of the Atmosphere. 



175 





Mean 



Rotation 



Rotation 





Apparent 



Rotation 



Rotation 





time. 



in zenith. 



in horizon 



. 



time. 



in zenith. 



in horizon 





h m 







o 





h m 



o 







, May 12. 



4 12 



301 



25 



1842, Dec. 24. 



12 3 



26! 





12. 



6 



27 



24! 



24. 



12 44 



28 





12. 



7 10 



30| 



28! 



24. 



1 32 



27 





12. 



7 35 



30* 



29 



24. 



2 24 



29 





12. 



7 45 



28! 



28! 



24. 



3 8 



27! 





Apparent time. 







24. 



3 39 



29! 





>, April 16. 



6 



23 Mud 



27. 



11 48 



26! 





16. 



6 33 



24 



21! 



27. 



12 



23! 



16! 



16. 



6 59 



27\ 



20! 



27. 



1 45 



24 





16. 



7 23 



2.9| 



24! 



27. 



2 35 



27! 





16. 



7 37 



32 



29! 



27. 



3 15 



29 





20. 



4 44 



27! 



17! 



28.. 



11 38 



29 





20. 



5 54 



28! 





28. 



11 48 



27! 



18! 



20. 



6 53 



28 





28. 



11 58 



27 



18! 



20. 



7 l 



29| 





28. 



2 31 



16! 



Hazy 



20. 



7 24 



30 





1843, Feb.16. 



3 28 



29! 



24! 



Sept. 13. 



5 28 



29| 





16. 



4 3 



29! 



26! 



13. 



5 31 



29 





16. 



5 4 



30 





13. 



6 38 



29| 













The great rotation, amounting to 32^° on the 13th of April, 

 1842, at 7 h 32 m , the greatest ever observed, was occasioned 

 by an unusually favourable state of the sky. I consider 30° 

 to be the maximum rotation in a normal state of the sky. 



Having, in a normal state of the atmosphere, fixed the locality 

 of the three neutral points, and determined the place and degree 

 of maximum polarization, we have the means of ascertaining 

 approximately the form of the lines of equal polarization, and 

 of constructing a map of them when the sun is in the horizon. 



In a paper published in Johnston's ' Physical Atlas/ in 1848*, 

 I have shown how this may be done, and have given two projec- 

 tions of these lines — one on a plane passing through the zenith of 

 the observer and perpendicular to the line joining the observer 

 and the sun, and the other on the plane of the horizon. These 

 two projections on a reduced scale, and without any of the numbers 

 upon the larger ones, are given in Piate II. It will be seen 

 from these projections, that the lines of equal polarization ap- 

 proximate to lemniscates, like the isochromatic lines in biaxal 

 crystals. 



On the Polarisation of Clouds and Exhalations. 



The polarization of clouds and other vapours presents some 

 interesting phenomena, and should be studied in climates more 

 genial than ours, 



* Reprinted, without the Piate, in the Philosophical Magazine, Decem- 

 ber 1847, vol. xxxi. p. 444. 



