During the day, and when the sun is shining, the time might be trans­mit­ted by the oc­cul­ta­tions of reflected solar light, which would be seen at any distance the curvature of the earth admitted.

The application of my Zenith Light might perhaps fulfil all the required conditions during daylight.

I have found that, even in the atmosphere of London, an opening only five inches square can be distinctly seen, and its oc­cul­ta­tions counted by the naked eye at the distance of a quarter of a mile. If the side of the opening were double the former, then the light trans­mit­ted to the eye would be four times as great, and the oc­cul­ta­tions might be observed at the distance of one mile.

The looking-glass employed must have its side nearly in {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.

Geological Theory of Isothermal Surfaces.

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.[63]

[63] In this inquiry I profited by the assistance of Mr. Head, now the Right Hon. Sir Edmund Head, Bart., K.C.B., late Governor-General of Canada. An abstract of my own ob­ser­va­tions was printed in the “Abstracts of Proceedings” of the Geological Society, vol. ii. p. 72. My friend’s historical views were printed in the “Transactions” of the Antiquarian Society.

〈TEMPLE OF SERAPIS.〉

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.