42 COSMOS. 



an accidental alteration in the therm ometrical scale which, 

 however, was adjusted by Arago in 1817 with his usual care, 

 or whether it indicates an actual increase of heat, is still 

 undecided. The mean temperature of the air at Paris is 

 51-478 F. Bravais is of opinion that the thermometer in 

 the Caves de V Observatoire stands below the limit of invari- 

 able temperature, although Cassini believes that he has founc? 

 a difference of j^ths of a degree (Fahr.) between the winter 

 and summer temperature, the higher temperature being found 

 to prevail in the winter.* If we now take the mean of 

 many observations of the temperature of the soil between 

 the parallels of Zurich (47 2'2') and Upsala (59 51 7 ), we 

 obtain an increase of 1 F. for every 40 feet. Differences 

 of latitude can not produce a difference of more than 1 2 or 

 15 feet, which is not marked by any regular alteration from 

 south to north, because the influence which the latitude un- 

 doubtedly exerts is masked within these narrow limits by 

 the influence of the conductive power of the soil, and by 

 errors of observation. 



As the terrestrial stratum in which we first cease to ob- 

 serve any alteration of temperature through the whole year 

 lies, according to the theory of the distribution of heat, so 

 much the nearer the surface, as the maxima and minima of 

 the mean annual temperature approximate to one another, a 

 consideration of this subject has led my friend Boussingault 

 to the ingenious and convenient method of determining the 

 mean temperature of a place within the tropical regions (es- 

 pecially between 10 degrees north and south of the equator) 

 by observing a thermometer which has been buried 8 or 12 

 inches below the surface of the soil in some well-protected 

 spot. At different hours and different months of the year, 

 as in the experiments of Captain Hall near the coast of the 

 Choco in Tumaco, those at Salaza in Quito, and those of 

 Boussingault in la Vega de Zupia, Marmato, and Anserma 

 Nuevo in the Cauca valley, the temperature scarcely varied 

 one tenth of a degree ; and almost within the same limits it 

 was identical with the mean temperature of the air at those 

 places in which it had been determined by horary observa- 

 tions. It was, moreover, very remarkable that this identity 



corrections by Gay-Lussac for Lavoisier's subterranean thermometer. 

 The mean of three readings, from June till August, was 53 -95 F. for 

 this thermometer, at a time when Gay-Lussac found the temperature 

 to be 53-32, which was therefore a difference of 0'63. 



* Cassini, in the Mem. de VAcad. des Sciences, 178G, p. 511. 



