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XXVII. On the Physical Properties of Water in relation to Ter- 

 restrial Climate. By Professor Hennessy, F.R.S. fyc. 



To the Editors of the Philosophical Magazine and Journal* 



Gentlemen, 



ALLOW me to express my satisfaction at seeing that conclu- 

 sions to which I have been led several years since, and 

 which have appeared in the pages of your Magazine as well as in 

 other publications, have been reproduced in your last Number by 

 Mr. James Croll. Your able contributor has already done me 

 the honour of sanctioning or adopting other results which I had 

 previously published, and I may therefore be excused for briefly 

 referring to them in the course of the present communication. 



In your Number for March 1859, S. 4. vol. xvii. p. 181, a 

 paper is printed in which the properties of water in their rela- 

 tion to terrestrial climate during different geological epochs is 

 the principal subject under discussion. After establishing, as I 

 venture to think, the superiority of water as a heat-distributor 

 over the earth's surface compared with the other materials of the 

 earth's coatiDg, I applied the results to explain the operation of 

 the Gulf-stream, and afterwards to the question of geological 

 climate. After a short discussion of Sir Charles LyelPs theory, 

 of the calorific influence of a belt of equatorial land combined 

 with circumpolar oceans, I made the following statement, which 

 I place side by side with a passage from Mr. CrolPs paper : — 



Mr. Croll (1867). Professor Hennessy (1859). 



" These, as well as many other " Not only are there physical 



considerations which might be grounds for adopting a somewhat 

 stated, seem to lead to the con- different conclusion [from Sir 

 elusion that, in order to raise the Charles Lyell's] (namely, that the 

 mean temperature of the whole most favourable condition for a 

 earth, water should be placed generally high terrestrial tem- 

 along the equator — and not land, perature would be in a compa- 

 as is generally believed" (p. 129). ratively equable distribution of 



land and water over equatorial 

 and extra- tropical regions, in- 

 stead of a concentration of land 

 in the former), but the study of 

 the present relations of sea and 

 land seems to strongly verify the 

 views on which this conclusion 

 is based" (p. 189). 



At p. 130 Mr. Croll refers to the equatorial regions as the 

 most important with reference to gains and losses of solar heat. 

 This I had already pointed out in section 6 of my paper, and in 

 a still more explicit manner in a paper read before the British 



P2 



