president's ADDrtESS. 627 



It is from the evaporation of the water of the ocean through 

 the agency of the sun's heat that is derived by far the greater 

 bulk of the rain and snow which fall on the globe. When water 

 is evaporated a large amount of heat is rendered latent and 

 locked up in the vapour, to be given out again on condensation. 

 Heat is measured in terms of the amount required to warm 

 water. The quantity of heat which will warm 1 lb. of water 

 1° F. is termed a heat unit, and is the British thermal standard. 

 Now water warmed from ordinary temperature, say 60° F., to 

 boiling point, which is 212° F., requires just 152 heat units per 

 lb. When a pound of water is evaporated, however, a very 

 much larger quantity of heat is necessary, for it requires 966 

 units to merely evaporate that amount without further increasing 

 its temperature. This means that just about 6J times the 

 amount of heat is required to change water into vapour as would 

 suffice to raise it from the ordinary temperature to boiling point. 

 It does not matter whether the water is boiling or not, its 

 evaporation at ordinary or any temperature requires practically 

 the same amount of heat, and the vapour on condensing gives up 

 the whole of this. As perhaps giving a better idea of the signi- 

 ficance of these figures, it may be mentioned in passing that the 

 quantity of heat required to evaporate one pound of water 

 represents energy equivalent to the force required to lift 

 3 J tons 100 feet above the earth's surface. Every pound of 

 water which falls as rain has therefore seized on and transported 

 to the area in which the rain is condensed sufficient heat to elevate 

 3J tons 100 feet. The imagination fails to properly grasp how 

 enormous must be the amount of heat required to vapourise the 

 great volume of water daily evaporated from the ocean. The 

 transference of such quantities of the sun's heat from the surface 

 of the ocean where it is received to the places where the clouds 

 are formed has a very great effect on climate ; in fact we may 

 safely say that the ocean forms the great storehouse of heat for 

 the habitable part of the globe's surface, and that, but for the sea 

 and the phenomena of evaporation and rain, the climatic condi- 

 tions would be such that the earth would be uninhabitable to 



