236 



SIR DAVID BREWSTER 



Mean Time. 



1842, May 14. 1^ 35" 



1842, Sept. 29. 4 37 



Very frequently the value of R was 



Rotation in 

 Zenith. 



301° 

 301 

 29 



Rotation in 

 Horizon. 



29° 



The following observations show the changes which take place in the maxi- 

 mum polarisation in a few hours : — 







Mean 



Rotation 



Rotation 







Apparent 



Rotation 



Rotation 







Time. 



in Zenith. 



in Horizon. 







Time. 



in Zenith. 



in Horizon 



1841, May 



12. 



4h 



12"> 



301° 



25° 



1842, Dec. 



24. 



12 



3 



26^ 





J) >> 



)J 



6 







27 



241 



J' S5 



)9 



12 



44 



28 





SJ J9 



»9 



7 



10 



301 



281 



)> !l 



99 



1 



32 



27 



... 



)5 !) 



?> 



7 



35 



301 



29 



>> J> 



99 



2 



24 



29 





>) J5 



5J 



7 



45 



281 



281 



»> >> 



99 



3 



8 



27^ 









Apparent Time 







» )) 



99 



3 



39 



291 





1842, April 



16. 



6 







23 



Much less 



'9 ?> 



27. 



11 



48 



261 



... 



>> » 



J) 



6 



33 



24 



2li 



)1 )) 



99 



12 







231 



161 



)> }) 



)) 



6 



59 



27i 



201 



!) )) 



99 



1 



45 



24 





55 >> 



)! 



7 



23 



291 



241 



)5 ;» 



99 



2 



35 



27i 





5) ;) 



)) 



7 



37 



32 



291 



)> 19 



99 



3 



15 



29 





M )1 



20. 



4 



44 



271 



m 



'» )> 



28. 



11 



38 



29 





JJ )) 



» 



5 



54 



281 





>) )> 



>' 



11 



48 



27^ 



18^ 



)) >) 



<) 



6 



53 



28" 





55 95 



5? 



11 



58 



27 



18^ 



t> )J 



)5 



7 



1 



291 





99 55 



J' 



2 



31 



161! 



Hazy 



)» >> 



5> 



7 



24 



30 





1843, Feb. 



16. 



3 



28 



29i 



241 



„ Sept. 



13. 



5 



58 



291 





55 )! 



95 



4 



3 



291 



261 



5S )) 



JS 



5 



31 



29 





59 99 



55 



5 



4 



30 





3J >J 



)) 



6 



38 



291 

















The great rotation, amounting to 32^" on the 13th April 1842 at 7^ 32"°, the 

 greatest ever observed, was occasioned by an unusually favourable state of the 

 sky. (See pp. 230 and 235.) I consider 30° as 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 polarisation, 

 we have the means of ascertaining approximately the form of the lines of equal 

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



In a paper published in Johnston's Physical Atlas, I have shown how this 

 may be done, and have given two projections 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 Plate XII. and in the Physical Atlas. It will be seen from 

 these projections that the lines of equal polarisation approximate to lemniscates. 

 like the isochromatic lines in biaxal crystals. 



On the Polarisation of Clouds and Exhalations. 

 The polarisation of clouds and other vapours presents some interesting pbe 

 nomena, and should be studied in climates more genial than ours. 



I 



