Is a Ship Canal Practicable? / Notes, Historical and Statistical, Upon the Projected Routes for an Interoceanic Ship Canal Between the Atlantic and Pacific Oceans, in Which is Included a Short Account of the Character and Influence of the Canal of Suez, and the Probable Effects Upon the Commerce of the World of the Two Canals, Regarded Either as Rivals, or as Parts of One System of Interoceanic Navigation

by S. T. Abert

BAROMETER.

Notwithstanding the improved formulæ, and more careful method of observation recommended by Lieut.-Col. Williamson, Corps Engineers, the barometer is subject to peculiar and anomalous variations, along the slopes of the Cordillera of the Isthmus and the Andes. Lieuts. Gibbon and Herndon refer to this phenomenon. Lieut. Maury attributed the effect to the damming or piling up of the trade-winds against the mountains. A recent traveler in the valley of the Amazon, I. Orton, observed the same phenomenon, but objects to Maury’s theory.

Sr. Moro makes the following observations: “If, under these circumstances (prevalent winds), barometrical observations are made simultaneously on both sides of the Sierra, on the side of the Gulf, they will exhibit a lower elevation than the true one, the error being greater as that station may happen to be lower down or more towards the north; but if time should admit of waiting until the weather be equally fine on both sides (which seldom happens), then the difference between the levels of the barometrical columns is insensible.”

Ventosa is peculiarly windy, and Nicaragua is subject to the Popagayos, a species of monsoon, upon the Pacific coast. But the more placid climate of the Atrato is similarly affected. “It is known as an established fact,” remarks Capt. Kennish, “that the clouds seldom pass over the Cordillera toward the Pacific, but are attracted by the mountains, and disgorge themselves on the Atlantic side; hence the reason of the perpetual rain, thunder, and lightning in the Atrato Valley, while on the Pacific coast there is scarcely any rain for eight months of the year.”

This unequal meteorological condition affects the barometer, and General Michler observed unaccountable discrepancies in the readings of two barometers when he reached the Hingador. With this exception, the results of this officer’s observation were as close an approximation to the truth as can be expected in a reconnaissance, but it is impossible to say what given observation may be affected by some unknown cause.

A comparison of hypsometric determinations with the same altitudes, ascertained by the spirit-level, will furnish some important elements for eliminating errors. But this operation doubles the labor of the surveyor, and time and cost of his explorations.

The errors of the barometer have led to singular inferences, and the errors of observers to many more. Humboldt, La Condamine, Boussingault, give a decreasing pressure along the Andes; and Orton, taking this statement for granted, asks, “Are the Andes sinking?” The evidence of geological and historical periods is, that the Andes and sea coast are rising. The exceptions to this rule are local, and perhaps only in appearance.

These objections to the use of this instrument only apply to situations where the spirit-level can not be used. To determine heights inaccessible to any other instrument, or for simultaneous observation of the meteorological condition of an extensive area of country, the portability of the barometer render it invaluable.

The height of the barometric column, on the Pacific slope of the Andes, according to Orton, is 29.930. He gives two values for the Atlantic side, 29.997 and 29.932. Michler gives the Atlantic coast of the Atrato 29.874.