" ‘Bvhpora- 
tion. 
—— 
Objections 
to this 
theory. 
EVAPORATION. 
other.” pits 
lution of water by air, is, that so ai — 
process to go on most ys 
Ct al the very reverse is 
? 
Hit 
. pe 
Hu 
Fes 
i 
uti! 
Hl 
" 
it 
t 
5S 
of water ipi be 
pls 5 po ha 
opposite states of electricity, 
this may 
strata are in 
219 
and thus mix rapidly and thoroughly togethér ; lence 
the reason of the very heavy showers which usually ac- 
company thunder storms, If the temperature of the 
atmosphere where the condensation takes place be suf. 
ficient to freeze the watery particles as they are formed, 
snow is produced ; but if they have time to collect into 
drops before they are frozen, they become hailstones. 
Southerly winds are commonly followed by rain, be- 
cause being warm, and saturated with aqueous vapour, 
they are cooled by coming into a colder climate; while 
northerly winds, by being exactly in an opposite con- 
dition, are dry and parching, and usually attended with 
fair weather. When the atmosphere is completely sa~ 
turated with moisture, and passes from a denser to.a 
rarer state, it suffers a diminution of temperature by its 
increased capacity for caloric, and being unable to hold 
the same quantity of water in solution, it deposits a 
portion of it.. It is owing to the same cause that a mist 
or cloud is formed = exhausting, a receiver with the 
air pump; and in manner may be explained the 
treason why fogs usually cover the tops of hills when a 
current - air, aie with eer ay is driven over 
them. air, which is ly transparent at the 
bottom of the vallies, reat its temperature is suf- 
ficient to make it dissolve completely the moisture with 
which it is charged, becomes opaque in-the more eleva~ 
ted regions, on account of the reduction of temperature 
which it undergoes, partly from change of position in 
Reroute, and partly from rarefaction in conse- 
quence of that change. On the contrary, when the at~- 
mosphere passes from a rarer to a more condensed 
state, instead of iting moisure, its solving power 
is increased ; and it dissolves either wholly or partially, 
the clouds which may have been previously formed. 
This is one reason why the rising and falling of the 
in the barometer become indications of the 
state of the weather. ¢ 
Dr Halley attempted to ascertain the quantity which 
evaporates we the surface of yeaa a oo 
peratures, ing a circular about eight 
inches pe pin Ni ~ 
experi mah onc equate fens of wartan yale i top 
; ees . ‘ 
about half a wine pint ; a square mile 6900 tons ; 
square 
189.) A surface of eight square inches, evaporated by 
air, without exposure to 
wind or sun, in the course of a whole year, 16292 
graino <6. water. alouh Gh cabin inalan, consequent 
ho depth of: mater evupoested in, tips tases sight 
allowed to receive the rain, and to suffer 
from the surface, as in ordinary ci 
register was kept of the water which made its way 
through the soil into the bottles; and a raingage of 
equal surface was placed close. by, for the sake of com- 
The following Table exhibits the results ob- 
Evapora- 
tion. 
_— 
Quantity of 
evaporation 
from the 
surface of 
