METEOROLOGY. 525 



proportion as one goes in one direction or the other from the heat maxi- 

 mum. (Z. 0. G. M., xvn, p. 21.) 



VI. — EVAPORATION, CONDENSATION, ETC. 



Stefan has given a summary of his investigations, 1874-1881, into the 

 laws of evaporation, from which we gather the following : 



(1.) The rate of evaporation is proportional to the logarithm of a frac- 

 tion whose denominator is the barometric pressure and whose numer- 

 ator is this pressure diminished by the vapor tension. 



(2.) The rate of evaporation out of a tube is inversely proportional to 

 the distance of the surface of the fluid below the open end of the tube. 



(3.) The rate of evaporation is independent of the diameter of the 

 tube. 



(4.) Within a closed tube the evaporation is observed by the bubbles 

 that form and rise to the surface, and it is found that the successive 

 intervals within which equal numbers of bubbles develop are to each 

 other as the successive uneven numbers. 



(5.) In hydrogen, evaporation proceeds four times as fast as in air. 



(6.) The amount of evaporation in given intervals of time increases as 

 the square root of the intervals. 



(7.) The amount of evaporation that ascends in a unit of time from a 

 circular surface into the air (assumed perfectly quiet) is proportional to 

 the circumference and not to the area of the circle, assuming that there 

 are no banks or walls to protect the edges. This is also true, to within 

 a tenth, of an elliptical surface of moderate eccentricity, i. e., whose ma- 

 jor axis is not more than four times greater than its minor axis. 



(8.) If now the vapor, instead of collecting close above the water sur- 

 face, rises and moves off to a distance by diffusion, then the stream lines 

 for the evaporation are hyperbolas, and those that start from the periph- 

 ery of the circular border of the basin constitute a hyperboloid of 

 revolution. Like all stream surfaces, this has the property that no va- 

 por penetrates through this hyperboloid so that it can be replaced by a 

 solid wall. Such a hyperbolic border to an evaporating dish will there- 

 fore not diminish the amount of evaporation in still air ; its proper con- 

 struction must be determined by Stefan's formula. As the water sur- 

 face sinks the evaporation diminishes in the ratio of r—h to h, where 

 r is the radius of the dish and h the linear sinking. For large values 



of h or where -^- is nearly unity, the condition is nearly the same as 



in a deep tube. Small surfaces evaporate more than large in proportion 

 to their area ; this latter is also true for the evaporation due to convec- 

 tion as well as diffusion. (Z. 0. G. M., xvn, p. 65). 



Stelling has published the results of observations by Dohrandt at 

 Nukuss on evaporation of water, and has discussed their connection 

 with temperature and wind velocity. He shows, first, that the observa- 

 tions are represented by Weilenmaun's formula somewhat better than 



