THE 

 LONDON AND EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE, 



SUPPLEMENT to VOL. XV. THIRD SERIES. 



LXXV. The Baherian Lecture.— On the Theory ofthe Astro- 

 nomical Refractions. By James Ivory, iC.H., M.A.^ F.R.S, 

 L. 4" -E., Instit, Reg. Sc. Paris, Corresp. et Reg. Sc. Gottin, 

 Corresp. 



[Continued from p. 395.] 



10. TPHE equation (C.) supposes that the atmosphere con- 

 -■- sists entirely of dry air : we have next to consider 

 what modification must be made when it contains a portion 

 of aqueous vapour. 



In the first place, when p' and r', the pressure and tem- 

 perature at the surface of the earth, are given, as they are in 

 the mean atmosphere which produces the refractions, the 

 quantity «, or the refractive power of the air, is not liable to 

 be altered by any possible mixture of aqueous vapour. For 

 if an addition of vapour to dry air diminish the refractive 

 power by making the density less, the greater action of the 

 vapour upon light is found almost exactly to compensate the 

 defect. Laplace first made this observation ; which has been 

 confirmed by MM. Biot and Arago, who have established by 

 experiments, that the refractive power of air, whether dry or 

 mixed with vapour, is the same, when the pressure and tem- 

 perature are the same. It thus appears that, as far as the 

 quantity a, or the refractive power of the air at the earth's 

 surface, is concerned, the astronomical refractions are inde- 

 pendent on the hygrometric condition of the atmosphere. 



But a mixture of vapour may produce changes in the ex- 

 pression of the refraction, by altering the coefficients or the 

 integrals. Now, if we attend to the formulas that have been 

 found for an atmosphere of moist air, and in the equation 

 (10.) make the same substitution as in the case of dry air, viz. 



= 07, 



Phil, Mag, S. 3, Vol. Iw^. Supplement, No, 99. 1839. 2 K 



^6^ 



