Xll 



INTRODUCTION. 

 2 3 



at the top, assuming, successively, the appearances of 1, 2, 3 r 



Generally called cumuli : or, if the upmoving current should be 

 riven out of its perpendicular motion by an upper current of air, 

 the clouds which might then form would be ragged and irregular, 

 called broken cumuli, as 4. These will always be higher than 

 the base of cumuli, but much lower than cirrus. While the cloud 

 continues to form and swell up above, its base will remain on the 

 same level, for the air below the base has to rise to the same 

 height before it becomes cold enough, by diminished pressure, to 

 begin to condense its vapor into water ; this will cause the base to 

 be flat, even after the cloud has acquired great perpendicular 

 height, and assumed the form of a sugar loaf. Other clouds, also, 

 for many miles around, formed by other ascending columns, will 

 assume similar appearances, and will moreover have their bases 

 all on the same or nearly the same horizontal level ; and the height 

 of these bases from the surface of the earth will be greatest about 

 two o'clock, when the dew point and temperature of the air are 

 the greatest distance apart. 1 The outspreading of the air in the 



1 On some Meteorological Phenomena observed in the Pyrenees, by M. Peytier. 

 The geodisic observations, says M. Peytier, that I have made with M. 

 Hossard in the western part of the chain of the Pyrenees, (from the Garronne 

 as far as Saint Jean de Luz,) during the years 1825, '26, and '27, having 

 placed me under the necessity of encamping on the principal mountains of 

 this part of the chain, I have had occasion to make some observations on 

 several meteorological phenomena of some interest, of which I will give an 

 account. 1st. On the clouds. It is extremely rare that there is not any 

 cloud on the chain of the Pyrenees ; thus, during the summer of 1826, 1 saw 



