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prevails as to the form and breadth of the isthmus (for example towards Nata), we have only to compare the maps of Lopez with those of Arrowsmith, and with the more recent ones of the Deposito Hydrografico of Madrid. The river Chagre, which flows into the sea of the Antilles to the west of Portobello, presents, notwithstanding its sinuosities and its rapids, great facility for commerce; its breadth is 120 toises at its mouth, and 20 toises near Cruces, where it begins to be navigable. It requires four or five days at present to ascend the Rio Chagre from its mouth to Cruces. If the waters are very high, the current must be struggled with for ten or twelve days. From Cruces to Panama merchandizes are transported on the backs of mules, for a space of five small leagues. The barometrical heights related in the travels of Ulloa * lead me to suppose that there exists in the Rio Chugre, from the sea of the Antilles to the Embarcadero, or Venta de Cruces, a difference of level of from 35 to 40 toises. This must appear a very small difference to those who have ascended the Rio Chagre; they forget that the force of the current depends as much on a great accumulation of water near the sources, as on the general descent of the river ; that is to say, of the descent of the Rio Chugre above Cruces. On comparing the barometrical survey of Ulloa

III

* Observations astronomiques d'Ulloa, p. 97.

with that made by myself in the river of Magdelen, we, perceive that the elevation of Cruces above the ocean, far from being small, is, on the contrary, very considerable. The fall of the Rio de la Madelena from Honda to the dyke of Mahates, near Barrancas, is nearly 170 toises *; and this distance nevertheless is not as we might suppose four times, but eight times, greater than that of Cruces at the fort of Chagre.

The engineers' in proposing to the court of Madrid that the river Chagre should serve for establishing a communication between the two oceans, have projected a canal from the venta de Cruces to Panama. This canal would have to pass through a hilly tract, of the height of which we are completely ignorant. We only know that, from Cruces, the ascent is at first rapid, and that there is then a descent for several hours towards the South Sea. It is very astonishing, that in crossing the isthmus neither La Condamine nov Don George Juan and Ulloa had the curiosity to observe their barometer, for the sake of informing us what is the height of the most elevated point on the route of the castle of Chagre at Panama. These illustrious savans sojourned three months in that interesting region for the commercial world ; but their stay has added little to the old observations which we owe to Dampier and to Wafer. However, it appears beyond a doubt that we find the principal Cordillera, or rather a range of hills that may be regarded as a prolongation of the Andes of New Grenada, towards the South Sea, between Cruces and Panama. It is from thence that the two oceans are said to be discernible at the same time, which would only require an absolute height of 290 metres *. However, Lionel Wafer complains that he could not enjoy this interesting spectacle. He assures us, moreover, that the hills which form the central chain are separated from one another by vallies which allow free course for passage of the rivers t. If this last assertion be founded, we might believe in the possibility of a canal from Cruces to Panama, of which the navigation would only be interrupted by a very few locks.

* 1098 feet. Trans.

There are other points where, according to memoirs drawn up in 1528, the isthmus has been proposed to be cut, for example in joining the sources of the rivers called Caimito and Rio Grande, with the Rio Trinidad. The eastern part of the isthmus is the narrowest, but the ground appears to be also most elevated there. This is at least what has been remarked in the frightful road travelled by the courier from Portobello to Panama,

* 947 English feet. Trans.

+ Description of the isthmus of America, 1729, p. 297. Near the town of Panama, a little to the north of the port, is the mountain of L'Ancon, which, according to a geometrical measurement, is 101 toises (646 feet) in beight. Ulloa, vol. i. p. 101.

a two days journey, which goes by the village of Pequeni, and is full of the greatest difficulties.

In every age and climate, of two neighbouring seas, the one has been considered as more elevated than the other. Traces of this vulgar opinion are to be found among the ancients. Strabo relates," that in his time the gulf of Corinth near Lechaeum was believed to be above the level of the sea of Cenchreae. He is of opinion * that it would be very dangerous to cut the isthmus of the Pelopo.' nesus in the place where the Corinthians, by means of particular machines, had established a portuge.' In America, the South Sea is generally supposed to be higher at the isthmus of Panama than the Atlantic ocean. After a struggle of several days against the current of the Rio Chagre, we naturally believe the ascent to be greater than the descent from the hills near Cruces to Panama. Nothing, in fact, can be more treacherous than the estimates which we are apt to form of the difference of level on a long and easy descent. I could hardly believe my own eyes at Peru, when I found, by means of a barometrical measurement, that the city of Lima was 91 toisest higher than the poit of Cal.' lao. An earthquake must cover entirely the rock of the isle San Lorenzo with water before the

Livias, lib.

* Strabo, lib. i. ed. Siebenkees, v. I. p. 146. 42. cap. 16.

+ 582 feet. Trans.
VOL. I.

ocean can reach the capital of Peru. The idea of a difference of level between the Atlantic and South Sea has been combated by Don George Juun, who found the height of the column of mercury the same at the mouth of the Chagre and at Panamu.

The imperfect on of the meteorological instruments then in ure, and the want of every sort of thermometric al correction of the calculation of heiglts, might also gie rise to doubts. These doubts have acquired additional force since the French enginters, in the expedition to Egypt, found the Red Sea six toises * ligher than the Mediterraneun. Til a geometrical survey be executed in the isthmus itself, we can only have recourse to barometrical measurements. Those made by me at the mouth of the Rio Sinu in the Atlantic Sea, and on the coast of the South Sea in Peru, prove, with every allowance for temperature, that if there is a difference of level between the two seas, it cannot exceed six or seven metres t.

When we consider the effect of the current of rotation 1, which carries the waters from east to west, and accumulates them towards the coast of Costa Ricca and Veragua, we are tempted to ad

* 38 feet. Trans. + 19 or 22 feet. Trans.

# I call current of rotation the general motion from east to west, observed in the part of the ocean comprised in the torrid zone.

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