340 ON THE METEOROLOGY OF IRELAND. 



We learn from these numbers that the annual variation of the 

 sea-temperature, at the surface, differs considerably from that of 

 the air above it, the difference consisting chiefly in a retardation 

 of the epochs of maximum and minimum. Thus the minimum 

 temperature occurs in the middle of February, and the maximum 

 in the middle of August, or about a month after the corre- 

 sponding epochs of the temperature of the air. The annual range 

 is also, as might have been expected, considerably less than that 

 of the air. These results accord sufficiently well with the conclu- 

 sions drawn by Ksenitz, from a comparison of the results of many 

 voyagers. 



But the most interesting result is that concerning the relation 

 between the temperature of the sea at the surface, and that of the 

 superincumbent air. Upon this subject the greatest discordance 

 exists in the statements of different observers. According to 

 Humboldt, the mean temperature of the Atlantic Ocean, at the 

 surface, is in all cases higher than that of the atmosphere above it. 

 This conclusion is confirmed by the observations of Peron and 

 Fitzroy, and is contradicted by those of Irving, Forster, and 

 Kotzebue. From an elaborate discussion of the observations of 

 many voyagers, Ksemtz infers that the temperature of the sea is less 

 than that of the air over the land in the lower latitudes, while in the 

 higher latitudes it is greater. The original conclusion of Humboldt, 

 however, seems to be placed beyond all doubt by the recent obser- 

 vations of Captain Duperrey, which appear to be more numerous, 

 and taken with more precautions to insure accuracy, than any 

 preceding. It seems now to be generally admitted that, in the 

 temperate and polar regions, the temperature of the sea is higher 

 than that of the air ; and the only question that remained was as 

 to the tropics. Now the observations of Duperrey were made all 

 round the globe, between 10 N. and 10 S. latitude ; and they 

 were taken at intervals of four hours, so as completely to eliminate 

 the effects of the diurnal change. From these observations it 

 appears that the temperature of the sea is higher than that of the 

 air within the zone already mentioned, the mean excess in the 

 Atlantic being 0'83 Fahr., and in the Great Ocean about half 

 that amount. 



The present observations possess much interest in connexion 

 with these questions. In order to perceive their bearing, I have, 

 in the Table which follows, given the half-yearly and yearly 



