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VIII. On the Constant of Refraction determined hy Observations with the Mural 

 Circle of the Armagh Observatory. By the Rev. T. R. Robinson, D. D., 

 Member of the Royal Irish Academy , and other Philosophical Societies. 



Read 11th January, 1841. 



IT may, perhaps, appear presumptuous in me to approach a subject which has 

 already occupied so many of the greatest masters of mathematical science, and 

 in the opinion of many is exhausted. But if we look without prejudice at the 

 labours of Laplace, Bessel, Ivory, and Plana, besides many others of less renown, 

 and carry our examination a little beyond the mere analytical work, we shall 

 find that the problem of astronomical refraction has not been rigorously solved 

 by theory, and I am even inclined to think never can be. All it appears to me 

 that theory can be expected to perform, is the supplying astronomers with ready 

 means of approximating to tables of refraction, which shall satisfy their observa- 

 tions ; and on the other hand they are bound to remember, that such tables, 

 however carefully verified for one observatory, may be defective when tried at 

 another. 



For in fact it is universally assumed in these investigations, that the atmos- 

 phere is arranged, with the surfaces of equal density spherical and concentric to 

 the earth ; this gives the differential of refraction in function of the density and 

 distance from the centre. Now, firstly, this fundamental hypothesis is not even 

 approximately true. Near the earth, the surfaces of equal temperature (and 

 therefore of equal density) must depend on the figure of the ground ; the air 

 over a hill must be very differently circumstanced in respect of heat, from that 

 at the same height over a deep valley. Forests, large bodies of water, and the 

 vicinity of cities must exert a similar disturbing influence, and that to an extent 

 which cannot be neglected. In a set of hourly observations, made some years 

 since on the altitude of my meridian mark, I found an increase of refraction, 



VOL. XIX. 2 A 



