1864.] ADDRESS BY SIR CHARLES LYELL. 399 



ing heat of the Sahara to melt the snows of the Appennines and 

 Alps. 



M. Denzler, in a memoir on this subject, observes that the Fohn 

 blew tempestuously at Algiers on the 17th July 1841, and then, 

 crossing the Mediterranean, reached Marseilles in six hours. In 

 five more hours it was at Geneva and the Valais, throwing down 

 a larsre extent of forest in the latter district, while in the cantons 

 of Zurich and the Grisons it suddenly turned the leaves of many 

 trees from green to yellow. In a few hours new mown grass was 

 dried and ready for the haystack; for although, passing over the 

 Alpine snows, the sirocco absorbs much moisture, it is still far be- 

 low the point of saturation when it reaches the sub- Alpine country 

 to the north of the great chain. MM. Escher and Denzler have 

 both of them observed on different occasions that a thickness of one 

 foot of snow has disappeared in four hours durinu the prevalence 

 of this wind. No wonder, therefore, that the Fohn is so much 

 dreaded for the sudden inundations which it sometimes causes. 

 The snow4ine of the Alps was seen by Mr. Irscher, the astrono- 

 mer, from his observatory at Neuchatel, by aid of the telescope, to 

 rise sensibly every day while this wind was blowing. Its influence 

 is by no means confined to the summer season, for in the winter of 

 1852 it visited Zurich at Christmas, and in a few days all the sur- 

 rounding country was stripped of its snow, even in the shadiest 

 places and on the crests of high ridges, I feel the better able to 

 appreciate the power of this wind from having myself witnessed in 

 Sicily, in 1828, its effect in dissolving, in the month of November, 

 the snows which then covered the summit and higher parts of 

 Mount Etna. I had been told that I should be unable to ascend 

 to the top of the highest cone till the following spring ; but in 

 thirty-six hours the hot breath of the sirocco stripped otf from the 

 mountain its whit3 mantle of snow and I ascended without diffi- 

 culty. 



It is well known that the number of days during which particular 

 winds prevail, from year to year, varies considerably. Between 

 the years 1812 and 1820 the Fohn was less felt in Switzerland than 

 usual ; and what was the consequence ? All the glaciers, during 

 those eight or nine years, increased in height, and crept down below 

 their former limits in their respective valleys. Many similar ex- 

 amples might be cited of the sensitiveness of the ice to slight vari- 

 ations of temperature. Captain Godwin-Austen has lately given 

 us a description of the gigantic glaciers of the western Himalaya 



