250 



SIR DAVID BREWSTER ON THE 



Under this impression I have been induced to submit to the Society the rest 

 of four years' observations which I made at St Andrews, and which, along with 

 those already published, will exhibit the optical condition of the atmosphere on 

 many days during every month of the year. 



1841, April 28.— Wind west ; fine day. 



Mean Time. 

 3 h p m. Polarisation a maximum in the plane passing through the sun and the 

 zenith, and at 88° 16' from the sun. 



When the sun, or the antisolar point, rose or set, 

 the neutral line of the polariscope bands, held and 

 moved vertically, was a hyperbola, as shown in 



fig. 1. 



■ :vSfr 



1841, April 30. 



Mean Time. 

 2 h 5 ro 



Polarisation a maximum in plane of 

 zenith and sun, and at 78° 25' from 

 sun. 



Horizc 



1841, May 6. 



Fig. 1. 



Mean Time. 

 3 h 30 m 



1841, May 8. 



Mean Time. 

 10 h 10m 



1841, May 9. 



Mean Time. 



12 h NOON. 



Polarisation, when a maximum, greater in plane of zenith and sun than 

 in any other plane. At sunset the difference small. The polarisa- 

 tion was greater in the S. horizon than at the same point in the 

 N. horizon, probably from the sky being there freer from haze. 



Polarisation, or R, = 25|°, and a maximum in plane of zenith and sun. 

 In the N.E., at an altitude of 40°, R = 14£°, and also much less in 

 S.W. horizon. 



Sky greenish blue. In plane of zenith and sun R = 13|°. At 4 h R = 

 24^° and 22|° in different places, and always greatest where the sky 

 was bluest. 



1841, May 11. 



Mean Time. 

 3 h 45 m p.m. R = 24^°, and a maximum in plane of zenith and sun. In other planes, 



R = 22J° 



1841, May 12. 



Mean Time. 



10 h 15 m a.m. The sky blue and unusually clear throughout the day. Barom. 301; 

 Therm., 9 h p.m. 48°. 

 R = 26i° in plane of zenith and sun and a maximum. In other 

 planes, 22i c . 

 11 40 R = 28J° in plane of zenith and sun. 



21^° in lower planes. 



