NOTEMBER 14, 1913] 



SCIENCE 



703 



the university shops. The method of observa- 

 tion is much the same as with the solar ther- 

 mometer, but the instrument is much more 

 sensitive and rapid in its action. Readings 

 are made in a few seconds and the instrument 

 will easily record an amount of heat as small 

 as 1 per cent, of that present at the surface. 

 The results obtained by the two instruments 

 are in substantial agreement. Observations 

 have been made on a stratum of water of con- 

 siderable thickness (1 m. or 0.5 m.) and have 

 usually dealt with strata beginning at 0.5 m. 

 or 1 m. below the surface — a depth at which 

 all, or nearly all, of the invisible part of the 

 spectrum has been absorbed. 



It has long been known that a stratum of 

 optically pure water 1 m. thick absorbs about 

 60 per cent, of the sun's energy, including 

 nearly all of that below the A line. In pure 

 water the absorption below one meter would 

 amount to less than 12 per cent, of the energy 

 present at a given depth in the 1 m. stratum 

 immediately subjacent. These figures are sub- 

 ject to variation, depending on the altitude 

 of the sun and the form of the energy spec- 

 trum. 



Lake water is optically very different from 

 pure water. The inland lakes of Wisconsin 

 are not very transparent; the transparency, as 

 shov?n by Secchi's disk, varying from less than 

 1 m. to about 7 m. The transparency is af- 

 fected both by turbidity, due to suspended 

 matter, and to stain, occasioned by matters ex- 

 tracted from peat, etc. 



Observations made on more than twenty-five 

 lakes showed that not more than 20 per cent, 

 of the sun's energy present at the surface is 

 found at a depth of 1 m., and the amount is 

 usually much less; sometimes as low as 2 per 

 cent, or 2.5 per cent. Not less than 30 per 

 cent, of the energy present at 1 m. is absorbed 

 by the stratum of water between 1 m. and 2 

 m.; usually as much as 40 per cent, to 50 per 

 cent, is absorbed; and the amount may be as 

 great as 85 per cent, to 95 per cent. The rate 

 of absorption per meter is substantially the 

 same in subjacent meters as it is between 1 m. 

 and 2 m. No readings have been made at a 

 greater depth than 6 m., since at greater 



depths the energy was always too small for 

 accurate measurement. 



From these observations it follows that the 

 heat of the sun's rays is practically absorbed 

 entirely by the upper meters of the lake. So 

 much as 1 per cent, of the energy present at 

 the surface is rarely found at a depth so great 

 as 5 m., and usually the 1 per cent, point is 

 reached between 3 m. and 4 m., or even higher. 

 It is quite impossible that an appreciable diur- 

 nal rise in temperature should be found in these 

 lakes at the depth of 5 m., and practically the 

 entire seasonal rise of temperature at 5 m. 

 and below is due to mechanical agencies — 

 chiefly, if not wholly, wind — rather than to 

 insolation. It follows also that there is in 

 general no relation between the depth to 

 which the heating of the sun's rays penetrates 

 and the thickness of the epilimnion. 



An interesting and (to me) unexpected re- 

 sult of these observations is the not uncommon 

 absence of correlation between the transpar- 

 ency of the water, as shown by Secchi's disk, 

 and the rate of absorption of energy. Stained 

 water may be much more transparent, as meas- 

 ured by the disk, than turbid water which is 

 not stained, but in such cases the rate of ab- 

 sorption of energy may be relatively, or abso- 

 lutely, greater in the stained water. For in- 

 stance, Marl lal^e, whose water is clear but 

 turbid with marl, had on August 21, 1912, a 

 transparency of 1.8 m. and a rate of absorp- 

 tion of the sun's energy below 1 m. of about 

 55 per cent, per m. On August 17, 1912, Otter 

 lake, a near neighbor, whose water is stained 

 but not turbid, had a transparency of 5.2 m. 

 and an absorption of about 54 per cent. Nu- 

 merous observations have been made, which 

 give similar results. It may be noted also that 

 bottom growing plants were found abundant 

 to substantially the same depth in these two 

 types of lakes. 



This work is still in progress and when 

 completed will be incorporated in a general 

 report on the temperatures of Wisconsin 



I may add that for three years past the 

 heat delivered by sun and sky at Madison has 

 been recorded at the United States Weather 



