The cause of the periodical swellings of the Orinoco acts equally on all the rivers that take rise in the torrid zone. After the vernal equinox, the cessation of the breezes announces the season of rains. The increase of the rivers (which may be considered as natural pluviometers) is in proportion to the quantity of water that falls in the different regions. This quantity, in the centre of the forests of the Upper Orinoco and the Rio Negro, appeared to me to exceed 90 or 100 inches annually. Such of the natives, therefore, as have lived beneath the misty sky of the Esmeralda and the Atabapo, know, without the smallest notion of natural philosophy, what Eudoxus and Eratosthenes knew heretofore,* that the inundations of the great rivers are owing solely to the equatorial rains. (* Strabo lib. 17 page 789. Diod. Sic. lib. l c. 5.) The following is the usual progress of the oscillations of the Orinoco. Immediately after the vernal equinox (the people say on the 25th of March) the commencement of the rising is perceived. It is at first only an inch in twenty-four hours; sometimes the river again sinks in April; it attains its maximum in July; remains at the same level from the end of July till the 25th of August; and then decreases progressively, but more slowly than it increased. It is at its minimum in January and February. In both worlds the rivers of the northern torrid zone attain the greatest height nearly at the same period. The Ganges, the Niger, and the Gambia reach the maximum, like the Orinoco, in the month of August.* (* Nearly forty or fifty days after the summer solstice.) The Nile is two months later, either on account of some local circumstances in the climate of Abyssinia, or of the length of its course, from the country of Berber, or 17.5 degrees of latitude, to the bifurcation of the delta. The Arabian geographers assert that in Sennaar and in Abyssinia the Nile begins to swell in the month of April (nearly as the Orinoco); the rise, however, does not become sensible at Cairo till toward the summer solstice; and the water attains its greatest height at the end of the month of September.* (* Nearly eighty or ninety days after the summer solstice.) The river keeps at the same level till the middle of October; and is at its minimum in April and May, a period when the rivers of Guiana begin to swell anew. It may be seen from this rapid statement, that, notwithstanding the retardation caused by the form of the natural channels, and by local climatic circumstances, the great phenomenon of the oscillations of the rivers of the torrid zone is everywhere the same. In the two zodiacs vulgarly called the Tartar and Chaldean, or Egyptian (in the zodiac which contains the sign of the Rat, an in that which contains those of the Fishes and Aquarius), particular constellations are consecrated to the periodical overflowings of the rivers. Real cycles, divisions of time, have been