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peculiar to that hemisphere. I shall have occasion to shew in another place how much the difference of temperature of countries situated to the north and south of the Equator has been exaggerated. The temperate part of South America has the climate of a peninsula which narrows towards the south ; and the summers are not so hot there, and the winters not so rude as in those countries which under the same latitude, in the northern hemisphere, widen towards the north. The mean temperature of Buenos Ayres differs but little from that of Cadiz, and the influence of the ice, the accumulation of which is undoubtedly greater at the south, than at the north pole, is hardly felt below the 48° of south latitude. We have already seen that the yellow fever in fact, first raged at Olinda in Brazil, in the southern hemisphere, and carried off a great number of Europeans. The same disease prevailed at Guayaquil in 1740, and in the beginning of this century at Monte Video, a port in other respects so celebrated for the salubrity of its climate.

For fifty years back, the vomito has never appeared on any point of the coast of the South Sea with the exception of the town of Panama. In this port as well as at the Callao*

* Leblond, Observations sur la fievre jaune, p. 204.

the beginning of the great epidemics is most frequently marked by the arrival of some vessels from Chili; not because that country which is one of the healthiest and happiest of the earth can transmit a disease which does not exist there, but because its inhabitants transplanted into the torrid zone, experience with the same violence as the inhabitants of the north, the fatal effects of an air excessively warm and vitiated from a mixture of putrid emanations. The town of Panama is situated on an arid tongie of land destitute of vegetation ; but the tide when it falls leaves, exposed for a great way into the bay a large extent of ground covered with fucus, ulvæ, and medusæ These heaps of marine plants and gelatinous mollusci remain on the shore exposed to the heat of the sun. The air is infected by the decomposition of so many organic substances; and miasmata of very little influence on the organs of the natives, have a powerful effect on individuals born in the cold regions of Europe, or in those of the two Americas., The causes of the insalubrity of the air are very different on the two coasts of the Isthmus. At Panama, where the vomito is endemical, and where the tides are very strong, the shore is considered as the origin of the infection.

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At Porto-Bello where remittent bitious fevers prevail, and where the tides are scarcely sensible, the patrid emanations spring from the very strength of the vegetation. A few years ago, the forests which cover the interior of the Isthmus, extended to the very igates of the town, and the monkies entered the gardens of Porto-Belo in bands for the fruit. The salubrity of the air has considerably increased, since, the governor Don Vicente Emparan, an enlightened administrator, gave orders for clearing away the wood in the neighbourhood.

The position of Vera Cruz bears more anaHogy to that of Panama and Carthagena, than to Porto-Bello and Omoa. The forests which cover the eastern slope of the Cordilleras, hardly 'extend to the farm of l'Encero, where a less dense wood commences, composed of Mimosa, Cornigera, Varronia, and Capparis Breynia, which progressively disappears at five or six leagues distance from the sea coast. The environs of Vera Cruz are frightfully arid. On arriving by the Xalapa road, we find near la Antigua, a few cocoa trees which ornament the gardens of that village ; and they are the last great trees to be discovered in the desert. The excessíve heat which prevails at Vera Cruz is increased by the hillocks of moving sands (meganos) formed by the impetuosity of the north winds, and which surround the town

on the south and south west side. These hitlocks which are of a conical form rise to the height of about 15 metres*; and being strongly heated in proportion to their mass, they preserve during night the temperature which they have acquired during the day. From a progressive accumulation of heat the centigrade thermometer plunged into the sand in the month of July, rises to 48° or 50°while the same instrument in the open air and in the shade keeps at 30°1. The meganos may be considered as so many ovens by which the ambient air, is heated; they not only act from radiating caloric in every sense, but also from their preventing by their being grouped together, a free circulation of air. The same cause which gives rise to them easily destroys them; and these hillocks change their places every year, as may be remarked especially in that part of the desert called Meganos de Cathalina, Meganos del Coyle, and Ventorillos.

But unfortunately for those of the inhabitants of Vera Cruz who are not seasoned to the

climate, the sandy plains by which the town · is surrounded, far from being entirely arid, are

intersected with marshy grounds in which the rain water which filtrates through the downs

* 49 feet. Trans.'
4 118°. 4' or 120° of Fahr.
$ 86° of Fabr.

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is collected. These reservoirs of muddy and stagnant water are considered by M. M. Comoto, Ximenez, Mociño and other intelligent physicians who have examined before me the causes of the insalubrity of Vera Cruz, as so many sources of infection. I shall not name here the marshes known by the name of the Cienega Boticaria, behind the powder magazine, the Laguna de la Hormiga, the Espartal, the Cienega de Arjóna, and the marsh of la Tembladera situated between the road of Rebenton and the Callejones de Aguas-largas. At the foot of the hillocks we find only small shrubs of croton and desmanthus, the euphorbia tithymaloides, the capraoia biflora, the jatropha with cotton-tree leaves, and ipomoca of which the stalk and flowers hardly rise above the arid sand which they cover. Wherever this sand is bathed by the water of the marshes which overflow in the rainy season, the vegetation becomes more vigorous. The rhizophora mangle, the coccoloba, pothos, arum, and other plants, which vegetate in a humid soil charged with saline particles, form scattered thickets.

These low and marshy places are the more to be feared as they are not constantly covered with water. A bed of dead leaves mixed with. fruits, roots, larvæ of aquatic insects, and other collections of animal matter, enter into fermentation, in proportion as they become

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