C >27 3 
take up your time in fo doing ; for if they carry ah 
appearance of truth, I need not make an apology for 
them ; if they appear falfe, they cannot injure thofe 
opinions, which they contradidl; and then I have 
but one apology to make, which is, to acquiefce, 
and lincerely alk your pardon for having troubled 
you with them. 
It is agreed, that the afcent of vapour and exha- 
lation through the air may be effedted two waysj 
by impulfe, and an alteration of their fpecific gra- 
vity. 
That vapour does not generally afcend by impulfe, 
may be proved by many familiar experiments, 'uiz. 
Put boiling water into a veffel, and then empty it, 
and hold the veifel with the apperture downwards : 
the vapour, which is afterwards expelled from the 
veffel, muft be in a diredtion downward j but we find, 
that as foon as it has got but a very little below the 
rim of the veifel, it has its diredtion altered, and 
afeends by the laws of fpecific gravity. The fame 
thing may be obferved in all boiling vefl'els, where 
the vapour is emitted in a diredtion downward ; or, 
in cold weather, when the vapour of a man’s breath 
may be feen, let him breath downward, and the 
diredtion of his breath will be prefently altered, as in 
the former cafe. Since then vapour does afcend 
without any other impulfe than that, which is inci- 
dent on all bodies afeending by the laws of fpecific 
gravity 5 it is necelfary to inquire, how the fpecific 
gravity of vapour is altered to caufe its afcent. This 
is generally fuppofed to be done by filling veficles of 
water with rarified air, until the diameter of the 
veficle be ten or more times the diameter of a drop 
of 
I 
