226 



SIR DAVID BREWSTER 



1842, November 14. — Barom 296. Fine frosty and clear morning. 



Apparent Time. 

 8^ SSO'A.M. 



R=:Maximum Polarisntion. Height of Neutral Point above 



In Zenith. In Horizon. the Antisolar Point. 



9510 



191° 



14° 7' 



1842, November 21. — Barom. 2977. Frosty morning. Thermometer 31' 



Apparent Time. 

 9^ 4™ A.M. 



Height of Neutral Point above 

 the Aritisoiar Point. 



18 36' 



1842, December 28.— Barom. 29-56. 



Apparent Time. 

 11'^ 40°' A.M. 



12 3 ., 



1 4 „ 



1 23 P.M. 



2 31 „ 



R=Maximum Polarisation. Height of Neutral Point above 



In Horizon. the Antisolar Point. 



In Zenith 



29° 

 27 



14^^ 



isr 



13° 41' 

 13 37 

 25 25! 

 27 30! 

 27 48! 



These remarkable results were doubtless owing to the causes which produced 

 the following phenomenon : — At I'' 4"*, when the neutral point started away from 

 the sun, a wiiite halo of 45^ appeared round the sun, and continued till 2^ SI"". 

 It was slightly brown on its inner rim. At l*" 23" the altitude of the halo was 

 32° 10', and the sun's altitude 8"^ 30', so that the radius of its outer rim was 

 23° 40'. 



When the vertical bands of the polariscope passed over the apex of the 

 halo, tkeii' intensity was increased, and when they passed over the halo in a 

 direction parallel to its horizontal diameter their intensity was diminished. As 

 the crystals of ice, by which the halo was produced, are doubly refracting, one 

 of the pencils must have been weaker than the other, an effect which would be 

 produced if the surfaces of the crystals were not perfectly smooth.* 



The polarisation of the sky was greatly reduced by the crystals of ice floating 

 in the air, which produced the halo. 



1843, March 25.— Barom. 29 97. 



Apparent Time. 

 5h 46ra 

 6 11 



R= Maximum Polarisation. 

 In Zenith. In Horizon. 



28" 

 29| 



Height of Neutral Point above 

 the Antisolar Point. 



18= 

 17 



25' 

 41 



1843, April 17. — Barom. 2984. Fine day ; wind east. 



Apparent Time. 



Q^ 33™ 

 7 7 

 7 33 



Height of Neutral Point above 

 the Antisolar Point. 



17° 



18 



18 



32' 



4 



44 



1843, April 29. — Barom. 29*63. Wind east. The heat of the rising sun very 

 great, and the vapour rising copiously from the ground. Everything more than 



* See Phil. Trans., 1819, p. 146. 



