CLIMATIC CONDITIONS OF THE UNITED STATES. 285 



tion to proceed at a higher rate than is obtained with greater pressure. 

 It is Ukewise true that an alteration in the water-content of the air 

 changes the rate at which evaporation may occur, ceteris paribus. 

 Furthermore, a change in the temperature or velocity of movement of 

 the air (wind) over the evaporating surface also alters the possible rate 

 of evaporation. It is thus seen that both physical and chemical changes 

 in the air exert an influence on the rate of evaporation from exposed 

 liquid or solid water. That a decrease in the amount of gas present 

 per given volume should accelerate evaporation is surely not a matter 

 to cause misunderstanding, for we regard the mixture as air, at what- 

 ever density it may occur. It thus appears that the evaporating power 

 of the air increases as the density of the air decreases, and this con- 

 sideration appears to clear up the whole difficulty above mentioned. 

 The usual popular quibble over the conception of limits arises here, 

 as elsewhere in physical science, when we consider the result of decreas- 

 ing the air-pressure to zero. In such a case the air approaches, and 

 finally should become, an absolutely empty space, without tempera- 

 ture and without chemical nature. Such an absolute vacuum would 

 have the highest possible evaporating power, in Livingston's sense, 

 and after such a condition had been reached (if it could be maintained) 

 the rate of evaporation from exposed liquid or solid water should be 

 controlled only by conditions resident in the liquid or solid itself. This 

 condition is impossible of attainment, of course, so that the evaporat- 

 ing power of the air never becomes infinite, but this consideration is 

 valuable in that it shows clearly how this power becomes greatest 

 when there is the least gas present in the air-space. The quibble arises 

 over the popular interpretation of the apparently paradoxical statement 

 that the evaporating power of the air is greatest when all the air has been 

 removed. Of course, when the limit is reached and the air-pressure is 

 actually zero, we have to broaden our definition of air so as to let the 

 term mean the space abutting against the evaporating surface into 

 which water vapor may diffuse. That this is necessary at the limit of 

 reduced pressure (which is never really attained) seems to be no reason 

 for changing our term, though if it seems desirable we are free to admit 

 that the term in question really denotes the evaporating power of the 

 circumambient space in which air usually occurs. 



Another objection to the term ''evaporating power of the air" is 

 parallel to the one always raised against the word suction. The water 

 vaporizes because of conditions resident in its solid or liquid phase, 

 and the energy thus transformed does not come (directly) from the 

 air-space. Just as the term ''suction," or sucking power, has to be 

 regarded as referring to the removal or decrease of a resistance, rather 

 than to the application of a driving force, so the evaporating power of 

 the air is to be regarded as proportional to the reciprocal of the measure 

 of the resistance offered by the air to evaporation. The resistance thus 



