260 Prof. Bischof on the Temperature of 



The means of determining the decrease of temperature with 

 the height, are : 



1st, Observations on the temperature of the air. 

 2d, Observations on the temperature of springs ; and, 

 Sd, Direct observations on the temperature of the soil. 

 The next question to be asked is, Whether the decrease of 

 temperature in the air, and that of the soil, in proportion to 

 the height, follow the same progression ? 



If, in general, according to Chap. VI, there is no reason for 

 supposing a difference to exist between the mean temperature 

 of the air and that of the soil, this question is already answered. 

 In this case the determination of the mean temperature of the 

 air or of the soil at various heights, would be sufficient to solve 

 the problem. That between the tropics there is no difference 

 between the mean temperature of the air and that of the soil, 

 we have seen in Chap. VI. In the temperate zone so few 

 observations have been made, that the means of comparison are 

 wanting. 



From all that has been said it follows, that between the tro- 

 pics, the decrease of temperature in the air may be found with 

 a tolerable degree of exactness, by single corresponding obser- 

 vations, made at different heights. The most accurate results, 

 however, are only to be got from the yearly mean. Von Hum- 

 boldt* takes the mean of all his observations between the tro- 

 pics at 654) feet difference of height for 2o.25 difference of tem- 

 perature, for tlie stratum of air comprised between the surface 

 of the sea and 3000 feet above it. As this philosopher made 

 most of his observations in the day-time, it may be assumed, 

 as we have said above, that they have yielded a somewhat too 

 rapid decrease of temperature. 



Let us compare this result with Boussingault's observations 

 on the decrease of the temperature of the soil in the Cordilleras, 

 between 11° north and 5° south latitude. This philosopher 

 ascertained the temperature of the soil at no less than 1 28 places, 

 at all heights between the surface of the sea and the limits of 

 perennial snow, after the manner already described (Chap. VI). 



" Reisen, ii. p. 492. Formerly Von Humboldt supposed it to be 726.6 ft. 

 Gilbert's Annal. xxiv. 48. 



