INTRODUCTION. Xlll 



upper parts of an ascending column will form an annulus all 

 round the cloud, under which the barometer will stand above the 

 mean; of course the air will sink downwards from its greater 

 weight in the annulus, and increase the velocity of the wind at the 

 surface of the earth, towards the centre of the ascending column, 

 while all round on the outside of the annulus there will be a gen- 

 tle wind outwards. Any general currents of air, which may ex- 

 ist at the time, will of course modify these motions, from the ob- 

 lique forces they would occasion. The upmoving current of air 



only four days without any clouds : the 12th of May, the Ibth of June, the 

 30th of July, and the 7th of August. 



It is in the morning, at the rising of the sun, that the mountains are seen 

 most frequently without clouds, but it is excessively rare that they are not 

 covered before the middle of the day. 



When the mountains are seen in the morning, we observe generally at one, 

 two, or three hours, more or less, after the rising of the sun, when the heat 

 begins to be felt, there form in the plains at the foot of the chain, some little 

 clouds, which rise gradually and reach the mountains. If we were placed 

 on a high mountain, we would see these little clouds form and rise sometimes 

 as rapidly as rockets. They group themselves in the mountains, where they 

 frequently form tempests; at other times, they rise slowly, assemble in a 

 mass, on the same level, and form a stratum more or less thick and more 

 or less elevated, that covers the plain and resembles a sea of white vapors. 



This stratum, thus formed, rises gradually during the day, (sometimes more 

 than 1000 metres,) and lowers in the evening and in the night. Often, this 

 stratum of clouds rising in the day and lowering at night, remains thus for 

 several days in succession. Very often the clouds dissolve in the night and 

 form again in the day some time after the rising of the sun. 



These clouds, when they rise in the mountains, generally follow the direc- 

 tion of the valleys, though the wind has not that direction. 



We remarked often in the high valleys some clouds leaning against the 

 two sides of the mountains, whilst the heavens were seen above from the 

 middle of the valley, between the two bands of clouds. 



We see again, frequently, a cap of clouds on some elevated peaks, when 

 there are not any on the mass of the chain. Often the clouds cover all one 

 side of the chain, whilst the other side is without any clouds ; and we re- 

 marked that the side toward France is more often covered than that toward 

 Spain. 



These clouds are sometimes seen to ride on the summit of the chain. This 

 happens, when pushed by the wind, the clouds attain the top of the chain, 

 where their weight causes them to fall back to the other side. 



We remarked also, sometimes, two strata of clouds moving in different 

 directions. There is then some probability of a change of weather. 



When there are two strata of clouds plainly shown, the upper is generally 

 higher than the summit of the chain ; it rarely touches the peaks. The dif- 

 ference of the level between the two strata is often very considerable. 



These clouds are not level in the rainy weather ; they are generally low 

 in time of rain, and much more elevated in storms. 



The composition of these clouds does not always appear to be the same; 

 sometimes they are light and transparent, sometimes they are thick and not 

 transparent, and sometimes they are dry, and at other times very humid. I 

 have seen the rain producing the rainbow ; this is when a part of the cloud 

 resolves itself into excessively fine rain. 



The clouds which produce these haloes, appear very elevated, very light, 

 and very transparent. 



C 



