of the Atmosphere. 293 



were made at the beginning of dawn and at the end of twilight, 

 with the view of determining the height of the atmosphere. 

 From the observations made at sea he obtained 320, and from 

 the observations made on land 340 kilometres, or 212 miles, as 

 the height of the atmosphere *, 



The most recent observations on the polarization of the atmo- 

 sphere were made by M. Andres Poey, between 1862 and 1864, 

 under the tropical sky of the Havannah. The observations them- 

 selves have not been published; but he states, as one of the 

 most important of their results, that " at sunrise and sunset the 

 system of atmospherical polarization ought necessarily to present 

 two planes of rectangular polarization — one vertical, passing 

 through the eye of the observer and the sun, and the other hori- 

 zontal, with four inversions of the signs, and four neutral points 

 90° from each other." 



M. Poey adopts my theory of atmospherical polarization, and 

 the analogy which I pointed out between the lines of equal po- 

 larization and the isochromatic lines of biaxal crystals, and be- 

 tween the same lines and those of uniaxal crystals when the 

 sun is in the zenith, the neutral points now meeting the sunf. 



It will be seen from the preceding details that the subject of 

 atmospherical polarization has become one of the most important 

 branches of optical meteorology. It has already thrown much 

 light on the constitution of the atmosphere; and when it has 

 been studied in different climates and at different altitudes above 

 the sea by Alpine travellers and scientific aeronauts, it will doubt- 

 less have still more valuable applications. 



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. 



18-11, April 28.— Wind west; fine day. 



Mean time. 



3 h p.m. Polarization a maximum in the plane passing through 

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



k Fig. 1. 4 



When the sun, or the anti- &£-. ■$&? 



solar point, rose or set, the ■ ' ■: Jj|f 



neutral line of the polariscope ^W^ & h a i g + ''• ''I 

 bands, held and moved ver- %^ ,..,- I [ I I I,,.v0f\^ 



tically, was an hyperbola, as V>"'- - ' J..-,L*s*& 



shown in fig. 1. 



MI 



horizon 



* Comptes Rendus, &c. vol. xlviii. pp. 109-112. 

 t Ibid. vol. lx. p. 781, April 17, 1865. 



