nrsALCBHious winds. 
407 
clouds, when the catia blows in the valley. This wind is 
dreaded by the inhabitants of Caracas ; it causes headache 
in persons whose nervous system is irritable. In order to 
shun its effects, people sometimes shut them selves up in 
their houses, as they do in Italy when the sirocco is blow- 
J ™.°ught I perceived, during my stay at Caracas, 
that the wind of Catia was purer (a little richer in oxygen) 
than the wind of T etarc. I even imagined that its purity 
might explain its exciting property. The wind of Petare 
coming from the east and south-east, by the eastern extremity 
of the valley of the G-nayra, brings from the mountains and 
the interior of the country, a drier air, which dissipates 
the clouds, and the summit of the Silla rises in all its 
beauty. 
We know that the modifications produced by winds 
in the composition of the air in various places, entirely 
escape our eudiometrical experiments, the most precise of 
which can estimate only as far as '0003° of oxygen. Che- 
mistry does not yet possess any means of distinguishing 
two jars of air, the one filled during the prevalence of the 
sirocco or the catia, and the other before these winds have 
commenced. It appears to me probable, that the singular 
effects of the catia, and of all those currents of air, to 
the influence of which popular opinion attaches so much 
importance, must be looked for rather in the changes 
of humidity and of temperature, than in chemical modi, 
fications. We need not trace miasms to Caracas from 
the unhealthy shore on the coast: it may be easily con- 
ceived that men accustomed to the drier air of the moun- 
tains and the interior, must be disagreeably affected when 
the very humid air of the sea, pressed through the gap 
of Tipe, reaches in an ascending current the high valley 
of Caracas, and, getting cooler by dilatation, and by contact 
with the adjacent strata, deposits a great portion of the 
water it contains. This inconstancy of climate, these some- 
w hat rapid transitions from dry and transparent to humid 
and misty air, are inconveniences which Caracas shares in 
common with the whole temperate region of the tropics — 
with all places situated between four and eight hundred 
toises of elevation, either on table-lands of small extent, 
or on the slope of the Cordilleras, as at Xalapa in Mexico, 
