refractive and dispersive Powers of the Atmosphere, S,c. 197 



that pure atmospheric air will vary gradually in its refrac- 

 tive powers according to the degree of heat; and remark- 

 able instances of the great elevation of the disk of the sun 

 above the horizon in cold climates, when calculation showed 

 him to be below it, have been adduced as striking exam- 

 ples. But it should be remembered by all, and it is already 

 known to meteorologists, that changes in temperature are 

 never unattended with changes in the quantity and modification 

 of aqueous vapours suspended in the air. And this circum- 

 stance must add considerably to the effect of low temperature, 

 and may impart to an atmosphere condensed by cold some great 

 and peculiar dispersive properties. The inordinate elevation 

 of the mast of ships at sea by refraction is an instance and 

 proof of the undoubted great effect of vapours occasionally 

 suspended in the air. The east wind is connected with the 

 arrival of peculiar vapours in it, and the vibration or irregular 

 movement of the star in the telescope may be owing to such 

 vapour being agitated by the electrical commotions that so 

 generally attend a change of wind from any other quarter to 

 one blowing from the east. 



Besides the general want of correspondence between the re- 

 sults of different observations, where it is necessary to subtract 

 a given quantity of refraction, we may make a particular ob- 

 servation on the still greater disagreement that is found to 

 exist in the declination of several particular stars, as published 

 by different observers ; for this disagreement, when applied 

 to certain individual stars, is referable to a compound cause; 

 for the light of the stars severally being composed of different 

 proportions of the primitive colours, the red ones, as Alde- 

 baran, Arcturus, and Betalgens, are less refrangible than Sirius, 

 Lyra, Capella, and the white stars in general. Of this I shall 

 treat in the third section. It may suffice to remark it slightly 

 here, in order that, when we are considering the varieties in 

 the atmosphere's dispersive property, we may also take into 

 account the compound effect produced by the various colours 

 of the stars whose light is thus variously dispersed. 



The projection of Aldebaran and other red stars on the 

 disk of the moon when the conjunction happens near to her 

 upper limb, as noticed by Mr. Lee*, is an illustration of the 

 above fact ; the greater proportionate refrangibility of her 

 white light, than that of the red light of the star, elevating her 

 apparent disk so as to cause the star to seem to be within it 

 just before or after the respective points of contact. 



When the wind first changes to become east, and when the 



• I wish my readers to peruse the whole of Mr. Lee's ingenious paper in 



Phil. Trans. ; for though it becomes necessary to cite it, in order to make 



due acknowledgements, nevertheless every individual Essay should be read 



< ntirc, and not in citations. air 



