of the Atmosphere. 165 



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 polarization of the sky was greatly reduced by the crys- 

 tals of ice floating in the air which produced the halo. 



1843, March 25.— Barom. 2997 in. 



R= maximum polarization. Height of neutral point 

 Apparent time. In zenith. In horizon. above the sun. 



h m , 



5 46 28 . . 18 25 



6 11 29^ .. 17 41 



1843, April 17. — Barom. 29*84 in. Fine day; wind east. 



6 33 .. .. 17 32 



-77 .. .. 18 4 



7 33 .. .. 18 44 



1843, April 29.— Barom. 2963 in. Wind east. The heat of 

 the rising sun very great, and the vapour rising copiously from 

 the ground. Everything more than a quarter of a mile distant 

 invisible. The sun shone occasionally, showing his pale white 

 disk. 



10 20 a.m. The bands were negative from the zenith through the 

 sun to the horizon, and positive to the opposite horizon. 

 The maximum polarization was 90° from the sun, but 

 was so weak that it was compensated by the refrac- 

 tion of a plate of ground glass, at an angle of incidence 

 of about 30°, the negative bands being then scarcely 

 visible. 

 2 55 p.m. The vapour still rising copiously, and the place of the sun 

 not visible. The bands were hardly visible, the polari- 

 zation, when a maximum, being compensated by the 

 refraction of a plate of glass at an angle of 5°. 



1843, May 3. —Barom. 29"65 in. Easterly haur. 



4 25 Mist flying before the sun, and the neutral point oscilla- 

 ting from near the sun to the zenith as the mist thickens. 



1843, June 21. — Barom. 29*75 in. Fine day; wind west. 



7 46 .. .. 15 55 



8 33 29 .. 17 57 



9 14 30 . . 16 28 



1844, February 3. — Snow covering the ground. 



4 11 



26 23 





17 55 



1844, April 15. 



5 54 



6 50 



.. 





25 6 



18 10 



1844, June 10.- 

 8 20 



—Barom. 2970 in. 

 24f 





18 14 





* See Phil. Trans. 1819, i 



). 146. 





