38't Meteorological Society. 



fixed stars, and on the declination of the latter. After some 

 general remarks on reflection, retraction, and prismatic di- 

 spersion, the author proceeds to consider the subjects just 

 mentioned, in three sections. In the first, " On the variation 

 in the refractive power of the atmosphere at different times of 

 the night and day, and on different occasions and seasons," 

 he ascribes that variation, principally, to the quantity and 

 nature of the aqueous vapour diffused in the air : and he sup- 

 ports this opinion by various observations on the planets and 

 stars, made at different times and seasons. In observing the 

 planets and brightest stars through prismatic glasses, he found 

 that the spectrum was less oblongated, whilst the red colour 

 was more distinctly apparent, at the period of the vapour 

 point, than at almost any other time of the same nights. On 

 other occasions, at the same period of evening, the violet and 

 in general the colours of the most refrangible rays were most 

 conspicuous, and the spectrum was more oblongated than or- 

 dinarily. Dr. Forster at length ascertained, that the greater 

 prevalence of the red in the spectrum uniformly accompanied 

 that state of the atmosphere when the cirrostratus diffused 

 itself after sunset ; whilst the more oblongated spectrum, with 

 the violet and most refrangible colours, attended an atmo- 

 sphere in which the condensing vapours assumed the form of 

 stratus. He infers from these and other observations, that the 

 changes in the qualities of the diffused vapour in the air must 

 produce great variation in the atmospherical refraction. In 

 the second section of his memoir, he suggests that local cir- 

 cumstances may produce great variation in the mean refrac- 

 tive power of the atmosphere at different places ; and that 

 the discordances in the places assigned to the fixed stars in 

 different catalogues of them, may have resulted from such 

 variation. In the third section entitled " Of varieties in the 

 composition and nature of the light of different stars, 

 considered as still further varying the effects of atmospheric 

 refraction, reflection, and dispersion," Dr. Forster details 

 a number of minute observations upon those varieties ; pro- 

 ceeds to inquire into their causes ; and concludes with an ac- 

 count of some experiments on the decomposition of the light 

 of the moon, the planets, and certain fixed stars.* 



A Memoir by Dr. Forster was also read, " On the great 

 Depression of Temperature which occurred in January 1820." 



The remarkable depression of temperature related in this 



* Part of Dr. Forster's paper was inserted in the Phil. Mag. for March ; 

 and the remainder will be found in the present number: both with consi- 

 derable additions by the author. 



paper 



