POLARISATION OF THE ATMOSPHERE. 265 



1842, March 2. — A wet day, the place of the sun being seen as a white spot. 



Apparent Time. 



2 h 20 m The polarisation everywhere extremely feeble. Babinet's neutral point was 

 nearly 75° above the horizon, or about 54° above the sea ! See March 16. 



1842, March 4.— Cloudy and sunshine. 



Apparent Time. 



l h 18 m R = 25i° in zenith, and 22J C at 30° alt. W. horizon. 

 1 48 One plate of glass at 60° incidence compensates the polarisa- 

 tion on the sea horizon opposite the sun. 

 3 h 53 m Arago. 



3 58 11° 13' 

 5 23 12 22 

 5 26 17 55 



Babinet. 

 5 38 R=27|° in zenith, and 22|-° in horizon. 17° 22' 



1842, March 7.— ll h 30 m R = 28J° in zenith, to 18^° in horizon, but at l h 30 m , 

 after showers of hail and rain, R = 22i° in zenith. 



1842. March 10. — Sky clear, and wind in west. 



Apparent Time 



ll h 0" R = 27J° in zenith. 



Neutral point seen below sun. 



R = 28° in zenith, 24 J° in horizon. 



Arago's neutral point not risen. 



The + bands scarcely seen in horizon. 



R = 28° in zenith. 



Arago. 

 12° 55' 

 13 56 



. „_ f The secondary neutral point just touching the sea 1 . . _ 

 1 horizon. J 



1842. March 13.— 12 h 36 m R = 26£° in zenith. Sky clear. 



Apparent Time. Arago. 



4h 27m The breach in the + bands not completed. 12° 15' 



4 39 But at 4 h 39™ the neutral point is formed. 12 6 



Both on the 10th and 13th Arago's neutral point is above the horizon, though 

 masked by the cause which produces the secondary neutral point. Over a space of 

 3£° above the sea horizon, the + bands almost wholly disappear before the — ones 

 are perceptible, and the neutral point is distinct on the sea horizon. 



1842, March 16. — Barom. 29-96, the sun occasionally shining through a 

 thickish haze in a sky without blue. Wind slight in south-west. 



Apparent Time. 



10 h 45 m Polarisation the same as on March 2 ; Babinet's neutral point 

 30° above the sun, or more than 60° above the horizon ! 



1842, March 17.— Barom. 29-77. Much rain last night. Wind west; white 

 clouds flying. 



Apparent Time. 



10 h 20m R = 26i° in zenith plane, and soon after 24£°. 

 10 50 R=20|- in zenith plane, and in a clear sky, over which 

 clouds have passed. 



11 



15 



3 



14 



4 







4 



10 



4 



13 



4 



15 



4 



20 



