Barometric Measurement of the Height of Cheviot. 73 



The important consideration of the decrement of heat, ac- 

 cording to the distance from the earth's surface, may now be 

 considered. From various measurements of heights by Messrs 

 Galbraith, Adie, Henderson, &c., it will appear, that, near the 

 surface of the earth, in this country, at about latitude ^^"^ N., 

 Fahrenheit's thermometer falls about 1° for 70 yards of ascent, 

 in heights not exceeding 2000 or tiOOO feet. Thus, the mea- 

 surements of 



Benlomond from Edinburgh gave ..... 243 feet 



Do.. from Rowardinnan (its base) . . . . 205 



Bennevis and its base . . . . . . . 216 



Carnethy and Edinburgh 183 



Cheviot and Holy Island 212 



Mean of the whole 212 



It may, therefore, be concluded, that, to depress the thermo- 

 meter 1°, near the surface of the earth, the ascent of 70 yards, 

 or 210 feet, in round numbers, must, in this climate, be pretty 

 near the truth. It is probable, however, that this does not hold 

 good either towards the equator or poles. 



By analyzing the results recorded in Ramond's work on the 

 barometric formula, it will be found that this increases with the 

 height. In heights of 5000 feet, the elevation necessary to de- 

 press the thermometer 1°, is about from 230 to 250 feet, ob- 

 tained by dividing the height by the difference of temperature*. 

 In fact, it may, a priori, be inferred, that at immense distances 

 in absolute space, from any of the celestial and planetary bodies, 

 where no contiguous body exists to influence the thermometer, 

 that it could be moved considerably in any direction, without 

 any change of its indications. From some late investigations 

 relative to astronomical refractions, similar inferences have been 



made. 



It may therefore be concluded, as well from experiment as 

 general reasoning, that the ascent for 1° increases with the 

 height, though the irregularity of this quantity derived from 

 different observations, so liable to be affected with extraneous 

 causes, renders an investigation of the law of increase difficult 

 and somewhat uncertain. From a consideration of various re- 



• Hence it follows, that at the highest point the depression for 1° must be 

 somewhat more than the mean for the whole, and that near the surface it 

 must be less. 



