53 



In the middle of the afternoon of Nov. 29th last year, the 

 sky being apparently very clear, I went to my transit in- 

 strument to ascertain the time from the meridian passage 

 of the star Alpha Aquil?e. But imagine my surprise when 

 the star utterly failed to appear in what seemed a clear blue 

 sky. I carefully examined all the adjustments of the in- 

 strument and my reckonings, and found all quite correct; 

 and my clock is never moi'e than a very few seconds in 

 error. As the sky appeared unusually blue I stepped out of 

 the observatory to see the whole sky, and to ascertain, if 

 possible, the cause of the non-appearance of the star. I 

 then found that over and in the neighbourhood of my meri- 

 dian the sky was almost an indigo blue, and immediately 

 from it to the south west it was tiuo;ed with ""reen. As the 

 sun went down clouds gathered on the horizon, the sky 

 vapours became moi'e visible, and there could be distinctly 

 traced from behind the clouds where the sun had set up to 

 the zenith all the principal prismatic colours. This was the 

 finest display of all the colourings that have recently ap- 

 peared in the sk}^ I naturally attributed the blue and 

 green to the presence of an unusual amount of the vapour 

 of water in the higher regions of the sky. Of course we 

 are all aware what other causes have been assigned for this 

 unusual phenomenon. 



If these colours arose from the refraction of the vapour 

 of water in the air, two things were requisite ; first, that 

 this vapour must, during these appearances, have been pre- 

 sent at a very unusual height in the atmosphere; and, 

 second, there must have been an excessive quantity present 

 relatively near the earth. That such was the case may ap- 

 pear from what has been shown in my paper " On the 

 Effects of Solar Radiation in Atmospheric "Vapour." That 

 all, or at any rate, the higher prismatic colours might be 

 visible, it was requisite that this vapour should extend to a 

 great height, and it is shown that this then was possible. 



