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LIL On the Comparative Temperature of the Northern and 

 Southern Hemispheres of the Earth. By Prof. Henry 

 Hennessy, F.R.S* 



MORE than twenty years have elapsed since I published 

 a systematic proof of the superiority of water over 

 land as a substance for absorbing and gradually diffusing 

 throughout its mass the heat received from the sun. The 

 superficial portions of the earth as a whole, including the 

 atmosphere in contact with the water, would be thus influ- 

 enced, and in this way the physical properties of water were 

 shown to have a predominant influence on terrestrial climate. 



The properties of water here referred to are : — (1) Its excep- 

 tionally high specific heat ; (2) its moderate permeability to 

 luminous rays of heat contrasted with its impermeability to 

 obscure heat ; and (3) its mobility. From Pfaundler's obser- 

 vations and experiments it appears that the specific heat of 

 the soil, which forms the general surface of dry land exposed 

 to the atmosphere, to sunshine and planetary space radiation, 

 varies from 0'19 up to 0*5. The majority of soils have spe- 

 cific heat of from 0*25 to 028. Thus the absorbing-power 

 of dry land for radiant heat is in all cases much less than that 

 of water. At night land more readily parts with the heat 

 acquired under sunshine, and from its immobility it cannot 

 directly transport by convection or by currents any of the 

 heat it acquires to other parts of the earth's surface. The 

 conclusions at which I arrived were entirely different from 

 those which were generally received anterior to the time of 

 the publication of my views. The generally received notions 

 at that time are best summed up in the words of Sir John 

 Herschel : — u The effect of land under sunshine is to throw 

 heat into the general atmosphere, and to distribute it by the 

 carrying-power of the air over the whole earth. Water is 

 much less effective in this respect, the heat penetrating its 

 depth and being there absorbed, so that the surface never 

 acquires a very elevated temperature." f 



These words occur in the latest edition of the work from 

 which they are quoted. They furnished Sir Charles Lyell 

 the groundwork of some of his ingenious reasonings regarding 

 Geological Climate ; but my conclusions were so different 

 that they might be Stly expressed in terms diametrically 



* Translated, with additions, from the Comptes Rendus of the Paris 

 Academy of Sciences for September 1882. 

 t ' Outlines of Astronomy.' 



