MEAN TEMPEBATUBE. 
17 
Hence it is probable, that, in the plains of the torrid zone, 
or in the valleys but little elevated, the mean heat of which 
is from 25 5° to 27°, the temperature of the bottom of the 
lakes can never be below 21° or 22°. If in the same zone 
the ocean contain at depths of seven or eight hundred 
fathoms, water the temperature of which is at 7°, that is to 
say, twelve or thirteen degrees colder than the maximum 
of the heat* of the equinoctial atmosphere over the sea, 
I think it must be considered as a direct proof of a sub- 
marine current, carrying the waters of the pole towards the 
equator. We will not here solve the delicate problem, as 
to the manner in which, within the tropics and in the tem- 
perate zone, (for example, in the Caribbean Sea and in the 
lakes of Switzerland,) these inferior strata of water, cooled 
to 4° or 7°, act upon the temperature of the stony strata of 
the globe which they cover ; and how these same strata, the 
primitive temperature of which is, within the tropics, 27°, 
and at the lake of Geneva 10°, react upon the half-frozen 
waters at the bottom of the lakes, and of the equinoctial 
ocean. These questions arc of the highest importance, both 
with regard to the economy of animals that live habitually 
at the bottom of fresh and salt waters, and to the theory 
of the distribution of heat in lands surrounded by vast and 
deep seas. 
The lake of Valencia is full of islands, which embellish 
the scenery by the picturesque form of their rocks, and the 
beauty of the vegetation with which they are covered: 
an advantage which this tropical lake possesses over those 
of the Alps. The islands are fifteen in number, distributed 
m three groups ;t without reckoning Morro and Cabrera, 
which are already joined to the shore. They are partly 
It is almost superfluous to observe that I am considering here only 
that part of the atmosphere lying on the ocean between 10° north and 10° 
south latitude. Towards the northern limits of the torrid zone, in latitude 
3 , whither the north winds bring with an extreme rapidity the cold air 
° Canada, the thermometer falls at sea as low as 16°, and even lower. 
T The position of these islands is as follows : northward, near the 
* lore, the Isla de Cura ; on the south-east, Burro, Horno, Otama, Sorro, 
^aiguira, Nuevos Peftones, or the Aparecidos ; on the north-west, Cabo 
<’ ^ n £ 0> 0! ^ ves * a °d Chamber# ; on the south-west, Brucha and 
u ebra. In the centre of the lake rise, like shoals or small detached rock*, 
a gre, Fraile, Peftasco, and Pan de Azucar. 
VOL. II. 
O 
