230 



BULLETIN OF THE BUREAU OF FISHERIES. 



the diffuse radiation in reaching a depth of loo cm. would be about 126 cm. as compared 

 with 115 cm. for the direct rays. The mean path for sky radiation during sunny hours 

 would be between the numbers given above, depending on the relative amount of the 

 sky radiation coming from areas close to the sun and, therefore, having approximately 

 the same length of path in the water as the sun's rays have. 



In the absence of knowledge of the amount of sky radiation at Seneca Lake, either 

 general or on the date of observation, no correction can be made for sky radiation. 

 Such correction can be made where observations are so numerous that it may safely be 

 assumed that sky radiation was the mean amount. This is the case with Lake Mendota, 

 and the best computation that can be made shows that the mean path of all rays to reach 

 a depth of 100 cm. in the period of April 15 to August 15 is about 1 18 cm. No essential 

 difference, therefore, is made in the results if all radiation is attributed to the sun with 

 a mean path of 1 15 cm., as has been done in the previous paragraphs. 



The observations on Canandaigua and Cayuga Lakes may be treated much more 

 briefly. They were taken at the same intervals as on Seneca Lake but to the depth of 

 5 m., which is ample for the determination of the rate of absorption. The results are 

 shown in figures 4 and 5, and summarized in Tables 13 and 14. 



Table 13. — Transmission op Radiation by Water of Cayuga Lake, Sheldrake Point, July 29, 

 1918, 1.45 TO 2.45 y. M., Government Time. (See fig. 4.) 



[Note.— Sky with cumulus clouds drifting across; clear between clouds. Transparency ol water 6.2 m. Transmission per 



meter about 66 f>er cent.] 



Depth in centi- 

 meters. 



300.. 

 300.. 



Per cent radiation remaining at depth 

 indicated. 



Observed 

 per cent. 



19. 2 

 19. 6 

 13.4 

 11-3 

 II. 4 

 8.1 

 8.6 



Mean 

 observed 

 p>er cent. 



12.8 

 8.4 



Computed per cent. 



Vertical 

 sun. 



13-3 



9.1 



Mean 

 sun. 



II. 9 

 7.6 



Per cent radiation remaining at depth 

 indicated. 



Depth in centi- 

 meters. 



Observed 

 1 per cent 



5-6 

 5-4 

 3-6 

 3-4 

 3 6 

 3-5 

 3-7 



Mean 

 observed 

 per cent. 



3-6 



Computed per cent. 



Vertical 

 sun. 



Mean 

 sun. 



O Ctm. 



Fig. 4.— Work cur\es tor Cayuga Lake, (See explanation, fig. 3.) 



The observations on Cayuga Lake are rendered somewhat irregular by the fact 

 that numerous white cumulus clouds were passing over the sky and work had to be 

 done when the sun was in the spaces between the clouds. Under these conditions the 



