14 PERIODS OF INCREASE AND DECREASE. 
line, between two great basins of rivers, that is crossed by 
the equator. The river Amazon, according to the informa- 
tion which I obtained on its banks, is much less regidar 
in the periods of its oscillations than the Orinoco ; it 
generally begins, however, to increase in December, and 
attains its maximum of height in March.* It sinks from 
the month of May, and is at its minimum of height in 
the months of July and August, at the time when the 
Lower Orinoco inundates all the surrounding land. As no 
river of America can cross the equator from south to north, 
on account of the general configuration of the ground, the 
risings of the Orinoco have an influence on the Amazon; 
but those of the Amazon do not alter the progress of the 
oscillations of the Orinoco. It results from these data, that 
in the two basins of the Amazon and the Orinoco, the con- 
cave and convex summits of the curve of progressive increase 
and decrease correspond very regularly with each other, 
since they exhibit the difference of six months, which results 
from the situation of the rivers in opposite hemispheres. 
The commencement of the risings only is less tardy in the 
Orinoco. This river increases sensibly as soon as the sun 
has crossed the equator; in the Amazon, on the contrary, 
the risings do not commence till two months after tl'ie 
equinox. It is known that in the forests north of the line 
the rains are earlier than in the less woody plains of the 
southern torrid zone. To this local cause is joined another, 
which acts perhaps equally on the tardy swellings of the 
Nile. The Amazon receives a great part of its waters from 
the Cordillera of the Andes, where the seasons, as every- 
where among mountains, follow a peculiar type, most fre- 
quently opposite to that of the low regions. 
The law of the increase and decrease of the Orinoco is 
more difficult to determine with respect to space, or to the 
magnitude of the oscillations, than with regard to time, or 
the period of the maxima and minima. Having been able 
to measure but imperfectly the risings of the river, I report, 
not without hesitation, estimates that differ much from each 
other.t Foreign pilots admit nmety feet for the ordinary 
* Nearly seventy or eighty days after our winter solstice, which is the 
■ummer solstice of the southern hemisphere, 
t Tuckey, Maritime Geoijr., vol. iv, p. 309. Hippistey, Eitped. it 
