710 Proceedings of the Royal Irish Academy. 



■with the same height, this effect will be still greater. If the whole 

 plain be elevated up to B, or a table-land of great extent, C L M D, 

 be superimposed, it is evident that the contact of contiguous atmo- 

 spheric exchanging currents will still more affect the temperature at 



B M 



5D 



E F 



B. Hence we must conclude that the temperature, in ascending 

 elevated ground, cannot follow a law independently of the nature of 

 the ground. At night the temperature should be much lower on 

 elevated ground than on low ground, or, in other words, the thermo- 

 metrical range should be greatly increased on mountains and high 

 grounds. This may also be thus illustrated : 



Suppose an elevated plain C D, also F E, by 1000 feet, a ther- 

 mometer at M will be influenced by the causes specified in connexion 

 with general temperature of the entire strata of air 1000 feet at F E. 

 But C D absorbs heat : it heats the strata in contact with and elevates 



M 



AN B E 



the thermometer. Let this be supposed to retain its place, and C D 

 to sink down until it reaches the point A B. A thermometer at N 

 would now indicate the temperature due to all the causes above 

 specified, together with the heating of the strata by the causes referred 

 to, while that at M would be deprived of the latter influences. 



General Baeyer remarks, from observations made on the Brocken, 

 and at Kupferkule, below it, that the variation of temperature between 

 these strata was not uniform : in fact a stratum of air intermediate 

 between both was warmer than either. This occurs from ascending 

 warm currents by day which retain much of their heat in consequence 

 of their small conductivity. [Convexion is less active as the cooler 

 strata above have also a small amount of density ; and thus when the 

 warmed air ascends to a certain height it is less liable to be lowered 

 in temperature by the descent of cooler particles through it from 

 above.] During the night the soil radiates its heat so rapidly as to 

 sensibly cool the air immediately above it ; consequently the greatest 

 differences of temperature between the intermediate stratum and the 

 ground stratum must occur about dawn. "When a mountain top 

 ascends abruptly, the warm stratum it radiates also into space, and 

 the air shows a low temperature all about it. The variation of 

 temperature from the intermediate stratum upwards was greater than 

 from the same stratum downwards. Similar variations of temperature 

 have been noticed by other observers. Arago quotes the results of 



