1880.] 



Biot and others on horizontal refraction. 



2.9 



It is to be regretted that the observations on stars rising or 

 setting near the prime vertical, made by Argelander at Konigsberg 

 in 1820, 1 ; and referred to in the conclusion of the preface to the 

 Tabuhe Regiomontance are not in print : though there is no reason 

 to think that they would furnish anything excejjtional. I know of 

 no others that would be of value in determining whether under 

 any circumstances horizontal refraction usually sinks to so low an 

 amount as it seems to have done in the opinion of the French 

 observers ; or as there seems reason to think, from my own obser- 

 vations of last year, it will sometimes do under very peculiar 

 circumstances. On that occasion it must be remembered that 

 the rays of the Sun were passing through strata of atmosphere 

 lying over a probably much colder portion of the sea than that near 

 my own position. A horizontal line drawn at any point on the 

 earth's surface will be at an elevation of 1000 yards at a distance of 

 about 70 miles, and for 2000 yards at about 100 miles distance: 

 but by the laws of refraction the track of any ray at either of these 

 distances must have been much lower than this : and though I feel 

 myself unable to suggest what its elevation at various distances 

 would have been, I am sure that the distance would have been so 

 great for no very high elevation as to bring it sensibly under 

 the direct influence of the Polar Ice. And although the natural 

 effect of cold is to increase refraction, the circumstances may have 

 been sufficiently abnormal to produce the effects observed. 



With these remarks, I feel justified in leaving the question 

 with such of my readers as may feel interested in it. 



(2) On the problem of two pulsating spheres in a fluid (Part 

 II.). By W. M. Hicks, M.A., Fellow of St John's College. 



In a paper read before the Society in the Michaelmas term of 

 last year,* I shewed how the force between two pulsating spheres 

 might be determined, by means of mass-images, and I calculated 



* Proceedings, Vol. m. pp. 276 — 285. 



