A LIMNOLOGICAL RECONNAISSANCE 23 



couples which can be alternately exposed to the sun and 

 covered. The electric current caused by the sun during ex- 

 posure is measured by a d'Arsonval galvanometer. The 

 swing of the galvanometer is directly proportional to the 

 heat energy in the solar radiation. If, therefore, the ther- 

 mal couples are exposed first at a depth of one meter below 

 the surface and then at a depth of two meters, the ratio of 

 the two readings will give the rate at which the stratum of 

 water transmits the solar radiation incident on its upper 

 surface. 



Readings were made in Okoboji lake to the depth of four 

 meters with the result shown in the following table: 



TABLE 14— OKOBOJI LAKE. READINGS OF PYRLIMNIMETER, 

 11.10-11.27 LOCAL TIME. SKY CLEAR. TRANSPAR- 

 ENCY OF WATER BY SECCHI'S DISC, 3.1 m. 



Depth, cm. Divisions Cal. Percent Depth Transmission, 

 of air per cent 



100-200 54 

 200 18.0 0.13 8.2 200-300 54 

 300 9.5 0.067 4.4 300-400 53 

 400 5.0 0.035 2.3 



Notes: The column marked divisions gives the swing of the galva- 

 nometer when the thermal couples are exposed to the sun. The column 

 marked Cal. gives the value of these readings in calories per square 

 centimeter per minute. The fourth column shows the value of the 

 readings in the water as a percentage of the readings in the air. The 

 last column shows the rate at which a stratum of water one meter 

 thick transmits the solar radiation incident on its upper surface. 



It appears that under the conditions of the observation 

 about 15.3% of the solar radiation reaching the surface of 

 the lake was present at the depth of 100 cm. Nearly 85% 

 therefore was either lost or absorbed before reaching that 

 depth. Such a result is a common one in lakes. The radia- 

 tion at 100 cm. rarely exceeds 20% of the total radiation 

 and often falls far below 15%. In absolutely pure water 

 some 43% may remain at that depth. But the stains al- 

 ways dissolved in lake water and the particles of organic 

 and inorganic matter suspended in it rapidly cut off the 

 radiation. 



It appears also that below the depth of one meter, about 

 53%-54% of the energy which reaches the surface of each 



