Prof. Hennessy on Terrestrial Climate. 181 



Such experiments, and many others which they obviously sug- 

 gest, appear to me to offer an interesting field of experiments 

 in physical optics ; and to those who are more practically ac- 

 quainted with this science than I can pretend to be, and who 

 may possess more delicate means of detecting minute effects, 

 I therefore leave them. 



XXVIII. Terrestrial Climate as influenced by the Distribution of 

 Land and Water at different Geological Epochs. By Henry 

 Hennessy, F.R.S., M.R.I. A., Professor of Natural Philo- 

 sophy in the Catholic University of Ireland^. 



11^ VERY point on the earth's surface is continually gaining 

 -i and losing heat ; and its actual temperatui'C at any given 

 moment depends on the difference between its gains and its losses. 

 If the outer coating of the earth were exclusively composed of 

 solid materials, terrestrial climate would depend principally on 

 the heat gained from sunshine and the heat radiated into space. 

 But as the earth is completely enveloped by an atmosphere, and 

 partly surrounded by a liquid, its thermal conditions must be 

 greatly influenced by the physical properties of these fluid cover- 

 ings. While the heating or cooling of a solid follows the clearly 

 defined, and comparatively well understood, laws of conduction 

 and radiation, the heating or cooling of gases and liquids is 

 further greatly modified by the mobility of their particles. The 

 changes of state which frequently take place in fluids^ whether 

 by evaporation or condensation, freezing or liquefaction, intro- 

 duce agencies which still further complicate the study of their 

 thermal relations. 



When we study the thermal conditions of a liquid distributed 

 over the terrestrial spheroid, it becomes manifest that these con- 

 ditions are influenced by the area, configuration, and physical 

 structure of such portions of the solid earth as rise above the 

 ocean and come in contact with the atmosphere, so as to con- 

 stitute the surface of the dry land. Upon this matter I propose 

 to develope certain views which are closely connected with those 

 I have already published relative to the distribution of heat over 

 such solid surfaces f. 



2. When a surface, covered with ordinary soil, receives the 

 rays of the sun, the heat thus acquired passes downwards, but 

 on arriving at a very small depth its intensity rapidly diminishes. 



* From the Atlantis for January 1859 ; communicated by the Author. 



t On the Distribution of Heat over Islands, &c., Phil. Mag. for October 

 1858, p. 24 1 . See also the Note on the Laws tiiat Regulate the Distribution 

 of Isothennal Lines, Atlantis, No. 3. p. 201. 



