234 BULLETIN OF THE BUREAU OF FISHERIES. 



water than i m. Schmidt (1908, p. 240) computes that about 27 per cent of the solar 

 energy is absorbed by i cm. of pure water and about 45 per cent by i dm. He uses 

 Langley's energy curve for the solar spectrum, which makes his figures somewhat 

 larger than would be the case in a curve for moderately high sun. In the curve which 

 we have used as standard (path of rays equals 1.5 atmospheres) about 43 per cent of 

 the energy would be absorbed by 25 cm. of pure water and 49 per cent by 50 cm. While 

 no great accuracy can be claimed for the figures shown by Seneca Lake of about 67 per 

 cent absorption for 25 cm. and 72 per cent for 50 cm., they are probably not greatly 

 in error. The differences between them and the data for pure water are much the 

 same as for greater depths. Thus more than one-half of the sun's energy is delivered 

 to the upper centimeters of water from which loss to the air is easy. But much of the 

 heat so delivered is distributed by the wind from the surface strata to deeper water, 

 especially in the early part of the wanning season when the lake is gaining heat rapidly. 

 From this source comes the greater part of the heat which the lake gains below i m. 

 in excess of that delivered by the sun. This heat amounts to 19,000 cal./cm.^ and much 

 of it must come from the 40,000 cal./cm.^, or more, absorbed by the upper 25 cm. of 

 the lake. During bright and windy days there must be thus moved down into the 

 lake by the wind much heat which is lost during cool periods when the whole upper 

 water of the lake cools down. 



It is true that on the whole the heat delivered by the sun to strata below the surface 

 is more likely to be retained, as the water above a stratum must be cooled to a lower 

 temperature than the deeper water before any heat can be lost by the latter. But 

 several times each season there is a general cooling of the upper water, when much 

 heat is lost, that placed by the sun as well as tliat placed by wind. 



At present, therefore, no accurate estimate can be made of the loss of sun-placed 

 heat at various depths. The subject must be left here with the general statement 

 that between 84 and 92 per cent of the work done in distributing heat through the water 

 of Seneca Lake is performed by the mnd, on the assumption that conditions of trans- 

 parency, etc., on August i were average ones. The amount really attributable to the 

 sun is probably as much as 10 to 12 per cent. More than this can not be said, both in 

 view of considerations presented above, and also in view of one other consideration 

 which the study of Lake Mendota has shown. In the early part of the warming period, 

 when gains of heat are rapid and when the deeper water is securing most of its heat, 

 the sun plays a small part in distributing the heat. Later in the summer the sun has 

 a much larger share of the work, when the epilimnion is forming, when gains of heat 

 are small (perhaps only 5 to 10 per cent of the incident radiation), and when these 

 gains are confined to the surface strata. 



The foregoing paragraphs have dealt with Seneca Lake alone. The same methods 

 may be applied to the other lakes with similar results. It is unnecessary to give the 

 details of the computations ; the results are shown in Table i6 (p. 235) and figures 4 

 and 5 (pp. 230, 231). 



