On the Evidence that the Earth’s Interior is Solid. (June, 
cates of the essential solidity of the earth, was Mr. William Hop- 
kins, who based his conclusion upon the phenomena of preces- 
sion and nutation. In order to mathematically investigate this 
question, he made two assumptions regarding the earth: 1. That 
it was composed of a homogeneous fluid mass contained in a 
homogeneous solid shell. 2. A heterogeneous fluid inclosed in 
a heterogeneous solid shell. In both cases the transition was as- 
sumed to be “ immediate between the entire solidity of the shell 
to the perfect fluidity of the interior mass.” He further assumed 
that the circulation would go on in every portion of the mass 
until it had lost its perfect fluidity throughout the entire mass at 
nearly the same instant. This of course could only be correct if 
the liquid was homogeneous. He did, however, state that a wis- 
cid condition of the liquid would prevent the descent of the 
cooled exterior portions towards the interior. With a globe con: 
stituted as above for his basis, Hopkins concluded that the phe- 
nomena of precession and nutation did not demand on the earth 
a crust of over 1000 miles in thickness, but they did require one 
at least 800 miles thick. In order to account for volcanic phe- 
nomena, he assumed that there were in the so 
of molten material whose origin was to be as 
fusibility of the material composing them, tha 
the surrounding solid crust.’ l 
Professor Hennessy later held? that in a fluid globe the p 
particles would sink through the lighter, while the lighter 
n of that forming 
mass would be reached, so that the globe ; 
series of spheroidal strata, each of uniform densi 
its own mass.” He further remarked : “ The exterior ding t d 
ré, acco g 1 
the fluid would cool first, until they would acqui 
: ` their cooling 
the particular circumstances which may influence ; general 3 
certain densities. If the effect of refrigeration be ney ae 
coo! 
acquired densities of the cooled matter. aa with the 
fluid would have its temperature reduced from i ge posi 
cold particles from above, and it would tend to ¢ we 
à . 4355 
Philos. Trans., 1839, pp. 381-423; 1840, PP- 193-208; 1842, PP 
? Philos. Trans., 1851, pp. 495-547. 
lidified crust, lakes 
cribed to a greatet 
