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Correspondence—Mr. O. Fisher. 237 
CORRESPONDENCE. 
—_—_—_@___ 
PROF. J. MILNE ON THE DISTRIBUTION OF VOLCANOS. 
Srr,—We are fortunate in having Prof. Milne residing near the 
voleanic band of the Pacific coast, and shall no doubt have our know- 
ledge of vulcanology advanced by his studies. There is cogency in the 
reasons which he adduces, to explain the occurrence of vents along 
the steep margins of oceans, through the thickening of the crust by 
the cooling effect of the water. He tells us that, ‘‘ without entering 
into any calculations on the subject, it is not at all unlikely that, as 
in one case we have land cooling beneath an atmosphere and the 
compensating effects of a sun, whilst in the other case we have land 
cooling beneath water, which, from all we know about deep-sea 
temperatures, is usually very cold, we should find any given isotherm 
at a much greater depth beneath the rocks, which form the bed of an 
ocean, as compared with the depth at which we find it beneath the 
rocks which form the land.” 
But why not “enter into calculation,” when, as in the present 
case, that is so easy, and add what this “much greater depth ” will be? 
Say that the mean temperature of England is 50° Fahr. And 
suppose the temperature of the sea-bottom to be 32°. The difference 
is 18°. Then, allowing an increase near the surface of one degree 
for every 50 or 60 feet of descent, the melting temperature of rock 
(whatever that may be) will be reached 18 times 50 or 60 feet lower 
beneath the ocean-bottom, than beneath England. This will make 
the crust only from 900 to 1080 feet thicker beneath the sea-bottom. 
But the melting temperature will be reached actually nearer to the 
solid surface beneath the sea-bottom, than beneath dry ground, 
which, as in parts of Siberia, is perpetually frozen. 
I cannot help suspecting that Professor Milne had in his mind a 
greater difference than the above, when he used the words ‘‘ much 
greater depth.” And I am sure that to geologists in general, whose 
ideas of time and space are formed upon a vast scale, these words 
would convey a much larger meaning than the modest fact. 
It is really time that scientific men and excellent geologists, like 
my friend Mr. 8. V. Wood, jun., should cease to quote with approval 
such an unphilosophic saying as that ‘figures may be made to prove 
anything.” Figures do but carry out to its legitimate conclusion 
some foregone assumption. If the result be wrong, not the figures, 
but the assumption is in fault. For instance, if there is any error in 
the result of the abstruse calculation which I have indicated above, 
it lies, not in the application of the multiplication table, but in the 
assumption that the temperature of the ground beneath land and 
sea alike increases near the surface by one degree Fahr. for every 
fifty or sixty feet of descent. 
If this assumption is doubted, I think I have shown why it is 
correct, in the Phil. Mag. for June 1879, p. 882. O. Fisumr. 
Harton, CAMBRIDGE, 10¢h April, 1880. 
