340 REV. JAMES FARQUHARSON ON THE ICE 



slowly than it had risen ; and the experiment was repeatedly renewed with similar 

 results, leaving no doubt that the heat, like the light, radiated through, and was re- 

 fracted by the water. If the fact is so in regard to the radiation of heat through a 

 mass of water four or five inches thick, where ought we to set the limits of thickness 

 of the mass through which it cannot pass ? Obviously only where the thickness is so 

 great, that the aggregation of the fluid, and of its minute impurities, prevents the 

 transmission of light, as in the deeps of the sea, but not within the ordinary depths 

 of our clear streams. 



Of the effect of radiation in cooling down the surface of the ground, and substances 

 placed upon it, during a clear sky, we cannot give a more lucid account than that of 

 M. Arago, in his paper " On the supposed Influence of the Moon on Vegetation." 

 " No one had supposed," says he, " before Dr. Wells, that terrestrial substances, ex- 

 cepting in the case of a very rapid evaporation, may acquire during the night a dif- 

 ferent temperature from that of the surrounding air. This important fact is now well 

 ascertained. On placing little masses of cotton down, &c. in the open air, it is fre- 

 quently observed that they acquire a temperature 6°, 7°, or even 8° centigr. below 



that of the surrounding atmosphere These differences of temperature between 



solid bodies and the atmosphere only rise to 6°, 7°, or 8° of the centesimal scale, when 

 the sky is perfectly clear. If the sky is clouded they become insensible." This lucid 

 statement, however, requires one modification ; for the greater cooling of the solid 

 substances, under a clear sky, takes place not only during the -night, but also during 

 the day, in places not directly exposed to the sun's rays. 



This radiation, as it passes freely through the transparent atmosphere, may, as we 

 learn from the above experiment, pass also through the transparent water, to cool 

 down the solid substances at the bottom below the temperature of the surrounding 

 fluid. That fluid is permeable to radiating heat as well as the atmosphere. The 

 application of the thermometer, in the hands of Dr. Wells, instructed us regarding 

 the cooling of the surface of the ground ; but the water of a river, placed under the 

 veiy same condition of a clear sky, fluid above and freezing below, is a great natural 

 thermometer, teaching us that a corresponding cooling is going on on the surface of 

 the solid opake substances of the bottom. In fact, if we may so speak, the pheno- 

 menon of the ground-gru is the result of an experiment in the water, entirely similar 

 to that of Dr. Wells on the land, performed by nature on a large scale, and pre- 

 sented to us for our interpretation and instruction. And when we look back to the 

 observations made in the month of January, we find the results of the modifications 

 of this great natural experiment corresponding with those of similar modifications of 

 the experiment on the dry land. 



The cooling of the surface of the ground by radiation, discovered by Dr. Wells, 

 takes place only under a clear sky. It is therefore greatly modified on parts of the 

 ground screened from a part of the sky by opake objects, as walls, trees, hedges. 

 In illustration of the extent to which a screening or shading body, near at hand. 



