WATER ITS RELATION TO METABOLISM 257 



the scorching power of the sun and to prevent the rapid scattering 

 into space of the heat gained by the earth. 



Owing to the high specific heat of water, enormous quantities of 

 sun-heat are stored up by the sea, while the temperature of the 

 latter nowhere rises above 85 0. Land, on the other hand, in the 

 tropics may be heated by the sun even up to 140 C. ! 



The heat contained in any given breadth of sunbeams is most 

 concentrated at the tropics, where at noon the earth's surface lies 

 directly transverse to the path of the sun's rays. Towards the poles 

 and in the winter the sun never rises high in the sky ; the rays fall 

 so obliquely that the given breadth of sunbeams is diffused over a 

 much wider area. The air, diathermic to the luminous rays of the 

 sun, and bad conductor of heat as it is, is warmed by contact with 

 the earth or sea, and the heat is conveyed from the lower to the 

 upper strata of the atmosphere by convection currents. The motive 

 power due to convection is enormously increased by evaporation. 

 The air, lightened by heat and the addition of vapour, ascends, and on 

 cooling parts with vapour which condenses as cloud ; the condensation 

 sets free the latent heat of vaporisation, and this heat serves to carry 

 the neighbouring air to still loftier altitudes. The process may be 

 repeated over and over again, until finally the cirrus clouds form at 

 a height of five or six miles. The origin of the vast motive power 

 of the winds (the pressure of whirlwinds reaches 100 Ibs. to the sq. 

 ft.) is brought home by the consideration that the energy required 

 to raise 1 Ib. of water 1 F. is equivalent to the energy of a pound 

 weight falling 783 ft., and that 564 Cal. are set free when 1 grm. 

 of steam at 100 C. condenses to water at 100 G. The water vapour 

 raised in copious abundance over the equatorial and tropical seas 

 is transported to the temperate and colder regions of the earth. The 

 winds arise from the displacement of warm moist air by colder and 

 heavier air, and are modified by the effect of the earth's rotation. 

 The south-west wind comes to us warmed by the sun-heated tracks 

 of the Atlantic, while the north-east sweeps from the ice and snow- 

 bound lands of the high latitudes of Europe. Condensation takes 

 place when warm air becomes cooled to the point of saturation. The 

 cooling is brought about by the mixture of cold and warm currents 

 of air, by warm air blowing upon cold land surfaces, by the sliding 

 of warm masses of air up the slopes of hills, where the air is not 

 only chilled, but expands on reaching higher altitudes regions of 

 lowered barometric pressure and from expansion cools. Such local 



r variations in the earth's structure enormously influence the rainfall. 

 The violence of tropical rains (the annual rainfall in the Bengal 

 mountains exceeds 650 inches) is explained by the following figures. 

 At 20 F. air takes up 1-3 grains of water per c. ft., at 60 F. 5-77, 



K 



