366 



Astronomical and Nautical Collectiom. 



obtained a modification of the law of uniform temperature, that 

 will give the refractions, to within about three degrees of the 

 horizon, with great exactness. Dr. Young has, by " adopting" 

 a law of variation of temperature advanced by Professor Leslie, 

 obtained an equation for refraction, the solution of which 

 gives the refractions with considerable exactness as far as the 

 horizon. 



The following method is derived from the formula, obtained 

 in the hypothesis of a density decreasing uniformly. However 

 great, within the usual limits, we may suppose the change of 

 density at the surface to be, there is no reason to suppose a 

 material change in the law of density in the atmosphere. 

 (Note. This reasoning may be fallacious, and it appears to be 

 very desirable, that the facts should be ascertained by a suf- 

 ficient number of observations, at given zenith distances near 

 the horizon.) At present we have not sufficient observations to 

 determine, whether the actual variations of refractions at low 

 altitudes are most conformable to the theory of Mr. Bessel, to 

 that of Dr. Young, or to that above given. 



TABLE L 



