of Heat over Islands. 251 



tribution of temperature, because in some extensive districts 

 observations are entirely wanting, although made in great num- 

 bers in certain localities. Although these isothermals may thus 

 require some ulterior modifications, I have no reason to doubt 

 their faithfulness in presenting a general view of the distribution 

 of temperature. The influence of distance from the coast, as 

 well as of latitude, is very distinctly visible ; and the arrange- 

 ment of the isothermals evidently conforms to the same laws as 

 those of Ireland. 



The mean temperatures of statioirs where observations have 

 been made have been recorded usually in connexion with certain 

 coordinates of each station, namely, its latitude, longitude, and 

 elevation above the level of the sea. 



While the influence of latitude and longitude is now more 

 justly estimated, that of elevation above the sea is sometimes 

 liable to be overrated. The decrease of temperature with height 

 has generally been calculafed from observations made on the 

 sides and summits of mountains, or by comparison of the thermal 

 conditions of successively overlying portions of the atmosphere 

 during the voyages of balloons. But a much slower rate of 

 decrease must take place in the temperature of the air which 

 touches gradually-rising and widely-spread surfaces, than along 

 nearly vertical lines or rapidly-ascending planes. The manner 

 in which the air becomes heated by contact with the ground is 

 sufficient to indicate the truth of this conclusion. Thus, although 

 some of the best determinations from observations made in bal- 

 loons give a decrease of 1° Fahr. for 276 feet, the decrease of 

 temperature on the sides of mountains has been estimated at 1° 

 m 355 feet, and at 1° in 433 feet on the surfaces of elevated 

 plains*. 



The mean elevation of the greater number even of the inland 

 stations of Great Britain whose temperature has been deter- 

 mined is probably less than 200 feet ; and almost all such 

 stations arc situated on gently undulating or low-lying grounds. 

 The highest station in Ireland is only 230 feet above the level 

 of the sea, and it is situated in the great central plain of the 

 island. 



As the heights of most of the inland and coast stations in 

 Irclaiul have been determined with more than ordinary cai'e by 

 actual levelling, we may estimate with remarkable precision the 

 elevation corresponding to a decrease of temperature of one 

 degree, on the supposition that vertical height above the sea, 

 and not liorizontal distance from its surface, produces the low 

 temperature of the interior as compared witli that of the regions 



* I'Jo metres ami 2.'J5 metres for 1° C. — Ka;mtz, Meleoroloyie, p. 215, 

 French eilitiuu. 



