24 THE AIR 



very high cirriform clouds, in which ice crystals re- 

 place globules of water. These halos present very 

 striking and beautiful effects, but the more complicated 

 formations of rings are rare. Careful observations 

 should be taken of all such phenomena, not only be- 

 cause they are often the first indication of an approach- 

 ing storm, but because of their intrinsic interest. Thus, 

 for example, a halo when observed should have the 

 angular diameter of the main ring measured by a sex- 

 tant, and the diameters of any fragmentary circles esti- 

 mated by measuring the visible arcs. The height of the 

 sun or moon above the horizon, the hour, and the state 

 of the sky as regards clouds, should all be noted. A 

 sun pillar is also a rare phenomenon ; this is a pillar of 

 light which, as the sun sets, shoots directly up into 

 the sky for a space of from 10 to 15 degrees, remaining 

 stationary for some little time. The light appears to 

 be reflected from the under-surface of ice crystals, as a 

 light may be reflected from a water surface. A very 

 rare and beautiful phenomenon may sometimes be 

 seen at sunrise or at sunset : pink rays springing up on 

 the horizon, opposite the sun, and sometimes reach- 

 ing to the zenith. The phenomenon, known as 

 the green ray at sunset and sunrise, is the brilliant 

 green colour which the line bounding the upper limb 

 of the sun assumes when it is just touching the water 

 horizon. It is curious that, although plainly visible 

 to the naked eye, the old navigators do not appear to 

 have noticed it. The theory of this appearance is 

 fully understood, but it may be used as a test of the 

 observers' vision. 



Amongst the electrical phenomena we have — apart 

 from lightning — the aurorae and St. Elmo's fires. 

 The sometimes radiant appearance of an aurora may 

 indicate a connection between this phenomenon and 



