FINGER LAKES OF NEW YORK. 2 27 



present at a deptli of i m. In the deeper i m. strata there was a loss of 28~to 29 per 

 cent of the energy present at the upper surface of the stratum. At 5 m. depth there 

 remained about 5.4 per cent, and at 10 m. about i per cent of the energy dehvered to 

 the surface. 



We may infer from such a set of observations the penetration of the sun's rays 

 during the whole day or during a longer period, assuming that the turbidity and color of 

 the water remain unchanged. In such a process it is not hard to secure results which 

 are correct in general, but it is impossible to secure minute accuracy. Certain, though 

 not all, of the facts which prevent minute accuracy will be mentioned. 



1 . Sunlight is a mixture of the direct rays of the sun and of rays reflected from the 

 sky. The percentage of sky radiation is very variable, being sometimes as low as 8 per 

 cent of the total radiation and rising nearly to 100 per cent when the sun barely shines 

 through haze or cloud. The quantity of energy reflected from a unit area of sky is also 

 variable and differs with the nature of the sky and the proximity of the area to the sun. 

 The mean percentage of sky radiation reflected from the surface of the water differs 

 from that of the direct rays, and the mean path in water of the rays from the sky differs 

 from that of the direct rays. It is practically impossible, under the conditions of ob- 

 servations on lakes, to determine either the amount or the distribution of the sky radia- 

 tion. It is, therefore, impossible to make full correction for the elements in the mixture 

 of direct and diffuse rays at the time of observations. 



2. It is also impossible to make such corrections for longer periods, since the aver- 

 age amount of sky radiation is still quite unknown for most places, and is not accurately 

 known anywhere. 



3. No correction has been made in the observations for radiation reflected from the 

 surface of the water, but the readings at i m., etc., have been compared directly with the 

 reading in air. The direct sun radiation, at the altitude of the sun when the observa- 

 tions were made, would lose about 2.1 per cent by reflection. The sky radiation would 

 lose 17.3 per cent if equal quantities came from equal areas of sky. This loss at the 

 surface, which can not be known accurately, has been balanced against the opposite 

 effect of the hemispherical glass cover of the sunshine receiver. There would be about 

 4 per cent of the sun's radiation reflected from this in air and about 0.5 per cent in water. 



In computing a standard curve of absorption for Seneca Lake, all radiation has 

 been referred directly to the sun, and the path of the rays in the water has been computed 

 on that basis, from the following elements: 



Time of observations, August i, 1918, 12.40 to i.ii sun time. 

 Corresponding altitude of sun, August i, 64.1 to 62.3°. 



Depth at which sun's rays pass through i m. of water, at first observation, 94.5 cm; at last observa- 

 tion, 93.7 cm.; mean 94.1 cm. 



On the curve of direct observations {A-A, fig. 2) are noted the readings at the dis- 

 tances corresponding to this path of the rays in water. These periods are plotted and 

 connected by a new curve, B-B, the curve for vertical sun. In this curve, which assumes 

 a sun in the zenith, the depth below the surface equals the length of path of the rays in 

 reaching that depth. This constitutes a standard curve, from which may be derived 

 the energy which remains at given depths below the surface at any time of the day or 

 year, provided the altitude of the sun is known and the corresponding length of the 

 path of its rays in water. It must be assumed also that all radiation comes directly 



