The elevation of the passes upon these routes should be definitely fixed. The instrument which must determine the question of practicability is the Wye spirit-level. If the capacity of the harbors are insufficient for the largest class of ships, or can not be made available at a reasonable cost, further examination is unnecessary. 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.