S24 



ing up the ice along the coasts, and that icebergs or sheets of ice 

 loaded with cargoes of boulders, clay, and g-ravel, were driven or 

 floated over the British Islands, whei'e they dropped their cargoes, 

 and, in many instances, stranded on the hill tops. At this period, 

 the author supposes that the relative levels of land and sea were 

 very different from what they are at present, a great part of the Bri- 

 tish Islands being then submerged. This theory, the author stated, 

 was not the result of much reflection or observation, and he merely 

 threw it out for the consideration of geologists. 



3. Contributions to Optical Meteorology. No. I. On the Po- 

 larization of the Light of the Sky. By Professor Forbes. 



The author began tty recapitulating the observations already made 

 known on this subject. 



The facts generally admitted (principally on the authority of M. 

 Arago) appear to be, (1.) That a clear sky reflects light polarized in 

 planes passing through the sun, the eye of the observer, and the point 

 of the sky observed. (2.) That this polarization is a maximum in a 

 zone 90° from the sun. (3.) That in the parts of the sky nearly op- 

 posite to the sun, this description ceases to be accurate ; for the po- 

 larization, in a vertical plane passing through the sun and the ob- 

 Berver, vanishes at an angle with the sun considerably less than 180°, 

 — perhaps 150° or 160° (varying according to circumstances), — and 

 gradually reappears in a plane perpendicular to the former, at greater 

 angles than this. (-4.) That the polarization is more intense in the 

 neighbourhood of the horizon than of the zenith. (5.) M. Babinet 

 lias recently remarked that, under certain circumstances, there is a 

 second neutral point in the neighbourhood of the sun. 



Professor Forbes 1ms verified these facts in nearly every particular, 

 by the aid of a modification of SavoTt's polariscope, constructed of 

 two plates of quartz, peculiarly cut and combined, together with Mr 

 Nicol's single-image calc-spar prism, which the author has substi- 

 tuted with great advantage for the tourmaline commonly used in 

 France. 



With this instrument he finds, (1.) That a uniformly cloudy sky 

 exhibits distinct traces of polarization. (2.) That rain-clouds gene- 

 rally polarize light ; but not, so far as he has observed, those charged 

 with snow. (3.) That the common rainbow entirely vanishes in one 

 position of Nicol's prism (the fact of its polarization was discovered 

 by Biot). (4.) That the polarization of moonlight reflected by the 

 ^ky is very sensible, and likewise the diffuse light, or burr, which 

 surrounds the moon iii cloudy weather. (5.) That the light reflect<"1 



