thermometer is at 28° or 29°, and by night at 23° or 24°; but at Acapulco I found the heat of the air by day 29° or 30°, during the night it kept at 26°, and from three o’clock in the morning, to sun-rise it suddenly fell to 17° or 18°. This change makes the strongest impression on the organs. No where under the tropics did I ever feel so great a coolness during the latter half of the night. It was like passing suddenly from summer to autumn; and the sun was hardly risen when we began again to complain of the heat. In a climate where the health principally depends on the functions of the skin, and where the organs are affected with the smallest changes of temperature", a cooling of the air to the extent of 10° or 12° occasions suppression of transpiration very dangerous to Europeans not seasoned to the climate. It has been falsely affirmed that the vomito never prevailed in any part of the southern hemisphere, and the cause of this phenomenon has been attributed to the cold believed to be peculiar to that hemisphere. I shall have oc

* The temperature of the air at Guayaquil keeps so uniformly between 29° and 32° centigrades, that the inhabitants complain of cold when the thermometer suddenly falls to 23° or 24°. These phenomena are very remarkable in a physiological point of view; and they prove that the excitability of the organs is increased by the uniformity and continued action of habitual stimulus.

casion 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 "

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 tongue 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, ulvae, and medusae. 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. At Porto-Bello, where remittent

bilious fevers prevail, and where the tides are scarcely sensible, the putrid 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 gates of the town, and the monkies entered the gardens of Porto-Bello in bands for the fruit. The salubrity of the air has considerably increased, since the governor Don Vicente Emparam, an enlightened administrator, gave orders for clearing away the wood in the neighbourhood.

The position of Vera Cruz bears more analogy 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, Cornisera, Varronis, 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 -targau, a few cocoa trees which ornament the gardeus of that village; and they are the last great trees to be discovered in the desert. The excessive beat 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 hillocks, 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° t, while the same instrument in the open air and in the shade keeps at 30°.f 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