PHYSICS. 379 



Hayford, John F., Northwestern University, Evanston, Illinois. Investiga- 

 tion of the laws of evaporation and stream-flow. (For previous reports 

 see Year Books Nos. 12-14.) 



The primary purpose of this investigation is to determine the 

 amount, and the relation to meteorological conditions, of the daily 

 evaporation from a large free- water surface, such as a lake or a large 

 reservoir. In using any one of the Great Lakes for this purpose it 

 is necessary to evaluate the income to the lake from all sources, the 

 outgo, and the change of content of the lake, day by day. The 

 change of content becomes known if the change in the mean level of 

 the whole lake surface is ascertained. The most serious difficulties 

 encountered in determining the change in the mean level of the whole 

 lake surface are those which arise from the fact that changes in the 

 barometric gradients over the lake and changes in the direction and 

 velocity of the wind produce fluctuations in the level of the water- 

 surface at the gage which is the station of observation. One must 

 evaluate these local changes of level with a high degree of accuracy 

 before it is possible to determine the outgo in the form of evaporation. 

 During the year covered by the report attention has been concentrated 

 almost exclusively on the determination of these local barometric 

 effects and local wind effects. 



The progress made during the year in determining the barometric 

 effects has consisted merely in securing better values for the eight con- 

 stants which express the barometric effects upon water-level at a given 

 station. No change has been made in the formula expressing the effects. 



As to wind effects, the work of the year has confirmed conclusively 

 the indications which had formerly been secured that it is necessary 

 to take into account the wind velocity and direction for each hour, 

 rather than the mean velocit}'' and prevailing direction for each day. 

 It has also been demonstrated that eight wind-directions must be 

 considered. Four directions are clearly insufficient. At the close 

 of the year an attempt is in progress to detennine the lag in the wind 

 effects, to determine whether the effects are cumulative, and to deter- 

 mine the exponent which expresses most accurately the relation 

 between the observed wind velocity and the disturbance of water- 

 level produced by the wind. The exponent was originally assumed, 

 in this investigation, to be 2; that is, the disturbance of water-level 

 was assumed to be proportional to the square of the wind velocity. 

 This attempt is being made by the use of the method of least squares 

 applied to observation equations in which the absolute term in each 

 equation is the change in water-level at a gage in one hour apparently 

 due mainly to changes in winds. The attempt has not progressed far 

 enough to justify confident statements as to its possible success and 

 as to the probable conclusions. The indications are that it will be 

 moderately successful, that the lag in wind effects is probably small, 



