Page:Passages from the Life of a Philosopher.djvu/480

464 the proportion of three to two, so that one of five feet by seven and a half ought to be seen at the distance of about eight or nine miles.

During one portion of my residence at Naples my attention was concentrated upon what in my opinion is the most remarkable building upon the face of the earth, the Temple of Serapis, at Puzzuoli.

It was obviously built at or above the level of the Mediterranean in order to profit by a hot spring which supplied its numerous baths. There is unmistakable evidence that it has subsided below the present level of the sea, at least twenty-five feet; that it must have remained there during many years; that it then rose gradually up, probably to its former level, and that during the last twenty years it has been again slowly subsiding.

The results of this survey led me in the following year to explain the various elevations and depressions of portions of the earth's surface, at different periods of time, by a theory which I have called the theory of the earth's isothermal surfaces.

I do not think the importance of that theory has been well understood by geologists, who are not always sufficiently acquainted with physical science. The late Sir Henry De la Beche perceived at an early period the great light those sciences might throw upon his own favourite pursuit, and