influenced by the Distribution of Land and Water. 187 



instances of the laws found to hold good in the British Isles^ 

 «nd almost all of the isothermals on the land would be closed 

 curves*. 



In the second case^ the ocean would acquire much less heat 

 from the sun^ and it would exercise a coohng influence on the 

 belt of intertropical land. But, as whatever evidence we possess 

 seems to indicate that intertropical seas owe their elevated tem- 

 perature not so much to the influence of thermal exchanges with 

 the air which has passed over the adjacent land as to the direct 

 influence of sunshine, we may conclude that, upon the whole, the 

 heat-bearing currents would in this case be less influential than 

 ia that which has been just considered. The heated air flowing 

 from the equatorial land should, by the agency of winds, in some 

 measure mitigate the temperature of the polar regions ; but we 

 have no reason for believing that this influence would be supe- 

 rior to that of the heat-bearing water-currents in our former 

 instance. 



If, now, we suppose the land to be equally distributed in 

 islands between the equatorial and polar regions, we shall have 

 conditions more or less favourable to the existence of oceanic 

 as well as of aerial heat-bearing currents ; and it seems not im- 

 possible that under such circumstances the entire surface of the 

 globe might enjoy the highest possible amount of general warmth, 

 by being best circumstanced for the accumulation, retention, 

 and distribution of the heat it receives from the sun. In this 

 case, as well as in the first which has been considered, warm 

 currents from the equatorial seas might freely bathe the coasts 

 of islands in higher latitudes, thus producing similar character- 

 istic cases of insular climate. The mean temperature of such 

 seas being higher than that of the air over the land, the iso- 

 thermal lines of the islands should be partly or entirely closed 

 cui-ves, having shapes dependent upon the outlines of the islands. 

 The greater the difference of atmospheric and water tempera- 

 ture, the more strictly should the isothermals conform to this 

 law. Thus, it is manifest that a nearly circular island with a 

 surface equal to that of Labrador, and lying in the same lati- 

 tude, would present a much greater diversity of climate between 

 its interior and its coasts, if the latter were bathed by sea-water 

 having a temjjcrature of 80° Fahrenheit, than if that tempera- 

 ture amounted only to 40°. As the manner in which the warm 

 air over the water would exchange its heat with the air over 

 the land would undoubtedly be by circulation, it would not 

 be easy to assign a distinct law for the difference of tem- 

 perature between the interior and the coast of the island ; but 

 it seems evident that this difference should, up to a certain limit, 

 • See Phil. Mag. for October 1868, p. 249. 

 02 



