228 



BULLETIN OP THE BUREAU OF FISHERIES. 



from the sun, or at least that the value of the sky radiation is the same as at the time 

 of observation. 



The results are stated in Table 12, vertical sun. 



The mean distribution of sunshine and cloud at Seneca Lake is not known, but at 

 Madison, Wis., the mean daily supply from sun and sky during the five months April 

 I to August 31 is 398 cal. The mean path of the rays during this period to reach a depth 

 of 100 cm. below the surface is 115 cm. In this computation allowance is made for 

 reflection from the surface in excess of 2.1 per cent; all radiation is supposed to come 

 from the sun ; and the form — though not the area — of the solar energy curve is supposed 

 to be constant. 



The points corresponding to this distance of 1 15 cm. per 100 cm. of depth are noted 

 on the curve for vertical sun, carried up to their proper place, and a third curve, C-C, 

 figure 2, is drawn, which is the curve for mean sun (Table 12). 



Table 12. — Transmission of Sun's Rays by Water of Seneca Lake, Aug. i, 1918. (f^eefig. 2, p. 226.) 



Depth in centimeters. 



100 

 300. 

 300 

 400, 



Soo, 



Per cent radiation reniaining 

 at depth indicated. 



observed 

 I)er cent. 



Computed per cent. 



Vertical 

 sun. 



21.9 

 15.6 



8.3 

 6.0 



Mean 

 sun. 



20. 7 

 14-3 

 9 9 



6.8 



4- 7 



Depth in centimeters. 



600. 



700. 



Soo 



900. 



1, 000. 



This curve of mean sun and Table 12 show that in Seneca Lake at i m. depth there 

 is found an average of about 20.7 per cent of the incident radiation and that each i m. 

 stratum below transmits less than 70 per cent of the radiation received by its upper 

 surface. The water has absorbed 99 per cent of the incident energy at about 9 m. as 

 compared with about 10 m. for the observed curve and 11 m. or more for the curve of 

 vertical sun. 



The difference in the three curves are not striking in this case; but if the observa- 

 tions had been made at an hour farther from noon, or later in the season, the difference 

 would have been correspondingly larger. 



It must not be supposed that this mean sun curve represents exactly the mean con- 

 ditions actually present during the period when the lake is warming. The transparency 

 of the water is variable and the sun's penetration varies with it. No account is taken 

 in this curve of the energy received during cloudy hours. Yet after all deductions are 

 made it remains true that the curve gives a generally correct picture of the actual direct 

 delivery of the sun's radiation to Seneca Lake so far as a single observation can give 

 this. Hardly more than 20 per cent of the incident energy is delivered to water below 

 the surface meter. Not over 5 per cent is delivered to a greater depth than 5 m. and not 

 over I per cent below 10 m. Even a considerable increase in transparency would leave 

 these figures, not unchanged, but of the same order of magnitude. 



Observations such as these are ordinarily made at times when the sky radiation is 

 relatively small — near noon of clear days. When, therefore, such an observation is 



