116 



Refractions of a Lyrae below the Pole. 



To the preceding observed refractions of a Lyras S.P. are 

 annexed the corrections to be applied to the refractions com- 

 puted by the French tables to give the observed refractions. 

 These corrections sufficiently point out the irregularities of 

 refraction at low altitudes. 



The French tables from 74" zenith distance to the horizon 

 may be considered less empirical than any other, since 

 they are deduced from a formula of Laplace assumed so, 

 that, partaking both of the arithmetical and geometrical 

 progressions of variation of density, it gives the diminution 

 of heat observed in ascending in the atmosphere. Gay 

 Lussac having ascended in a balloon to a considerable height 

 found the diminution of temperature nearly as resulted from 

 Laplace's formula. 



