134 Proceedings of the Royal Physical Society. 
of which the crust is composed contract uniformly by dimi- 
nution of temperature, and at the rate of t2V¥* foi" every 
180° Fahr., which is the rate of contraction of flint glass. 
Then we have 180° : i^Vs • • 1500 : gf-f^ = tc^t^. and 
T^i-y^ of 45000 yards (taking this as the thickness of the 
crust) = 30048 yards, the amount which the radius would 
be shortened by a loss of 3000° of surface temperature. The 
circumference of the earth would thus be reduced by 30048 
X 3-1416 X 2 = 1886-976 yards,t so that a point on the 
surface at the equator would have its diurnal orbit lessened 
to this extent. Since the space moved over by the crust in 
its daily axial revolution would thus be diminished, whilst 
its momentum continued the same as before, a shortening 
of the day to a certain amount would necessarily take place. 
Suppose now the original diurnal period of the earth to have 
been 24 hours, then 43,781,760 yards (or 24,876 miles, being 
the circumference of the earth) : 1887 yards : : 86164" (24 
hours) : 3"'7136, which would be the amount of the daily 
acceleration of the crust, but for the retarding influence of 
the friction of its lower surface upon the molten interior. 
5. The loss of heat at the surface being assumed = 3000°, 
the corresponding contraction will be ji-^s, and the super- 
ficial contraction of a s-reat circle of the earth = i^Ai^ • or 
O ^ 7 4"8 8 ' 
say ^A^-S- =1 333-J- miles, which would be the aggregate 
width of the fissures opened on the whole circumference 
of the earth by a reduction of temperature to this amount. 
This is equivalent to 195 J yards for every degree of 
Fahrenheit. 
6. By this refrigeration of the crust, and the consequent 
contraction of all its parts, its capacity would obviously be 
diminished, so that it would grasp the internal molten mass 
with greater and greater force, until the tension became ^ex- 
treme, when fissuring would ensue and molten matter would 
be erupted, until the capacity of the crust and the quantity of 
* According to the very careful experiments of Mr Adie, the contraction of 
a column of Ratho greenstone cooled 1000° was 4^ per 1000. For 180° this 
would amount to i ^^^f^^ gTir, or a little exceeding that of flint glass, so that 
the rate above assumed is perhaps not very far from the truth. 
t Or 1-2578 yards for each degree of Fahrenheit. 
