Progress in Science. 
[October, 
558 
Such a result was, no doubt, to be found by observation in places which had 
been overflown by lava in the memory of man or a few years further back ; 
but if it were found for the whole earth, it would limit the whole of geological 
history to within 10,000 years, or, at all events, would interpose an absolute 
barrier against the continuous descent of life on the earth from earlier periods 
than 100,000 years ago. Therefore, although search in particular localities 
for a diminution of the rate of augmentation of underground temperature in 
depths of less than a kilometre might be of intense interest, as helping us to 
fix the dates of extindt volcanic adtions which had taken place within 10,000 
years or so, we know enough from thoroughly sure geological evidence not to 
expedt to find it, except in particular localities, and to feel quite sure that we 
should not find it under any considerable portion of the earth’s surface. If we 
admit as possible any such discontinuity within goo, 000 years, we might be 
prepared to find a sensible diminution of the rate at three kilometres’ depth, 
but not at anything less than 30 kilometres if geologists validly claim as much 
as go, 000, 000 of years for the length of the time with which their science is 
concerned. Now, this implied a temperature of 100 deg. C. at the depth of 
30 kilometres, allowed something less than 2000 deg. for the temperature at 
60 kilometres, and did not require much more than 4000 deg. C. at any 
depth, however great, but did require at the great depths a temperature of, at 
all events, not less than about 4000 deg. C. It would not take much 
‘ hurrying-up ’ of the adtions with which they are concerned to satisfy 
geologists with the more moderate estimate of 50,000,000 of years. This 
would imply, at least, about 3000 deg. C. for the limiting temperature at 
great depths. If the adtual substance of the earth, whatever it might be, 
rocky or metallic, at depths of from 60 to 100 kilometres, under the pressure 
actually there experienced by it, could be solid at temperatures of from 3000 
deg. to 4000 deg., then we might hold the former estimate (go, 000, 000) to be 
as prouable as the latter (50,000,000), so far as evidence from underground 
temperature could guide us; if 4000 deg. would melt the earth’s substance at 
a depth of 100 kilometres, we must rejedt the former estima e, though we 
might still admit the latter ; if 3000 deg. would melt the substance at a depth 
of 60 kilometres, we should be compelled to conclude that 50,000,000 of years 
was an over estimate. Whatever might be its age, we might be quite sure the 
earth was solid in its interior ; not absolutely throughout its whole volume, for 
there certainly were spaces in volcanic regions occupied by liquid lava ; but 
whatever portion of the whole mass was liquid, whether the waters of the 
ocean or melted matter in the interior, these portions were small in com- 
parison with the whole ; and we must utterly rejedt any geological hypothesis 
which, whether for explaining underground heat or ancient upheavals and sub- 
sidences of the solid crust, or earthquakes, or existing volcanoes, assumed the 
solid earth to be a shell of 30 or 100, or 500, or 1000 kilometres thickness, 
resting on an interior liquid mass. This conclusion was first arrived at by 
Hopkins, who might, therefore, properly be called the discoverer of the earth’s 
solidity. He was led to it by a consideration of the phenonema of precession 
and nutation, and gave it as shown to be highly probable, if not absolutely 
demonstrated, by his confessedly imperfedt and tentative dedudtion, but a 
rigorous application of the perfedt hydrodynamical equations leads still 
more decidedly to the same conclusion. Sir William Thomson asked those 
who possessed the *• Transactions of the Royal Society” for 1862, and of 
Thomson and Tait’s “ Natural Philosophy,” Vol. 1, to draw the pen through 
sedtions 23-31 of his paper on the “ Rigidity of the Earth,” in the former, and 
through everything in sedtions 847-8qg of the latter, which referred to the effedt 
on precession and nutation of an elastic yielding of the earth’s surface, for he 
had convinced himself that those conclusions at which he had arrived by a 
non-mathematical short cut were grievously wrong. 
A little consideration sufficed to show him that a very slight deviation of 
the inner surface of the shell from perfedt sphericity would suffice, in virtue 
of the quasi-rigid;ty due to vortex motion, to hold back the shell from taking 
sensibly more precession than it would give to the liquid, and to cause the 
liquid (homogeneous or heterogeneous) and the shell to have sensibly the 
