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Firft, The vapours paffing the tropics into colder 
regions, have their furrounding fire condenfed by de- 
grees j which muft increafe their fpecific gravity, and 
leffen their repulfive power j by which means they 
muft both defcend, and approach each other, till at 
laft they form denfe vifible clouds ^ and thefe clouds 
are alfo accumulated by other fucceeding vapours, of 
like fpecific gravity, till they form clouds, which are 
often feveral hundred yards in depth ; which I have 
often feen, in paffing through them up the fides of 
very high mountains. In clouds of fuch depth, I 
think, the coalition of their particles, to form drops, 
may arife from their motion, and the order of fpe- 
cific gravity. For the air being a great deal denfer, 
and of greater fpecific gravity,, at the bottom of fuch 
a cloud than it is at the topj and the particles of 
vapour, with their fire, being pretty near an equal 
fpecific gravity j thofe particles,, which are below, 
muft neceffarily be forced upwards, and thofe above 
muft as neceflarily defcend, till they all meet at that 
altitude, whereat their fpecific gravity would make 
them float, if they were not prevented merely by 
their repulfive power : but this repulfive power muft 
be greatly adtcd upon, if not quite overcome, in 
fome places by this prelfure of the air and of the 
vapours. But fuppofing the particles brought into a 
very near approach by this preflure, it will be fcarcely 
poffible for them, upon any motion of the clouds, 
to keep their ftated diftances without impinging up- 
on one another.. And whenever this happens, it is 
eafy to ffiew how rain is formed. For where-ever 
two or more particles are forced to- approach within: 
their repulfive powers, they will run into contadl and 
fpliericity 
