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page 103 ---Cape
Verde Islands---June 14, 1817---navigation---
The wind, however, drove us with increased rapidity past this delightful
group of islands. In a few days we were in the latitude of Cape Verde.
On the evening of the 14th of June, we descried the Island
of Boa Vista, which appeared like a long and rather low land;
the most southern point of the island bore N. by W. twelve miles. distant.
None of the other islands were visible, the sky being covered the whole
day with grey clouds. In the channel, between the Cape Verde Islands and the continent of Africa,
a thick white fog prevails for the greater part of the year, particularly
near the coast, and probably arises from the combination of the exhalations
from the sea, with the impalpable dust brought by the N.E. wind from the
neighboring sandy desert; besides this, the islands themselves, that lie
scattered about in the quarter, may , perhaps, contribute to collect and
condense the vapours rising from the ocean. Navigators, therefore, seldom
have a pure sky for their observations in this channel; and they now prefer,
on the voyage to the Cape, New Holland, India,
and America,
to steer to the west, in sight of the islands; whereas, earlier voyagers
kept close to the continent.
page 104 ---Tropic
of Cancer ---navigation--June 12, 1817---scientific instruments
and measurements---
Those ships which go through
the channel keep in longitude 19° and 20° W. of Greenwich;
and, during the months in which the sun is in the south, find it
advantageous to hold near the continent, where northerly winds usually
blow. By this means they avoid the shoal call Banco de Porgas,
the existence of which, however, has lately been called in question; as
also, the dangerous reef (query, of coral?) Boneta, which is
said to lie two miles E. by N. of the most northern point of the island
of Boa Vista.
The nearer we approached the Cape
Verde Islands, the more different did the character of the elements
become. Even in the latitude of the Canaries,
we experienced rapid changes in the temperature of the air, and those
sudden distinct gusts and whirlwinds which are here frequently observed.
It was not till about the 11th or 12th of June, in the longitude of 21°
51' W. of Paris,
when we crossed the Tropic
of Cancer, beyond those islands, that the N. and E. wind which
had hitherto alternated, united in a N.E., and afterwards in a N.N.E.
wind, which blew day and night with equal strength towards the equator;
with this steady N.E. wind we made a hundred and fifty miles in four and
twenty hours. We perceived a similar change in the temperature of the
air and water, as in the saltness of the sea, and other natural phenomena.
North of the tropic the temperature of the air changed day and night,
always differing at least one degree from that of the water; but now there
was a smaller difference, and alternately an almost equal decrease
and increase; in the same manner the instrument always indicated a constant
decrease in the saltness of the sea, but so, that here too, the water
taken from some depth was more salt than that at the surface.
 
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