Ch. VI. SOUTH AMERICA. 73 



at fome diftance from it ; fo that thefe vapours are 

 to be coniidered, on one hand, as yielding to the 

 current of the air, and, on the other, as afcending 

 in proportion to the rarefadion they have received 

 from the rays of the fun. Hence it follows, that 

 thefe vapours cannot be thofe v^hich are moll con- 

 denfed, as the difficulty of rifing is always propor- 

 tional to the degree of condenfation *, and at the fame 

 time their magnitude would render them more fuf- 

 ctpcible of the impulfe of the wind. So that thefe 

 conlcquently being the moft fubtiie or tenuious, on 

 having paifed that region, the celerity v/ith which 

 they were before carried upwards is decreafed, and 

 great numbers of them being united from that lofty 

 mift which is feen after the cloud is totally diffipa- 

 ted. This miR cannot be converted into rain ; for 

 having paifed above the region proper for its forma- 

 tion, all the parts become congCHicd, and their 

 weight can never be increafed fufficiently to over- 

 come the refiftance of the air Vv^hich fupports them ; 

 for the quantity of thofe which have overcome this 

 obftacle being inconfiderable, they cannot be united 

 with a fuffirient quantity of others to withftand the 

 continual diffipation occafioncd by the adion of the 

 rays of the fun. Nor can they defcend in either 

 fnow or hail, as might be expefed from their pre- 

 fent ftate. Befides following always, tho' with lefs 

 velocity, the current of the wind, any fuch concre- 

 tion of them as to form a thick cloud is prevented 

 for as we have already obferved, thefe mifts are fo 

 tenuious, as to afford in the day time a confufed view 

 of the dilk of the fun, and of the (lars in the night. 



In order to render the premifes agreeable to ob- 

 fervation, one difficulty ftiU remains, nam.ely, that 

 thole lofty mifts are feen only in winter, and not in 

 fummer. But this, in my opinion, muft naturally 

 be the confequence ; for befides the general reafon 

 that the ftronger influence of the rays of the fun dif- 

 " ■ - perfes 



