LABORS OF THE JOINT COMMITTEE. 151 



short. It may be added that this is the cloud which forms 

 the thunder shower in summer, and is known in the books 

 by the name of cumulus. When this cloud rises with its 

 top to a considerable height, it frequently enables the ob- 

 server to ascertain the direction of the upper current of air, 

 for the top of the cloud will lean in the direction of that 

 current. 



120. Our correspondents will be richly rewarded by ob- 

 serving this cloud with great care. It generally begins in 

 clear days to form (when the complement of the dew point 

 is not great) pretty early in the morning, and as generally 

 disappears in the evening; sometimes it appears thin and 

 ragged in its borders, and sometimes its borders are well 

 defined, and its top and sides as white and apparently as 

 dense as snow. The tops of very lofty ones generally lean 

 towards the east or north of east at Philadelphia, because 

 this is the direction in which the upper current moves ; 

 and, from the theory of the trade winds, it is presumed that 

 in the latitude of New Orleans the tops generally lean 

 towards the north, and in lower latitudes towards the north 

 west. When the wind is from the south west they are 

 more likely at Philadelphia to rise higher and lean less. 



121. Sometimes they are observed to swell out above, as 

 if they were blown with a large bellows below ; and occa- 

 sionally, in such times, a thin film of cloud is seen to form 

 in the clear sky above them, at a short distance, which 

 gradually, by the upward motion of the cloud below, is 

 overtaken by that cloud, and then seems to spread over it, 

 as a thin veil over a bank of snow. This thin cloud is 

 formed by the upward motion of the other lifting up the 

 air above it, and cooling it by expansion, and is denomi- 

 nated a cap. When these caps form, it generally rains. 

 Is there any thing in the appearance of these clouds while 

 forming which will indicate whether they will rain or not 

 with absolute certainty? When the top of this cloud rises 

 to a very great perpendicular height, the barometer un- 



