LIFE IX THE ALPS. 329 



yet the displacement in a single year cannot be much 

 more than a hair's-breadth. Looking abroad, we discern 

 still larger evidences of imperceptible motion. We find 

 rocks cracked across, cavities formed, crevasses gaping, 

 and separated from each other by wrinkled soil; the 

 whole leaving no doubt upon the mind as to the slow 

 motion of the whole surface of this mountain down- 

 wards. And when we examine the outlines of the hills, 

 we find that this sliding down must, in former ages, have 

 occurred on a vast scale. The torrents which furrow 

 the mountains, and which sometimes cut great chasms 

 in their sides, carry, in the long run, both sliding stones 

 and moving earth to lower levels. 



We have now come to our concluding observation. 

 On the 25th of September, 1890, my friend M. E. 

 Sarasin, of Geneva, and myself, had the good fortune 

 to witness a rare and beautiful phenomenon. The sun 

 was sloping to the west, and the valley below us, in 

 which lies the great Aletsch glacier, was filled by a 

 dense fog. Standing in a certain position, with the 

 sun behind us, we saw, swept through the fog in front 

 of us, a grand colourless arch of light. It occupied a 

 position close to that which an ordinary coloured rain- 

 bow might have occupied. Twice in England, and 

 twice only, I have seen this wonderful luminous band 

 the first time, in company with my wife, on the high 

 moorland of Hind Head. The white bow was first de- 

 scribed by the Spanish navigator, d'Ulloa, after whom 

 it is named. Its explanation can only be briefly in- 

 dicated here. Along with the true rainbow, and within 

 it, there are usually produced a number of other bows, 

 by what Dr. Young named the " interference of light." 

 They are called supernumerary bows. When the rain- 

 drops are all of the same size, and exceedingly small, 



