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XXXIV. Memoir on the Variations of the reflective, refractive 

 and dispersivePowers oftheAtmosphere,Sfc. By T. Forster, 

 M.B. F.L.S. Member of the Astronomical and the Metem-o- 

 logical Societies of London, and Corresponding Member of 

 the Academy of Natural Sciences at Philadelphia, fyc* 



T N the infancy of every science, the first great object to be 

 -* achieved is the accumulation of phaenomena appertaining 

 thereto. In proportion as we are enabled to lay out these in 

 their natural order and arrangement, in the same proportion 

 we advance the science ; and where we are enabled to unravel 

 the particular causes of each, we promote a knowledge of it 

 in the highest degree. 



In the following observations it will be found that I have 

 been enabled by observation and reflection to collect a con- 

 siderable number of facts, and to place them in some sort of 

 arrangement. And though much progress has not yet been 

 made on the development of their precise causes, yet such as 

 has been already known has been arranged in a way which is 

 likely to facilitate and direct the future inquiries of more able 

 and industrious speculators than myself; and it is with a view 

 of thus engaging the cooperation of the many intelligent 

 members which compose the Meteorological Society, that I 

 have ventured to obtrude the ensuing crude observations on 

 their notice at this early stage of our investigations. 



The refractive power of the atmosphere has been long well 

 known, and corrective tables of mean refraction intended 

 to be applied to astronomical observations have been em- 

 ployed for many years. In my endeavour, therefore, to be 

 brief, in order not to become obscure, I shall avoid as much 

 as possible the reiteration of observations already before the 

 public, as the reader may refer immediately to Bradley's 

 tables, and to various works on refraction. But though the 

 refraction of light by the intervention of the atmosphere is 

 well known, that part of the subject which more immediately 

 belongs to meteorologists, has been particularly neglected; 

 I mean the Variation in the Atmosphere's reflective, refractive 

 and dispersive powers, resulting from the diffusion therein of 

 different modifications of cloud at different times and places. 

 And the various effects which this aforesaid variation produces, 

 considered as a fluctuation in the prismatic propeiiy of the air 

 through which the modified light of the sun, moon and stars se- 

 verally is transmitted to the inhabitants of our globe. 



The object of this paper is therefore to show the liability of the 



* Read before the Meteorological Society of London in February and 

 March 1824, and published by permission. 



atmosphere 



