CHAP. 2. 18. STRUCTURE OF CLOUDS, &c. 95 



cumuli, which were flying along in the wind, 

 have by any accidental inosculation of the 

 clouds, or from any other cause, changed to 

 cumulostratus : the change has been uniformly 

 attended with a retardation of the motion of 

 the cloud. This probably arises from its having 

 been increased in density in proportion to the 

 surface presented to the wind. 



SECTION XVIII. 



Of the Structure and Buoyancy of Clouds. 



THE first step towards a perfect knowledge 

 of any science, is to have an accurate and well 

 arranged detail of particular appearances. From 

 effects thus laid out in order, we proceed to 

 examine what may have been their causes. It 

 may be proper now to examine, whether the 

 particles of clouds remain afloat in the air, or 

 only gravitate very slowly to the ground ? In 

 other words, on what peculiarity of structure 

 does their comparative levity depend ? Expe- 

 rience being deficient, conjecture supplies its 

 place, and supposes an adequate cause. 



M. de Luc, and M. de Saussure, have sup- 



