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t 
And, laftly, how water, whofe parts appear from the 
asolipile to be capable of immeafurable expanlion, 
Ihould by heat alone become Ipecifically lighter than 
the common atmofphere, without having recourfe to 
a fhell incloling air, or other afhftant machinery ? 
and when raifed, I am perfuaded we fhall find, that 
to fupport them floating, perhaps many days, in the 
atmofphere, is not a knot fo intricate, as to oblige us 
to conjure up a new divinity to unravel it. 
But before we proceed to this fecond part of our 
talk, it will be neceffary previoufly to confider, firft, 
how fmall a degree of heat is required to detach 
or raife the vapour of water from its parent-fluid. 
In the coldeft day, I might fay the coldeft night, 
of winter, when the weather is not frofty or very 
damp, wet linen or paper will become dry in the 
courfe of a few hours. A greater degree of heat 
muff indeed caufe a quicker evaporation. But I am 
perfuaded, that was it not for the preffure of the fu- 
perincumbent fluid, greatly lefs than that of boiling 
water would inftantly difperfe the whole fo heated 
into vapour. 
Secondly, That in the opinion of Sir Ifaac Newton, 
well illuftrated by the late lamented Mr. Melvil, the 
fun-beams appear only to communicate heat to bodies 
by which they are refradted, reflected, or obftrudted ; 
whence, by their impulfe, a reabtion or vibration is 
caufed in the parts of luch impacted bodies. 
This is fupported by the experiment of ap- 
proaching fome light body, or blowing fmoke near 
the focus of the largeft glares ; and from obferving, 
that thefe do not afcend, it is evident the air is not 
fo much as warmed by the paflage of thofe beams 
thro’ it, yet would inftantly calcine or vitrify every 
opake 
