10 Prof. Marcet on the Comparative Effects of Nocturnal Radiation 



caloric from the higher regions of the atmosphere. The cooling of 

 the surface of the earth naturally gives rise to a corresponding 

 diminution of temperature of the stratum of air in its immediate 

 vicinity ; the effect is transmitted to the stratum above, though 

 naturally in a less degree, and so on from one stratum to another, 

 until a height be attained at which the temperature of the atmo- 

 sphere is found to be equal to that of the stratum of air in con- 

 tact with the earth. 



I had often thought of inquiring whether the effects of noc- 

 turnal radiation, tending as they do to produce a gradual in- 

 crease of temperature on ascending above the earth's surface, 

 are entirely dependent on the radiation of the ground, properly 

 so called^, or whether they would be equally perceptible above a 

 large sheet of water, such as the sea or a lake. The exceptionally 

 fine clear weather of October last afforded me a favourable oppor- 

 tunity for making some experiments on the effects of nocturnal 

 radiation from the surface of the lake of Geneva. 



Let me, however, be allowed to remark that experiment alone 

 could determine to what extent a large surface of water is 

 capable of producing, by the radiation of its caloric, the whole, 

 or at least a part of the effects due to the nocturnal radiation 

 of the earth. Water, it is well known, possesses a consider- 

 able radiating power ; Leslie, in his { Researches on Heat/ 

 found it to be equal to that of lampblack, and superior to that 

 of paper. It is not, therefore, because water does not radiate suf- 

 ficiently that we can be authorized to conclude a priori that the 

 nocturnal increase of temperature is not as likely to take place 

 over a liquid surface as over the solid ground; but there is an- 

 other fact, depending upon the peculiar constitution of liquids, 

 which must also be taken into consideration. The particles of 

 liquids, it is well known, are essentially moveable, and their dif- 

 ferent strata subject to a constant interchange of position when 

 affected by the slightest changes of temperature. The conse- 

 quence is, that the moment the upper surface of a given extent of 

 water has begun to cool by the effect of nocturnal radiation, it 

 will become denser than the stratum immediately below it; it 

 will therefore descend and be replaced by this stratum, which, 

 becoming heavier in its turn, will be replaced by the following, 

 and in the same way successively from one stratum to the other; 

 so that in fact there is no reason why the temperature of the sur- 

 face of the water should undergo any appreciable change. Under 

 these circumstances it will, I think, be admitted that the effect 

 of nocturnal radiation (inasmuch as it would tend to lower the 

 temperature, first of the surface of the water, and next that of the 

 stratum of air in immediate contact with this surface) will become, 

 if not entirely imperceptible, at least far less apparent than on 



