676 



BULLETIN OF THE BUKEAU OF FISHERIES 



I loiow of no direct measurements of the depth to which the infra red rays do 

 actually carry heat into the sea water in measurable amount under the conditions 

 of turbidity actually existing at sea, but even distilled water is so nearly opaque to 

 them that they are almost entirely absorbed (for practical purposes, entirely so) in 

 one meter, and their penetration into the sea is certainly less. That is to say, nearly 

 half of the sun's direct radiant heat is expended, theoretically, upon this thin surface 

 film. 



According to a calculation carried out in the physical laboratory of Harvard 

 University through the kindness of Prof. Theodore Lyman, 58 per cent of the energy 

 conveyed by the visible part of the solar spectrum would be absorbed by passage 

 through 9 meters more (i. e., a total of 10 meters) of perfectly clear distilled water, 

 so that only about 20 per cent of the total solar energy entering the water would 

 penetrate as deep as 10 meters, this small residual lying chiefly in the blue-green 

 part of the spectrum. Certainly less than 1 per cent could penetrate as deep as 200 

 meters — chiefly in the ultra violet. Probably this calculation would apply equally 

 to pure salt water. The sea, however, is never clear; and in boreal coastwise waters 

 such as the Gulf of Maine, which are always comparatively turbid, the fine particles 

 in suspension — silt or plankton — absorb so much of the sun's rays that the penetra- 

 tion of heat is much reduced. 



It is, of course, with the depth to which the water of the gulf is measurably 

 warmed by the direct penetration of solar radiation under conditions actually pre- 

 vailing there that we are now concerned. This may be approximated by experi- 

 ments that have been made in other seas. In the comparatively clear water of the 

 Mediterranean, off Monaco, Grein's (1913) measurements^' of the penetration of 

 different parts of the solar spectrum showed that the wave lengths as long as the 

 blue-green, and longer, were virtually all absorbed in the upper 50 meters, red-yellow 

 in the upper 10 meters, as appears in the following table condensed from his 

 account. 



Intensity of light penetrating to different depths, taking the amount at 1 meter as 100 



Translated into terms of solar energy, this means that at least 70 per cent of all 

 the radiant solar heat that penetrated as deep as 1 meter was absorbed at a depth of 

 10 meters; and as nearly all of the energy of the infra red certainly was absorbed in 

 that upper meter of water, it is not likely that more than 13 per cent of the solar 

 heat that entered the water at all reached as deep as 10 meters by direct radiation, 



siThese experiments were made with a "revolving photometer," for description of which, and of the method by which the 

 degree of blackening of the photographic plates was measured, see Grein (lOlS;;. 



