Prof. Marcet on the Effects of Nocturnal Radiation, 9 



even from antediluvian times. But it is still more probable that 

 it has been calculated backwards, a matter of no difficulty when 

 once the precession of the equinoxes was known, and the epoch 

 when it occurred assigned with more or less accuracy according 

 to the means of calculation. 



7. This reconstruction of the celestial sphere in the time of 

 the Emperor Yao, even supposing he was a historical character, 

 appears to me to be based upon such uncertain data as to make 

 it altogether untrustworthy. If the data were more exact, the 

 case would be different. Unless, however, reliable data can be 

 procured, it seems a pity to prostrate the science of astronomy 

 by using it in such a cause, lending its name and high character 

 to prop up the conclusions of vague and uncertain traditions, 

 enveloped too often in fable and falsehood. 



J. H. Pratt. 



Calcutta, October 19, 1861. 



II. Experiments on the Comparative Effects of Nocturnal Radia- 

 tion from the Surface of the Ground and over a large Sheet of 

 Water. By Professor Marcet of the Academy of Geneva*. 



IT is acknowledged that about the period of sunset, provided 

 the sky be clear, the temperature of the air in contact 

 with the earth's surface is cooler than that of the atmosphere 

 at a certain height above the ground. This fact was first noticed 

 by Pictet and Six, towards the end of the last century; but as 

 the theory of radiant heat had not yet been established, no 

 satisfactory explanation of the phenomenon w T as offered until 

 1814, when Dr. Wells published his valuable ' Essay on Dew/ 

 Many years afterwards (in 1842) a series of observations, on 

 the same subject, I had made in the neighbourhood of Geneva, 

 was published in the eighth volume of the Memoires de la Societe 

 de Physique et d'Histoire Natwelle, tending to prove that during 

 clear and calm nights the atmosphere becomes gradually warmer 

 on ascending above the surface of the earth, until a certain 

 height be attained, which varies according to circumstances, but 

 is generally not less than from thirty to forty yards. The obser- 

 vations I made in 1842 have just been fully confirmed by an 

 elaborate treatise on the subject published by Professor C. 

 Martins of Montpellierf. The results obtained are, no doubt, 

 attributable to the gradual cooling of the earth's surface arising 

 from its nocturnal radiation into empty space; which radiation, 

 when the sky is clear, is not compensated by the transmission of 



* Communicated by the Author. 



t Vide Memoires de VAcademie des Sciences et Belles Lettres de Mont- 

 pellier, vol. v. 



