There are no direct observations, by which to judge of
the mean temperature of the year in these southern islands.
But after reading the above accounts, it will readily be granted
that it must be very low. Even in Georgia, in lat. 54°-55°,
it is not improbable that the soil is perpetually frozen
at a few feet beneath the surface. At Deception Island in
lat. 62°-63° from the preservation of the dead body alluded
to, and the interstratification of ice with the volcanic ashes,
we may feel almost sure that such must be the case. In the
northern hemisphere, it is only on the great continents that
so low a mean temperature is found in corresponding latitudes.
In North America, according to Richardson,# north
of lat. 56°, the thaw does not penetrate to a greater depth
than tliree feet. In the Steppes of Siberia, Humholdtf
states that to the northward of 62°, the ground between
twelve and fifteen feet below the surface is always frozen.
In the space, however, between these two great northern continents,
the line of perpetual congelation rises considerably
tow'ards the north.
I t is a remarkable meteorological fact, that in the
northern and southern hemispheres, a low mean temperature,
in latitudes without the frigid zone, is the result of a
directly opposite condition of things. In the northern hemisphere
the atmosphere is rendered extremely cold, from the
radiation of a large extent of country during a long winter ; nor
is it moderated by the warmer currents of any neighbouring
sea: hence the extreme cold of the winter more than
counterbalances the heat of summer. In the southern
hemisphere, on the other hand, although the winter is
moderate, the summer is cold ; for a sky constantly clouded
rarely permits the rays of the sun to warm the surface (itself
a bad absorbent) of the great ocean : hence, the mean
temperature of the year falls below the freezing point. ^ It
will at once be evident, that a kind of vegetation which
requires an equable temperature, will approach much nearer
* A p p e n d ix to B a c k ’s E x p e d itio n .
f F ra gm e n s A s ia tiq u e s , vo l. ii., p . 3 86.
r1" ,
..
ft'
!
the line of perpetual congelation in a climate such as this of
the southern hemisphere, than in the opposite one subject to
extremes.
The height of the plane of perpetual snow in any country,
seems chiefiy to be determined by the extreme heat of
summer, rather than by the mean of the year. As the
summer in Tierra del Fuego is so very wretched, we ought
not to feel surprised at the fact stated by Capt. King,
—that in the Strait of Magellan, the line descends to about
3500 or 4000 feet. In the northern hemisphere, we must
travel about fourteen degrees nearer the pole to meet with
so low a limit, namely, between lat. 67° and 7 0 ° on the
mountains of Norway.
In the Cordillera of South America, between latitudes
41° and 43° 30', the culminant peaks have altitudes pretty
nearly equal. Several were measured by the officers of the
Beagle with considerable care, by angles of elevation, the positions
of the mountains being accurately known. Osorno is
7550 feet; mountain south of Osorno 5609; Minchinmadiva
7046; northern end of same range 6862 ; Corcovado 7510;
Yntales 6725. Not only these points, but a great part of
the range* was thickly clothed with snow, in the beginning
of February (answering to our August), which descended
some way down the mountains, and presented to a distant
beholder a perfectly horizontal line. We were assured that
the snow, which it appeared must inevitably be the case,
remained throughout the year. On January 26th, after a
week of uncommonly fine weather, Mr. King measured with
a pocket sextant, the angle of this line with the summit
of the Corcovado; and subtracting the result from the total
height, the snow-line was found to descend to 4480 feet.
It is possible that there may have existed some unknown
cause of error; but as the average height of the few highest
M r. S iiliv an , w h o s u rv e y e d th is p a r t o f C h ilo e , in fo rm s me, tb a t
b e tw e e n O so rn o a n d Y n ta le s , th e r e a r e p ro b ab ly m a n y m o u n ta in s w h ich
ris e to a h e ig h t o f n e a rly 6 0 0 0 f e e t. H e says h e d o e s n o t re c o lle c t an y
o n e sum m it, which (d u r in g J a n u a r y ) was n o t co v e red w ith sn ow.
T 2