LECTURE ON CLIMATE. 
3 
fog, or mist, is first produced, after which, the 
aqueous particles coalescing, form themselves, in 
virtue of the attraction of cohesion, into spherules, 
and fall by their gravity to the earth, producing 
the phenomenon of rain.’’ This account, how- 
ever, is a meagre one, for the reason and manner 
of the condensation is not explained ; 1 prefer, 
therefore, Dr. Hutton’s description. According 
to this, rain depends on the great principle 
“ that the quantity of moisture which air can 
hold, increases in a much faster ratio than its 
temperature .for if we have two equal bulks of 
air saturated with moisture, and of the different 
temperatures of 15 deg., and 45 deg., driven 
together and mixed, the resulting temperature 
would evidently be 30 deg., now the air at 15 
deg. can contain 200 parts of moisture, and that 
at 4 5 deg. 800 : the aggregate being 1 000, or either 
half 500 ; at a temperature of 30 deg., however, 
this portion is only able to contain 400 parts of 
moisture, the remaining 100 parts being precipi- 
tated, forming clouds or descending in rain ; on 
this principle then, modified by previous damp- 
ness, and the fact of complete or incomplete 
mixture, depends the formation of clouds and 
rain : it will therefore be readily understood, how 
in warm weather, with variable wind and squalls, 
large quantities of rain should be precipitated by 
the violent mixture of clouds of unequal tempera- 
ture ; while a continuance of steady wind is ac- 
companied by dry weather. Clouds then, as we 
have just seen, are minute particles of water, con- 
densed from vapour by a lower temperature, and 
which from their minuteness float like fine dust in 
the air ; fogs are clouds resting on water, and 
clouds fogs, suspended in the air. But besides 
the ordinary rain fall, we have frequently during 
the night, and often in considerable quantity, the 
deposit of an insensible rain called dew : this 
being dissipated by the evaporation, which is 
always going on, especially in the warmth of the 
sun. Thus we have a beautiful accord and har- 
mony, on the one hand, vapour rising up in fogs 
and clouds from the surface of oceans and rivers, 
wafted on the land, and descending upon the 
earth to be again returned to the rivers and seas ; 
whilst upon a smaller scale, and in the silence of 
night, we notice the dew descending, and partly 
compensating for the evaporation of the day. 
Now as it is interesting to know the exact pro- 
portion between, and quantity of, the evaporation 
and dew, I have had constructed, by a clever 
maker in Sydney, (A. Tornaghi,) a very simple 
apparatus, to measure the amount of evaporation ; 
it is merely a cylinder containing water, having a 
metallic pointer, which can be raised or depressed 
by a screw above j theupperpart of the pointer being 
provided with a scale, marked to inches and tenths, 
and vernier, exactly like the barometer, but 
read downwards ; the cylinder being almost 
filled with water, the pointer is depressed, 
until its fine extremity touches the surface of the 
water ; the scale is then read : at any subsequent 
time the pointer is again depressed to the sur- | 
face of the water, and the difference, as read on 1 
the scale, gives the amount of evaporation in the 
time between the two readings ; any rain that 
may have fallen being added to the result ; but I 
found, that though this little instrument worked 
perfectly well, yet the result could not be cor- 
rect, as the heavy dew which fell, fell in the cy- 
linder, as on all around, and ought to be, like 
rain, added to the quantity of evaporation. To 
obtain absolute correctness then, I have had a 
small dew guage constructed, by which the quan- 
tity of dew deposited can be readily determined, 
and added to the second reading of the evapora- 
tor ; by these means I find the evaporation of 
forty days and nights to be 3 inches two-tenths; 
whilst the dew deposited during the same time 
has been three-tenths of an inch ; the evapora- 
tion thus being two inches and nine-tenths in 
excess of the dew deposited. 
The formation of nail and snow are wrapped in 
some obscurity : the hypothesis of Volta, re- 
specting the formation of hail being somewhat 
probable, as well as ingenious, is here given : 
premising that hail is invariably seen with and 
during thunder and lightning, and is therefore 
the effect of sudden electrical changes in clouds 
charged with vapour, Volta supposed that two 
clouds of different temperatures, the one charged 
with positive, the other with negative electricity, 
approached one another, until the one was verti- 
cal above the other ; the difference of tempera- 
ture having produced condensation in the upper 
cloud, with sufficient cold to produce ice, small 
hailstones are formed, and fall upon the lower 
cloud, but as in tbe well known experiment, 
where two elder pith balls suspended with silk, 
and charged with opposite kinds of electricity, 
repel each other, so the small hailstones are re- 
elled by the oppositely charged lower cloud 
ack to the upper cloud, their size somewhat in • 
creased by the congelation of additional vapour ; • 
arrived at the upper cloud, they acquire again 
additional size, and are thrown back to the lower 
cloud ; and this is again and again repeated, until 
their weight becomes so great, that they resist 
the electric attraction, and fall by their gravity 
to the earth. 
At a temperature of 32 deg. Fahrenheit water 
under ordinary circumstances freezes, or becomes 
ice; in a perfectly still place, however, 
tbe commencement of freezing is often 
delayed, until the water is somewhat be- 
low ‘ freezing, and on the other hand 
hoar frost is seen, with the thermometer 
several degrees above 32 deg. : as this last fact 
is an important one to gardeners and agricul- 
turists, a short explanation is added, hoar frost 
(or frozen dew), when seen in a surrounding 
temperature above 32 deg., is occasioned by the 
power, which plants and various bodies have, 
of radiating heat ; thus grass and plants gen - 
erally are powerful radiators, stones and earth 
bad radiators, glass a good radiator : and thus 
we see hoar frost on the window panes, and on 
plants, while the bare earth is untouched; for 
these bodies when no longer under the influence 
