Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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These Naturalists Found It 88.7
Degrees, The Barometer At Nineteen Inches One Line.
In the kingdom
of New Grenada, at the chapel of Guadaloupe, near Santa-Fe de
Bogota, I have seen
Water boil at 89.9 degrees, under a pressure of
19 inches 1.9 lines, At Tambores, in the province of Popayan, Senor
Caldas found the heat of boiling water 89.5 degrees, the barometer
being at 18 inches 11.6 lines. These results might lead us to
suspect, that, in the experiment of M. Lamanon, the water had not
reached the maximum of its temperature.
Day was beginning to dawn when we left the ice-cavern. We observed,
during the twilight, a phenomenon which is not unusual on high
mountains, but which the position of the volcano we were scaling
rendered very striking. A layer of white and fleecy clouds
concealed from us the sight of the ocean, and the lower region of
the island. This layer did not appear above 800 toises high; the
clouds were so uniformly spread, and kept so perfect a level, that
they wore the appearance of a vast plain covered with snow. The
colossal pyramid of the peak, the volcanic summits of Lancerota, of
Forteventura, and the isle of Palma, were like rocks amidst this
vast sea of vapours, and their black tints were in fine contrast
with the whiteness of the clouds.
While we were climbing over the broken lavas of the Malpays, we
perceived a very curious optical phenomenon, which lasted eight
minutes. We thought we saw on the east side small rockets thrown
into the air. Luminous points, about seven or eight degrees above
the horizon, appeared first to move in a vertical direction; but
their motion was gradually changed into a horizontal oscillation.
Our fellow-travellers, our guides even, were astonished at this
phenomenon, without our having made any remark on it to them. We
thought, at first sight, that these luminous points, which floated
in the air, indicated some new eruption of the great volcano of
Lancerota; for we recollected that Bouguer and La Condamine, in
scaling the volcano of Pichincha, were witnesses of the eruption of
Cotopaxi. But the illusion soon ceased, and we found that the
luminous points were the images of several stars magnified by the
vapours. These images remained motionless at intervals, they then
seemed to rise perpendicularly, descended sideways, and returned to
the point whence they had departed. This motion lasted one or two
seconds. Though we had no exact means of measuring the extent of
the lateral shifting, we did not the less distinctly observe the
path of the luminous point. It did not appear double from an effect
of mirage, and left no trace of light behind. Bringing, with the
telescope of a small sextant by Troughton, the stars into contact
with the lofty summit of a mountain in Lancerota, I observed that
the oscillation was constantly directed towards the same point,
that is to say, towards that part of the horizon where the disk of
the sun was to appear; and that, making allowance for the motion of
the star in its declination, the image returned always to the same
place. These appearances of lateral refraction ceased long before
daylight rendered the stars quite invisible. I have faithfully
related what we saw during the twilight, without undertaking to
explain this extraordinary phenomenon, of which I published an
account in Baron Zach's Astronomical Journal, twelve years ago. The
motion of the vesicular vapours, caused by the rising of the sun;
the mingling of several layers of air, the temperature and density
of which were very different, no doubt contributed to produce an
apparent movement of the stars in the horizontal direction. We see
something similar in the strong undulations of the solar disk, when
it cuts the horizon; but these undulations seldom exceed twenty
seconds, while the lateral motion of the stars, observed at the
peak, at more than 1800 toises, was easily distinguished by the
naked eye, and seemed to exceed all that we have thought it
possible to consider hitherto as the effect of the refraction of
the light of the stars. On the top of the Andes, at Antisana, I
observed the sun-rise, and passed the whole night at the height of
2100 toises, without noting any appearance resembling this
phenomenon.
I was anxious to make an exact observation of the instant of
sun-rising at an elevation so considerable as that we had reached
on the peak of Teneriffe. No traveller, furnished with instruments,
had as yet taken such an observation. I had a telescope and a
chronometer, which I knew to be exceedingly correct. In the part
where the sun was to appear the horizon was free from vapour. We
perceived the upper limb at 4 hours 48 minutes 55 seconds apparent
time, and what is very remarkable, the first luminous point of the
disk appeared immediately in contact with the limit of the horizon,
consequently we saw the true horizon; that is to say, a part of the
sea farther distant than 43 leagues. It is proved by calculation
that, under the same parallel in the plain, the rising would have
begun at 5 hours 1 minute 50.4 seconds, or 11 minutes 51.3 seconds
later than at the height of the peak. The difference observed was
12 minutes 55 seconds, which arose no doubt from the uncertainty of
the refraction for a zenith distance, of which observations are
wanting.
We were surprised at the extreme slowness with which the lower limb
of the sun seemed to detach itself from the horizon. This limb was
not visible till 4 hours 56 minutes 56 seconds. The disc of the
sun, much flattened, was well defined; during the ascent there was
neither double image nor lengthening of the lower limb. The
duration of the sun's rising being triple that which we might have
expected in this latitude, we must suppose that a fog-bank, very
uniformly extended, concealed the true horizon, and followed the
sun in its ascent.
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