Theory of Fog-Bows. 461 



(12), (13). The suggestion that we have to deal with perfect 

 spheres of ice may be put on one side. In the first place, I 

 have never heard of such being observed ; and, secondly, if 

 such were to exist for a moment, either evaporation or 

 deposition would soon destroy the perfection of form. Nor do 

 I think it probable that such small drops could be raised much 

 above the temperature of the air by the sun's radiation. 

 Indeed I have often seen the air in Davos, when only a few 

 degrees below freezing-point, filled with floating crystals of 

 ice sparkling in the brilliant sunshine. I am thus led to 

 think that these drops on Ben Nevis were originally formed 

 in air above the freezing-point, and, being free both from jars 

 and from contact with foreign bodies, have been cooled far 

 below 32° F. without congelation. Drops of water suspended 

 in oil of the same density have been cooled by Dufour to 

 — 4°F. If this opinion be correct, the drops on striking any 

 object should freeze instantaneously. 



I quote the following from an article in ' Nature ' (April 9, 

 1885), by Mr. Omond : — " In addition to the actual fall of 

 snow, hail, &c, there is on Ben Nevis a form of solid precipita- 

 tion scarcely known on lower ground, but of almost daily 

 occurrence here. In ordinary weather the top of the hill is 

 enveloped in drifting fog, and, when the temperature of the 

 air and ground is below freezing, this fog deposits small 

 crystalline particles of ice on every surface that obstructs its 

 passage. These particles on a wall or large sloping surface . . . 

 combine to form long feathery crystals ; but on a post or 

 similar small body they take a shape more like fir-cones with 

 the point to windward.'''' The deposit only forms on the wind- 

 ward side of each object. Its rate of growth may exceed an 

 inch per hour. As far as I can judge from the above descrip- 

 tion the deposit is just what might be expected from liquid 

 particles below 32° F., drifting with the wind. Jf the fog 

 particles were dry ice, how should they stick together to form 

 the feathers ? I may suggest further, that in the upward de- 

 flexion of the wind by the slopes of the mountain we have an 

 excellent reason for the cooling of the air after the drops were 

 formed. 



Hotel Buol, Davos, 

 April 1890. 



