100 Tzvcnty-sex'cnth Annual Meeting 



of the sun. The questions I liave been trying to determine are 

 tliese : How far does tlie heat of the sun penetrate into the water, 

 and how does it get down to the depth which it actuahy reaches? 



The heat of the sun falls on the surface of the lake, and there 

 are three ways in which the heat may penetrate through the sur- 

 face to the deeper water. In the first place, it may go down by 

 conduction ; the warm water warming by conduction the stratum 

 immediately below it. This method is practically of no import- 

 ance. The power of the water to conduct heat downward is so 

 small that it may be entirely neglected. 



The second way in which the heat may get down is by the 

 direct action of the sun shining down into the water, penetrating 

 it and warming it as it goes. This method means a good deal 

 more than conduction, although it means a great deal less than 

 is ordinarily supposed. By far the greater part of the heat of the 

 sun is stopped by the first layers of the water and gets no further. 

 All the heat that belongs to the dark portion of the sun's rays is 

 stopped by a very thin layer of water, and that part which is in the 

 luminous portion of the spectrum is very rapidly absorbed, especi- 

 ally if there are plants or other opaque particles in the water. If, 

 then, the temperature of the water depended on the penetration 

 of the sun's rays, and if the water were entirely undisturbed by 

 the wind, we should find a high temperature only to a very small 

 distance from the surface, and then we should find a very rapid 

 change to cold water below. 



But as a matter of fact, our lakes are exposed to the action 

 of the wind, and this Action constitutes the most important means 

 of distributing the heat of the sun to the layers of water below the 

 surface. The action of the wind sets up currents in the water, 

 which distribute to a greater or less depth the heat which the sur- 

 face secures from the sun. As a matter of fact, we find in the 

 middle of summer a layer of water, often 20 or 30 feet in thick- 

 ness, which has been almost uniformly warmed by the sun. The 

 thickness of this layer depends not on the depth to which the 

 sun's rays penetrate the water, but on the action of the wind dis- 

 tributing to a greater or less depth the surface layers which have 

 absorbed the heat of the sun. 



It follows from this method of distribution that the depth to 

 which the water is warmed will depend upon the action of the 

 wind, and if lakes in the same region are compared, which are 

 equally exposed to the influence of the sun and wind, the amount 

 of warm water on the surface and the depth which the heat of 

 the sun will reach will depend very largely upon the area of the 



