126 M. H. de Saussure on the Humming Sound 



tain was something disquieting. This phenomenon lasted some 

 five or six minutes ; soon the rain and the thunder recommenced 

 anew. When we had reached the upper limits of the forests 

 the storm became more endurable, though there fell one of 

 those diluvial rains characteristic of the hot season within the 

 tropics. M. F. Craveri, an Italian physician established at 

 Mexico, who had made the ascent of the Nevado de Toluca before 

 me at the commencement of the rainy season, told me that he had 

 been witness of the same facts, which he remembered with terror. 

 The electrical phenomena were yet more violent. On the 19th 

 of May, 1845, that traveller ascended the Nevado de Toluca by 

 the south-east side, starting from Tenango, and descended by 

 the north-west slope on Toluca. The south-west side of the 

 mountain was free from snow at that season. 



The electrical phenomenon was brought on suddenly by a 

 cloud coming from the west, and which perhaps had been gene- 

 rated on the snow-fields of that slope. Scarcely were the tra- 

 vellers enveloped in it ere they felt the sensation electricity 

 produces ; and this was almost immediately followed by a dull 

 sound. They felt at all their extremities, at the fingers, nose, 

 and ears, confused electrical currents. The fear which seized 

 them, then in the midst of those lofty solitudes, made them im- 

 mediately commence the descent with hasty steps. The thun- 

 der did not yet growl ; but at the end of five minutes there fell a 

 snow like rice, and, the cloud communicating its electricity to 

 the soil, there arose from it the same sound that I have before 

 described. This sound was very loud, and appeared general over 

 the mountain. The long hair of the Indians became stiff and 

 erect, giving an enormous size to the head of these people. The 

 sight of this phenomenon added to the panic of those who had 

 expected pleasure from this expedition. The so singular sound 

 which is heard in the rocks of mountains at the moment of the 

 electrical phenomenon deserves to be studied by competent phy- 

 sicists. It resembles the taps which little pebbles produce when 

 they knock together as they are alternately attracted and repelled 

 by electricity. But it seems certain to me that it arises from a 

 kind of crepitation or crackling of the electricity which escapes 

 from the rough points of the rocky soil. 



A third observation of the same kind is due to M. Craveri, 

 who was surprised by the same kind of storm near the summit of 

 Popocatepetl, on the 15th of September 1855, with this differ- 

 ence, that, the scene taking place on snow-fields, the sound of 

 crackling of the soil w^as not produced. 



Here are facts bearing on the point that have come to my 

 knowledge : — 



In 1767 H. B. de Saussure visited the summit of the Brevent 



