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d) The rale of evaporation of moisture from damp, porous sub- 

 stances of the same material is proportional to the extent of the 

 surface presented to the air, without regard to the relative thick- 

 ness of the substances. 



e) The rate of evaporation from different substances mainly 

 depends upon the roughness of, or inequalities on, their surfaces, 

 the evaporation going on most rapidly from the roughest or most 

 uneven surfaces ; in fact, the best radiators are the best vaporizers 

 of moisture. 



f ) The evaporation from equal surfaces composed of the same 

 material is the same, or very nearly the same, in a quiescent atmos- 

 phere, whatever may be the inclination of the surfaces: thus a 

 horizontal plate with its damp face upward evaporates as much 

 as one with its damp face downward. 



g) The rate of evaporation from a damp surface (namely, a 

 horizontal surface facing upward) is very much affected by the 

 elevation at which the surface is placed above the ground. 



h) The rate of evaporation is affected by the radiation of sur- 

 rounding bodies. 



i) The diffusion of vapor from a damp surface through a 

 variable column of air varies (approximately) in the inverse ratio 

 of the depth of the column, the temperature being constant. 



j) The amount of vapor diffused varies directly as the tension 

 of the vapor at a given temperature, and inversely as the depth 

 of the column of air through which the vapor has to pass. 



k) The time in which a given volume of dry air becomes sat- 

 urated with vapor, or saturated within a given percentage, is 

 nearly independent of the temperature if the source of vapor is 

 constant. 



1) The times in which different volumes of dry air become sat- 

 urated with watery vapor, or saturated within a given per cent, 

 are nearly proportional to the volumes. 



m) The vapor already formed diffuses itself in the atmosphere 

 much more rapidly than it is formed from the surface of the 

 water. (This assumes, of course, that there are no convection 

 currents of air to affect the evaporation or the diffusion.) 



Effect of wind and other meteorological elements. That the 

 velocity of the wind must have a very material effect upon evap- 

 oration, and hence upon the runoff of streams, is at once apparent 

 on inspection of .Mr FitzGerald's evaporation formula, given in a 

 preceding section. Again, on exaihining the annual summaries 



