Personal Narrative of Travels to the Equinoctial Regions of America, During the Year 1799-1804 — Volume 2 — Background and Themes

Granite Dikes and the Work of Water

The narrative repeatedly returns to the behavior of water on granite. At the cataracts, Humboldt observes that the pongos—rocky funnels worn by the current—are enlarged by the perpetual motion of quartz pebbles and sand. He contrasts these with the pongos of the Amazon, which are cut into conglomerate or red sandstone and are therefore more easily destroyed. One such pongo at Rentama collapsed eighty years before his visit, leaving the riverbed dry for hours.

The authors note that the Indians of Atures dispute the claim that the rocks of the raudal change their aspect; instead, they say, the partial torrents shift their course, carrying more or less water to one bank or the other. Humboldt speculates that the causes of these changes may lie far upstream, for in rivers, as in arteries, all movements propagate to great distances. This analogy between hydrology and physiology is characteristic of his explanatory style.

The Nocturnal Amplification of Sound

One of the most striking passages in the volume concerns the noise of the Great Cataracts. Humboldt reports that the sound is three times as loud by night as by day, a phenomenon that had already struck the local missionaries and Indians. He methodically reviews possible explanations: the velocity of sound decreases with temperature; intensity diminishes in agitated air and at higher altitudes; carbonic acid gas conducts sound more feebly than a mixture of azote and oxygen. None of these facts, he admits, satisfactorily explains the observation.

He then considers a hypothesis based on masking: during the day, the hum of insects, the song of birds, and the rustling of leaves create a uniform background noise that reduces the perceived intensity of the cataract. But this reasoning fails in the Orinoco forests, where the hum of insects is louder by night than by day. Humboldt finally suggests that the sun's uneven heating of the soil creates currents of air of different density, which interfere with the propagation of sound. The passage is a model of scientific candor—he presents a puzzle, tests several hypotheses, and leaves the question open.

Bifurcations and the Intertwining of Rivers

The volume's table of contents announces a preoccupation with river systems: the intertwining and bifurcations of the Apure and Arauca, the junction of the Apure with the Orinoco, and the famous Cassiquiare canal that connects the Orinoco to the Rio Negro. Humboldt treats these waterways not as static features but as dynamic networks whose branches shift over time. He notes that the partial torrents into which the great river divides itself at the cataracts change their direction, carrying sometimes more, sometimes less water toward one bank.

This attention to bifurcation extends to the human geography: missions, villages, and portages are situated at these nodes. The narrative moves from the mission of Atures to the portage from the Tuamini to the Rio Negro, always alert to the ways in which water both connects and separates. The reader is invited to see the landscape as a system of anastomosing channels, where a change upstream can alter the course of a river leagues away.

Readers approaching this volume should come prepared for a narrative that moves slowly, pausing to measure a torta or to debate the cause of a sound. The rewards lie in the precision of the observations and the honesty of the reasoning. Humboldt does not claim to have resolved every puzzle; he invites the reader to share in the process of inquiry. The journey is as much intellectual as geographical, and the landscape is rendered with a clarity that makes the reader feel the spray of the cataract and the hum of the insects.