102 Mr. J. Croll on Ocean-currents in relation to 



annual temperature of the ocean ought to be greater than that of 

 the land in equatorial regions as well as in temperate and polar 

 regions. This will appear obvious for the following reasons : — 

 (1) The ground stores up heat only by the slow process of con- 

 duction, whereas water, by the mobility of its particles and its 

 transparency for heat-rays, especially those from the sun, be- 

 comes heated to a considerable depth rapidly. The quantity of 

 heat stored up in the ground is comparatively small; the quan- 

 tity stored up in the ocean is great. (2) The air is probably 

 heated more rapidly by contact with the ground than with the 

 ocean; but, on the other hand, it is heated far more rapidly by 

 radiation from the ocean than from the land. The aqueous va- 

 pour of the air is to a great extent diathermanous to radiation 

 from the ground, while it absorbs the rays from water and thus 

 becomes heated. (3) The air radiates back a considerable por- 

 tion of its heat, and the ocean absorbs this radiation from the air 

 more readily than the ground does. The ocean will not reflect the 

 heat from the aqueous vapour of the air, but absorbs it, while the 

 ground does the opposite. Radiation from the air, therefore, tends 

 more readily to heat the ocean than it does the land. (4) The 

 aqueous vapour of the air acts as a screen to prevent the loss by 

 radiation from water, while it allows radiation from the ground to 

 pass more freely into space ; the atmosphere over the ocean conse- 

 quently throws back a greater amount of heat than is thrown 

 back by the atmosphere over the land. The sea in this case has 

 a much greater difficulty than the land has in getting quit of the 

 heat received from the sun ; in other words, the land tends to 

 lose its heat more rapidly than the sea. The consequence of 

 all these circumstances is that the ocean must stand at a higher 

 mean temperature than the land. A state of equilibrium is never 

 gained until the rate at which a body is receiving heat is equal 

 to the rate at which it is losing it ; but as equal surfaces of sea 

 and land receive from the sun the same amount of heat, it there- 

 fore follows that, in order that the sea may get quit of its heat 

 as rapidly as the land, it must stand at a higher temperature than 

 the land. The temperature of the sea must continue to rise till 

 the amount of heat thrown off into space equals that received 

 from the sun; when this point is reached, equilibrium is esta- 

 blished and the temperature remains stationary. But, owing to 

 the greater difficulty that the sea has in getting rid of its heat, 

 the mean temperature of equilibrium of the ocean must be higher 

 than that of the land ; consequently the mean temperature of 

 the ocean, and also of the air immediately over it, in tropical 

 regions should be higher than the mean temperature of the land 

 and the air over it. 



We find, however, when we examine M. Dove's charts of 



