108 M, Ramond*s tnstructiom for the Application of [AuC« 



observer the lesser calculations incident to them, which I have 

 made it my business to supersede ; and all these methods have 

 the inconvenience of requiring the exclusive use of certain mea- 

 sures, or obliging the calculator to go through reductions which 

 greatly lengthen the operations, and multiply the causes of 

 error. The logarithmic method is perfectly independent of dif- 

 ferent systems of measures. The observer must, it is true, burden 

 himself with a set of logarithmic tables, but it is very easy to 

 separate that part of them which contains the series of numbers 

 wanted, and this joined with the tables here given can never be 

 considered a great incumbrance to a traveller who takes the 

 trouble of carrying a barometer to the summits of mountains. 



Isolated Observations. 



With respect to the decrease of temperature as we ascend in 

 the atmosphere, nothing seems certain. Near the surface of the 

 earth, the decrease of temperature is commonly very slow ; some- 

 times however very rapid. The rate of decrease is commonly 

 accelerated at a certain height, and the maximum of accelera- 

 tion is found in a stratum of air whose absolute elevation seems 

 to vary with the chmate. Near the equator, M. de Humboldt 

 has found it between 8,200 and 11,480 feet. In the Pyrenees, 

 I have found it between 6000 and 9000 feet : above this, it pro- 

 ceeds again more slowly ; and this general disposition of circum-» 

 •tances is again modified and disturbed in a thousand ways by 

 the influence of seasons, of situations, of winds, of ascending and 

 descending currents, of the sun, clouds, rain, &c. ; so that when 

 we form an opinion of the rate of decrease from observations at 

 two or three points of a measured scale of elevation, we com- 

 monly find the law defective for all intermediate points. The 

 supposition of an uniform decrease adopted in all our formula? is 

 a mean value which accords with the greater number of cases, 

 and holds, as it were, an even balance between a multitude of 

 opposite results. This supposition is in all respects sufficient to 

 answer the purpose for which it is chiefly wanted, the measure- 

 ment of mountains ; about which the air subjected to the reac- 

 tion of the earth, exhibits effects very different from those pro- 

 duced in its state of absolute independence; and as far as obser- 

 vation has yet gone, this mode of proceeding is justified by the 

 accuracy of our measurements, as well as recommended by its 

 simplicity. 



The extreme irregularity which affects the decrease of tem- 

 perature in the stratum of air next the earth, is one of those 

 obstacles which must always present themselves in any attempt 

 to determine elevations exactly, without corresponding and 

 simultaneous observations of the barometer and thermometer. 

 It may not be useless to make a few remarks on observations of 

 this kind, as more confidence has been reposed in them by some 

 philosophers than they appear to me to deserve. 



