342 ON THE METEOROLOGY OF IRELAND. 



temperature of the sea, on the average of the entire year, exceeds 

 that of the air over the coasts by2'0 ; while the latter temperature 

 exceeds that of the air inland (for the same latitude and longitude) 

 by l-8. The total excess of the sea-temperature above that of 

 the air amounts, therefore, to 3'8 Fahrenheit. 



This excess, which appears to be much greater than has been 

 observed elsewhere, is to be ascribed, mainly, to the influence of 

 the gulf-stream upon the temperature of that part of the ocean 

 which bathes our shores. But there is likewise another cause 

 which undoubtedly contributes to the effect. It has been shown 

 by Mayer and Joule, that heat is generated by the friction of 

 fluids in motion, and the latter experimentalist has established 

 the important physical law, that there is a definite relation 

 between the heat so produced, and the mechanical power ex- 

 pended by the moving mass. Mr. Eankine has applied this 

 principle to explain the fact, observed by M. Eenou, namely, that 

 the temperature of the river Loire at Vendome is higher than that 

 of the air above it ; and it is obvious that a similar explanation 

 is applicable to the phenomenon under consideration. There is 

 no doubt as to the reality of the cause ; the only question is as 

 to the magnitude of the effect to be ascribed to it. That such 

 effect is, at all events, sensible, I infer from two circumstances. 

 The first of these is, that the phenomenon of the excess of sea- 

 temperature appears to be general, and must, therefore, be the 

 effect of some general cause ; the second is, that on the coasts of 

 Ireland there is no sensible difference between the amount of the 

 excess on the eastern and on the ivestern shores. 



Should the effect of this cause be found to be sensible, and its 

 amount be determined, our views of the cycle of meteorological 

 phenomena would be much enlarged. The elevation of tempera- 

 ture rarefies the air ; the denser air flows in to supply the partial 

 vacuum, and wind is produced ; and finally, this wind, both by its 

 own motion, and by that of the ocean which is so subject to its 

 power, restores again the heat which had been converted. Thus the 

 normal condition of temperature is preserved, not only throughout 

 the changes which render it latent and sensible, in the generation 

 and condensation of vapour, but also in its conversion into 

 mechanical power, and its reproduction, in the phenomena of the 

 tempest and of the billowy sea. 



