62 
in these regions, and they consequently enjoy a much milder 
temperature. The winds proceeding from the north, being as 
it were confined within the great valley formed by the inter- 
ruption of the grand chain of mountains between Tarifa and 
Petapa, descend with considerable force through the gaps of 
Tarifa and Chivela. The Venta de Chicapa, situated opposite 
the former, receives these currents at times v.ith great force 
and they are also often felt by travellers on arriving in front 
of the Portillo de la Chivela. 
The climate of the elevated section of this part of the Isth- 
mus is so diiferent from that of the plains, that when the 
thermometer st£mds in the latter place at 30°, it scarcely rises 
to 13" at Chivela or Tarifa. In all the heights surroimding 
these estates, we find the pine ocote, the presence of which 
clearly indicates a very temperate climate. This considerable 
difference of tenjperature appears much more strange con- 
sidering that the absolute elevation of Tarifa is not more than 
650 feet, whilst its latitude is less than 17". Our surprise 
ceases, however, when it is remembered that Tarifa partici- 
pates in the temperature of the Sierra, which, near to it, 
reaches suddenly an elevation of more than 7,500 feet. 
The summits of, the Sierra Madre are almost constantly 
enveloped by the clouds, which, coming from the Atlantic 
there discharge themselves, a circumstance that accounts for 
the rivers of the Isthmus having an almost constant body of 
water during the greater part of the year. At Guichicovi 
and Santa Maria Chimalapa it rains almost incessantly, and 
often also at San Miguel. Whenever such is the case the 
table-land of Tarifa is covered with a kind of thin mist, which 
disappears upon reaching the Portillo where the reigning wind 
becomes more powerful. This phenomenon may be con- 
sidered permanent, the suspended mist rarely extending be- 
yond the table-land of Tarifti. Thus the Venta enjoys an 
almost constant unclouded atmosphere, and Kiin seldom falls 
in the plains. 
It cannot fail to be observed, that among the advantages 
offered by the isthmus of Tchunntepcc for the execution of 
the proposed work, that of a mild and healthy climate, pre- 
