A LIMNOLOGICAL RECONNAISSANCE 31 



shows large gains at all depths as compared with July 18. 

 The total gains to the depth of 25 m. amount to nearly 2600 

 cal. per sq. cm. of surface, or more than 300 cal. per day. 

 This sum must be considerably more than half the heat de- 

 livered by the sun, which at Madison averaged 460 cal. for 

 the days in question. A gain of this magnitude in July 

 seems impossible. Such gains are not infrequently made 

 in April vrhen the temperature of the water is low, so that 

 distribution of heat is easy and when it is also above that 

 of the air so that losses from the surface are at a minimum. 

 In July the mean daily gain of lake Mendota is 69 cal. and 

 in July, 1915, it was 97 cal. It seems clear therefore that 

 the apparent rise of temperature between July 18 and July 

 26 can not be a real one. It is possible, since the volume 

 of the bottom water is so small, that it may be displaced 

 by wind in such a way as to cause this apparent rise, or the 

 apparent fall between June 28 and July 18. (Tilton '17, 

 p. 39). If so, these oscillations of temperature are excep- 

 tionally great and are themselves worth study. 



The alternative conclusions can not be avoided, either 

 that Okoboji lake is exceptional to a wholly unprecedented 

 degree, or that the readings of the minimum thermometer 

 need further correction. It is best therefore to defer de- 

 tailed discussion of them until a careful study has been 

 made of the heat exchanges of the lake. In reaching this 

 conclusion we ought not to fail to recognize the great serv- 

 ice done to limnology by Dr. Tilton's observations. He 

 recorded for the first time the bottom temperatures in a 

 deep lake whose basin is of such form as to give rise to 

 very high temperatures at the bottom. He thus shov/ed 

 where exceptional opportunity lay for the study of the ef- 

 fect of the form of the basin on the temperature of lakes 

 and perhaps in other respects. 



We may be pardoned an illustration of the influence of 

 Dr. Tilton's records. There is in Wisconsin a small body of 

 water. Rock lake, whose dimensions are: Area, 496 ha.; 

 maximum depth, 20.4 m. ; mean depth, 6.1 m. ; volume devel- 

 opment, 0.90. (Juday '14, p. 34, map VII). It is therefore 

 almost a morphometric copy of Okoboji lake on a small 



