4 
LECTURE ON CLIMATE. 
of the sun, radiate, or throw off, their heat, and 
soon sink below 32 deg. in a temperature 
several degrees above it ; on these cold surfaces 
the dew descends, and is frozen, producing 
those needles and crystals of ice, with which 
we are familiar. It was iaid just now, that hoar 
frost was frozen dew ; this however is not in- 
variably true, for besides parting with their 
heat, plants throw out moisture from their own 
bodies which may become frozen, this is often 
the case with buds of trees in early spring, and 
blight ensues ; the obvious plan to pursue 
where practicable, to save plants or flowers 
from frost, is to cover them over, which checks 
the power of radiation and protects their cold 
surface from the dew. 1 have now, as an im- 
portant element in the matter of which I am 
treating, to speak of winds ; and first for a 
definition, what is wind? and next to seek the 
cause — how are winds produced ? Winds are 
sensible currents in the atmosphere, propagated 
either by compression or by rarefaction ; de- 
veloped directly in or inversely to the direc- 
tion in which they blow, by the one or other 
condition of origin. The general causes are 
stated as follows 1. The ascent of the air 
over certain tracts heated by the sun. 2. 
Evaporation causing an actual increase in the 
volume of the atmosphere. 3. Rain, snow, &c. 
causing an actual decrease in its volume by the 
destruction of the vapour. It is easy to under- 
stand that if a portion of water be removed 
from a reservoir, the surrounding water will 
flow in and restore the equilibrium ; or if by 
force we impel some of this fluid in a certain 
direction, an equal quantity will move away in 
a contrary direction ; and also if a portion of 
the fluid. rarefied by heat or condensed by cold, 
ascends in the one instance, and in the other 
descends, a counter current being the natural 
and visible consequence, so with the invisible 
atmospheric air similar effects are found to 
follow the same causes. With this knowledge 
then, we view the wind, not as the 
“ fickle breeze,” but as the atmospheric 
ocean, obeying proved and well defined laws. 
The trade winds, and the land and sea breeze, 
particularly demand some attention ; it has 
already been stated, th'at the largest quantity of 
rain precipitated, is in tropical countries, where 
there are heat, squalls, and variable winds; The 
sudden conversion then of large quantities of 
vapour into fluid, and its precipitation on the 
earth, as well as the rarification of other portions 
by the heat of the sun, and its ascent, cause a 
local vacnum, and the adjacent air rushes in to 
re-establish the equilibrium ; as an illustration of 
the magnitude of this vacuum, Dr. Lardner in- 
stances a fall of in ter tropical rain, of an inch in 
depth, and extending over a hundred square 
leagues ; and states, that the vapour from which 
such a quantity of rain would be produced by 
condensation, would, at a temperature only of 50 
degrees, occupy a volume 100,000 times greater 
that that of the fluid ; tho extent of the vacuum 
being, a volume of 200 cubic miles, or a column 
whose base is a square mile, and height 200 miles. 
But the cold air rushing in from either pole, to 
fill this vacuum, would be due north, and south, 
whereas the trade winds are N.E., and S.E. ; 
th's easting is caused by the diurnal rotation of 
the earth, which is in a direction from VV. to E. ; 
the particles of air being as it were, left behind, 
and so acquiring an easterly direction. A des- 
cription of the trade wind is so simply and well 
given by Lieut. Maury, an American, that I 
shall add it, merely stating first, that about the 
equator, and also at the edge of the tropics, the 
opposing winds meet, neutralize each other, and 
form calm regions, and called the Calm Belts of 
the Equator, of Cancer, and of Capricorn. 
Queensland is situated about the calm 
belt of Capricorn. And therefore with- 
out the influence of the regular trades. 
Maury says “From the parallel of about 
30 degrees north and south, nearly to the 
equator, we have extending entirely around the 
earth, two zones of perpetual wind, viz., the zone 
of north -east trades, on this side, and of south- 
east on that ; with slight interruptions they 
blow perpetually, and are as steady and as con- 
stant as the currents of the Mississipi River ; 
always moving in the same direction, except when 
they are turned aside by a desert here and there, 
to blow as monsoons, or as land and sea 
breezes. As these two main currents of 
air, are constantly flowing from the poles 
towards the equator, we are safe in assuming, 
that the air which they keep in motion, must 
return by some channel to the place towards the 
poles, whence it came in order to supply the 
trades, if this were not so, these winds would 
soon exhaust the polar regions of atmosphere, 
and pile it up about the equator, and then cease 
to blow, for the want of air to make wind of. 
This return current, therefore, must be in the 
upper regions of the atmosphere, at least until it 
passes over those parallels, between which the 
trade winds are always blowing on the surface. 
The return current must also move in the direction 
opposite to that wind, the place of which it is 
intended to supply. These direct and counter- 
currents are also made to move in a sort of spiral 
or etoxodronfic curve, turning to the west as they 
go from the poles to the equator, and in the op- 
posite direction as they move from the equator 
towards the poles. This turning is caused by 
the rotation of the earth on its axis. The earth 
we know moves from west to east, now if w^ 
imagine a particle of atmosphere at the north 
pole, where it is at rest, to be put in motion in a 
straight line towards the equator, we can easily 
see how this particle of air, coming from the 
very axis of diurnal rotation, where it did not 
partake , of the diurnal motion of the earth, 
would, in consequence of its vis inertice , find, 
as it travels south, the earth slipping from under 
it, as it were, and thus it would appear to be 
coming from the north-east, and going towards 
the south-west ; in other words it would be a 
