102 ROYAL SOCIETY OF CANADA 



givie a more lucid account than that of M. Arago, in his paper " On 

 the supposed Influence of the Moon on Vegetation." " Ko 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 different temperature from that of the surrounding air. 

 This important fact is now well ascertained. On placing little masses 

 of cotton down, etc., in the open air, it is frequently observed that 

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

 the surrounding atmosphere .... These differences of temper- 

 ature 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 sl<y 

 is clouded they become insensible." This lucid statement, however, 

 requires one modification ; fox the gricater cooling of the solid sub- 

 stances, 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 atmos- 

 phere, 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 per- 

 meable 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 very 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 phenomenon 

 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 presented 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 ds 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, modifies 

 the radiation, I shall detail some observations I made on the 7th of 

 January last, incidentally in the first instance, but then extended, in 

 reference to the observations on the ground-gru, which I was making 

 at the time. Having occasion that day to dig into recently hoed 



