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page xli:-
kinds of weather, though the resistance of similar winds
from neighbouring hills may prevent its being taken notice
of.
It may be remarked of this wind, that it generally blows
from Cross-fell longest in the Spring, when the sun has
somewhat warmed the air beneath, and does not cease till it
has effectually cooled it: thus it sometimes continues for a
fortnight or three weeks, which I consider as a peculiarity
of the Helm-Wind of Cross-Fell. As such therefore, though I
have not leisure to attempt the discussion of its various
phaenomena, such as that of the bar, &c. I hope I shall
not do a thing unpleasant to the philosophic reader, if I
attempt to give an account of its general cause, which I
shall do on the following simple principles.
The chymists know, that evaporation produces cold, and that
of AEther is remarkable enough. To those who have not seen
their method, or heard of it, I shall mention that of the
brewers, who throw quick-lime into their large cauldrons
when they want to cool them suddenly; and the effect, I
apprehend, depends upon similar principles, though exhibited
in a different manner: any man, however, may have a cheap
proof of the cold produced by evaporation, by plunging his
hand into water, and holding it up to the sun or wind, or
both, at any season, to dry. Even waving all these as causes
of the cold to be expected from vapours, the fogs hovering
upon rivers are a sensible proof that the thing is so; which
any one must have experienced still more forcibly, who has
met with a rainy cloud, or a mere fog, upon the mountains in
warm weather, and will have felt that such a cloud, even at
a distance, can obtrude a piercing cold upon the warmest day
in Summer. Now, whether it is by attraction, (which I think
it is,) or any thing else, that hills detain clouds which
wander in the region of their summits, belongs not to my
purpose, since it is a thing known universally to exist
wherever there are hills: and Cross-fell, probably from its
vast length, height, and loneliness, is peculiarly powerful
in detaining them; for it intercepts many, and there is no
hill near whose attraction, or effects upon the winds,
&c. may serve to remove them: thus they gather not into
that shadowy indiscriminate mass of vapours which settle on
hills before and during rains, but such as I have described.
It will be easily conceived, from what is premised, that
such an assembly of vapours will produce a proportional
degree of cold around it; whereas the height of any hills in
Britain, without such a cause, or a brisk wind, is not
sufficient to produce any great degree of it. I have felt a
suffocating warmth on the tops of some of them in a calm
day, even when drifts of snow still remained, and were far
beneath me. Such cold, therefore, must occasion a
corresponding density in that portion of air on which it
operates; and when the sun, by that more-continued action of
his rays which belongs to valleys, has warmed the air in the
country beneath in the day, or left it so in the evening,
that air must be proportionally rarefied; and becoming
lighter than that in a higher region, or at least lighter
with respect to its situation, the equilibrium which fluids
wish to preserve will consequently be interrupted: the
higher air will thenceforward rush down to keep up this
equilibrium, and in its course occasion the wind of which we
are speaking.
I believe this account will correspond with all the
circumstances attending the Helm-Wind of Cross-Fell. I think
it necessary to mention again, that it blows until it has
entirely cooled the air of the adjoining country, (for
itself is very chill,) and is almost always terminated by a
rain, which restores, or to which succeeds a general warmth,
and into which the Helm seems to resolve itself.
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