497 



stances which fell under my own observation when in Switzerland in 

 1867. 



When the Fohn rushes into the central and northern Alpine valleys 

 it has generally been observed as a dry, warm wind, and it is this 

 peculiar dryness which first suggested its desert origin. This dryness 

 of the air in the valleys is, however, by no means universal ; and it is 

 always accompanied by falls of rain and snow on the mountains, sub- 

 sequently followed by moist precipitations in the valleys themselves. 

 Hence it follows, that the Fohn, when first impinging on the mass of 

 the Alps, is not a dry, but a moist wind. The first portions of the aerial 

 current of which it is composed, having been stripped of their moisture, by 

 condensation and precipitation among the higher summits and ridges, 

 descend in a dry state into the Alpine valleys. Together with its 

 moisture, it has lost heat when expanding in traversing the summits of 

 the mountains ; but on descending to the lower valleys, it is again com- 

 pressed, and thus gives out sensible heat, and becomes known as a warm 

 as well as a dry wind. This explanation of its physical characters 

 seems to be generally admitted among the meterologists who have most 

 carefully studied the phenomenon. The next question that arises is, 

 whether it is probable that warm currents of air derived from the 

 Sahara, or from some other southerly source, would follow the precise 

 direction required for sweeping over the Alps. The condition of the 

 Sahara as an originator of warm aerial currents is one essentially con- 

 nected with the diurnal fluctuation of temperature.* By day its surface 

 acquires intense heat, the greater part of which it loses by radiation 

 during the night. "We should thus expect a priori, that its disturbing 

 influence on the atmosphere should be fluctuating and violent, rather 

 than extensive, such as might accompany a more constant source of heat. 

 Moreover, the column of heated air rising upwards from the heated soil 

 of the Sahara, and tending to flow meridionally northwards, owing to 

 the earth's rotation, would be gradually deflected, and move towards 

 the north-east, so as to blow towards Turkey, Asia Minor, and the 

 Black Sea, rather than Switzerland. 



The general direction of the Fohn points to an origin west of the 

 Sahara, in the Atlantic Ocean, off the coast of Morocco. This part of the 

 Atlantic is traversed by a branch of the great north-western equatorial 

 thermal current, of which another branch strikes our shores. The 

 bifurcation occurs south of the Azores, and one portion proceeds towards 

 the north-west coast of Africa and south-west coast of Europe. In order 

 to determine whether the Fohn originates from the Atlantic or the 

 Sahara, a careful tracing out of its passage over intermediate countries must 

 be made; and this has been in a great measure effected by Professor Wild, 

 of St. Petersburg ; Professor Dufour, of Lausanne ; Dr. Hann, and other 

 metereologists. M. Wild concludes that every time an equatorial 



f * See " On the Temperature of the Lower Regions of the Earth's Atmosphere," 

 "Transactions," vol. xxiv., pp. 402, 408. 



