186 Prof. Hennessy on Terrestrial Climate as 



the entire oceanic covering of our planet, compared to its atmo- 

 spheric coating. 



In comparing the calorific influence of the land on distant 

 regions with the agency of the sea, it should therefore be re- 

 membered that, while the latter stores up heat, and acts by night 

 as well as by day, the action of the land is eflPective only as long 

 as the sun's rays are impinging upon it. 



4. Let us endeavour to apply these conclusions to the question 

 of the influence of the distribution of land and water upon 

 general terrestrial temperature. As the amount of solar heat 

 received by any point on the earth's surface is a function of the 

 latitude, it follows that the distribution of land and water at dif- 

 ferent latitudes must be studied in order to obtain its influence 

 on temperature. This distribution may be supposed to take 

 place in an endless variety of ways, of which the following three 

 cases are the most important : — 



1. Preponderance of land towards the poles, and of water 

 towards the equator. 2. Preponderance of land towards the 

 equator, and of water towards the poles. 3. Equable distribu- 

 tion of land and water in polar and equatorial regions. 



At the present day three-fourths of the earth's surface are 

 covered with water, so that all the dry land has been truly cha- 

 racterized as an assemblage of large and small islands placed in 

 a great ocean. If we suppose, with Sir Charles Lyell*, that, in 

 the question now under consideration, the proportion of sea to 

 land is the same as at present, each of the above three cases ia 

 susceptible of two principal divisions, according as the islands 

 composing the land happen to be few and large, or numerous 

 and small. If all the dry land on the globe were collected into 

 a single vast continent, the climatological conditions of the 

 earth, all other things remaining the same, would be very dif- 

 ferent from what would take place if the land were broken up 

 and spread out in numberless islands. Whatever may be the 

 supposed distribution of land and water, it is manifest that its 

 chief influence on the general temperature at the surface of 

 our planet would result from the action of aerial and oceanic 

 currents. 



In the first case above referred to, the belt of equatorial ocean 

 would probably acq\iire a high temperature ; and although the 

 circumpolar islands would possess very rigorous climates in their 

 interior, portions of their coasts might be washed by heat-bear- 

 ing currents, just as the north-western coast of Europe is washed 

 by the Gulf- stream at the present day. The superiority of mean 

 temperature of the ocean might in this case be so great that the 

 distribution of heat over the islands would present remarkable 

 * Principles of Geology, chap. vii. 9th ed. p. 101. 



