Although, according to Bessel, 25,000 cubic miles of water flow in
every six hours from one quarter of the earth to another, and the
temperature is augmented by the ebb and flow of every tide, all
that we know with certainty is, that the _resultant effect_ of all
the thermal agencies to which the earth is exposed has undergone
no perceptible change within the historic period. We owe this fine
deduction to Arago. In order that the _date palm_ should ripen its
fruit, the mean temperature of the place must exceed 70 deg. Fahr.;
and, on the other hand, the _vine_ cannot be cultivated successfully
when the temperature is 72 deg. or upwards. Hence the mean temperature
of any place at which these two plants flourished and bore fruit must
lie between these narrow limits, _i. e._ could not differ from 71 deg.
Fahr. by more than a single degree. Now from the Bible we learn that
both plants were _simultaneously_ cultivated in the central valleys
of Palestine in the time of Moses; and its then temperature is thus
definitively determined. It is the same at the present time; so that
the mean temperature of this portion of the globe has not sensibly
altered in the course of thirty-three centuries.