5'.

"From an after-investigation, I am led to believe that the cause of
this phenomenon arises from the fact that the trade-winds are dammed
up by the Andes, and that the atmosphere in those parts is, from this
cause, compressed, and, consequently, heavier than it is further from
the mountains, though over a less elevated portion of the earth. The
discovery of this fact has led me to place little reliance in the
indications of the barometer for elevation, at the eastern foot of
the Andes. It is reasonable, however, to suppose that this cause
would no longer operate at Egas, nearly one thousand miles below the
mouth of the Huallaga."

The report of Lieutenant Gibbon, is also exceedingly instructive.
Separating from Lieutenant Herndon at Tarma, upon the Andes, he pursued a
southern course, along the eastern slopes of the chain from 11 deg. 30'
south, almost to 18 deg. south, at Ohuro, making a journey of about 7 deg. 30' of
latitude.

A considerable portion of this journey was over eastern and less elevated
portions of the Andes; but little below, however, the line of perpetual
snow. Here, during the dry season, he met with frequent showers and fogs
from the eastward, but left them as he descended into the plains upon the
table-land. There he found the dry season more distinctly marked; but
occasional irregularities were found upon the table-lands, as every where
upon corresponding elevations. The S. E. trades, however, were there
obvious, during the dry season, notwithstanding the irregularities. The
rainy season, from December to May, he spent at Cochabamba, and at its
close he traveled north down the Madeira and its tributaries, to the
Amazon. Although scarcely consistent with my prescribed limits, I can not
forbear making a few extracts. Thus, when on the mountains, east of
Huanvelica, in the N. E. counter-trade, he says:

"Our course is to the eastward. The snow-capped mountains are in
sight to the west. Temperature of a spring 48 deg.; air 44 deg.. Lightning
flashes all around us; as the wind whirls from _north-east_ to
south-west, rain and snow-flakes become hail, half the size of peas.
Thunder roars and echoes through the mountains; the mules hang their
heads, and travel slowly; the thinly-clad aboriginal walks shivering
as he drives the train ahead; the dark cumulus cloud seems to wrap
itself around us."

Again, at the