PHYSICS. 399 



which the conjugate interference paths of the successive rays of a 

 pencil of hght may be modified systematically. Though the work is 

 not yet completed, the experiments made thus far have been very 

 favorable, the whole object being to obtain interference patterns suffi- 

 ciently cramped to be available for adjustment. Incidentally, the 

 frequent transitions of elliptic into hyperbolic types of interference 

 between the extreme cases of horizontal and vertical Unes are interest- 

 ing theoretical results. 



Finally Professor Barus has succeeded in obtaining what appears to 

 be a very interesting and new departure in optics, viz, forced vibra- 

 tions, exhibited by light waves of shghtly different wave-lengths. 



Haj^ord, John F., Northwestern University, Evanston, Illinois. Investiga- 

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

 Year Books Nos. 12 and 13.) 



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 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 first evaluate these 

 local changes of level with a high degree of accuracy before it is pos- 

 sible to determine the outgo in the form of evaporation. During the 

 year just ended, attention has been concentrated largely upon this 

 phase of the problem, with steadily increasing success. The knowledge 

 which is thus being secured of the laws of the barometric effects and 

 wind effects bids fair to be of considerable value. Each one of the four 

 Great Lakes is being used separately in the investigation. Important 

 checks against erroneous conclusions are thus obtained. 



At first it was believed to be sufficient for the purpose of the investi- 

 gation to compute the mean barometric gradients for each day over the 

 lake concerned, and to determine by the proper least-squares computa- 

 tions the effect upon the water-level at the gage of the change from 

 day to day in this mean gradient. It has been found, however, that 

 in order to express the phenomena adequately one must take account 

 of the changes in barometric gradients in each successive 12-hour 

 period. Also, it has been found that at least 8 constants must be 

 determined to express the barometric effects at a station adequately, 



