86 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



the numbers are less than those deduced from astronomical observa- 

 tions, owing probably to our ignorance of the physical data at great 

 heights, particularly of the true law of decrease of temperature with 

 the height in the free atmosphere. For instance, only 57 kilometres 

 are found for the height of the stratum in which the pressure is not 

 more than the tenth of a millimetre. 



