influenced by the Distribution of Land and Water. 183 



pical regions, while the warmest are those blowing from the pole. 

 The former pass over extensive surfaces of heat-radiating, and 

 therefore heat-losing land, while the latter traverse the heat- 

 retaining ocean. In the summer (at least by day) the opposite 

 phsenomena are observed, of warm winds from the north and 

 cold from the south. Combined observations on the wind and 

 on tcDiperature, by day and night, would further elucidate a 

 problem which, in the words of the writer, " cannot be solved 

 without greatly adding to the stock of our knowledge." While 

 the feeble conducting power of the solid portions of the earth's 

 coating allows but a small portion of the sun's heat to pass 

 beneath the surface, so that whatever warmth is thus received 

 on that surface during the day is readily radiated into space 

 during the night, a liquid mass, similarly exposed to sunshine 

 and subsequent nocturnal radiation, possesses peculiar properties 

 which greatly influence the differences between its thermal losses 

 and gains. The most important of these properties are — (1) the 

 great capacity of water for heat, by which it gradually accumu- 

 lates and slowly parts with whatever warmth it has received; 

 and (2) the intermobility of its particles, by which exchanges of 

 temperature in different parts of the liquid mass are essentially 

 promoted. 



Let us consider the effect of the sun's rays on a globe covered 

 with water, and we shall soon perceive that a more energetic pro- 

 cess than that of conduction accompanies the exchange of tem- 

 perature between the different portions of the fluid. The water 

 which receives the vertical rays of the sun will be more heated 

 than the waters which receive its rays at more oblique inclina- 

 tions. Not only the amount of warmth received over a given 

 area, but also the depth to which the rays of heat penetrate 

 below the surface, depends upon the angles made by these rays 

 with the vertical. Inequalities of surface temperature, depend- 

 ing on the latitude, the hour-angle, and the sun's longitude, 

 should thus result. The more heated waters would expand, 

 and tend to spread over the cooler waters in other regions. Cur- 

 rents should arise from the mutual actions and reactions of the 

 unequally heated portions of the flnid. The colder currents 

 would usually tend to flow beneath the warmer, unless at tem- 

 peratures approaching that of the maximum density of water, 

 and thus a process of circulation would be established by which 

 the temperature acquired by the su])crficial strata of the water 

 should be ultimately propagated to a certain depth below the 

 surface. Evaporation would also take place, and by the con- 

 densation of vapour a certain portion of the heat received by the 

 water would be imparted, in the formation of clouds, to the su- 

 perincumbent atmosphere. 



