286 ON THE POLARISATION OF THE ATMOSPHERE. 



1845, January 31— Therm. 18° at 10 h p.m. of the 30th; 8 h a.m., therm. 12°. 

 Barom 2954. rising. Fine frosty, clear day ; ground covered with snow. 



Mean Time. 



12 h 55 m R = 26|° in zenith and in horizon. 



12 55 Arago's neutral point in N, hor. ; sun's alt. then 14° 45'. 



1 40 Arago's neutral point still in horizon. 



1 40 R = 26|-° in zenith. 



1845, February 1.— Barom. 2980. Frosty day; cloudy till 3 h . 



Mean Time. Above Horizon. 



3 h 30 m Alt. Arago's neutral point, 8° 15' 



R=20J° maximum polarisation in zenith to 14|° in hor. 

 3 45 Alt. Arago's neutral point, 14 30 



R=22^° maximum polarisation. 



1845, April 8.— Barom. 2906. 



Mean Time. Above Horizon. 



5 h 35™ Alt. Arago's neutral point, 9° 10' 



6 1 „ „ 20 20 



5 39 Alt. Babinet's neutral point, 25 10 



6 4 „ „ „ 15 25 



1845, April 15.— See " Edinburgh Transactions," vol. xxiii. p. 220. 

 1845, July 14. — On the top of Scuirmore, near Glenquoich. 



Mgan Time. Above Horizon. 



5h 24 m Altitude of Arago's neutral point, 6° 40' 



R = 23|° maximum polarisation in zenith plane. 



1845, September 6. — Barom. 30-10. Fine day; a milky sky. 



R=17° maximum polarisation in zenith plane. 



Above Horizon. 



R = 23° Arago's neutral point. 11° 0' 



Arago's secondary neutral point seen in horizon. 13 24 



Arago's neutral point, 19 10 



Babinet's R = 28^° maximum polarisation, 24° in ( 24 



horizon. \ 20 24 



1850, July 1, 15, 29.— See " Edinburgh Transactions," vol. xxiii. p. 237. 

 1850, July 9. — Barom. 29-79, rising. Fine clear sky. 



Mean Time. 

 6 h 56 m Bands just visible at the land horizon. 



7 5 Bands invisible close to land horizon. 



7 21 During the previous 21 minutes no trace of the + 



bands was seen. At 7 h 21 m they were seen, and 

 became rapidly brighter. 



The positive action which here produced the secondary neutral point was not 

 strong enough to produce it by exhibiting the + bands counteracting the — ones, 

 at some height above the horizon ; but it was strong enough to neutralise them 

 for 21 minutes, and to weaken them greatly when they did appear. 



Mean Time 



l h 



10 m 



5 



31 



5 



41 



6 



36 



5 



43" 



6 



28 



