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heated by the rays of a burning sun. In another place I shall mention the experiments made by me while I staid at Cumana, on the action of the roots of the mangle on the ambient air, so long as they remain slightly moistened and exposed to the light; and these experiments will clear up in some degree the remarkable phenomenon antiently observed in both Indies, that of all the places where the manchineel and the mangle vegetate with vigour, the most unhealthy are those where : the roots of those trees are not constantly covered with water. The putrefaction of vegetable matter is in general the more to be dreaded under the tropics, as the number of astringent plants is very considereble there, and as these plants contain in their bark and roots much animal matter combined with tan*.
If there are undoubted existing causes of the insalubrity of the air in the soil which surrounds Vera Cruz, it cannot however, be denied that there are others within the very town itself. The population of Vera Cruz is too great for the small extent of ground which the city occupies. Sixteen thousand inhabitants are confined within a space of 500,000
* Vauquelin, on the tan of gelatine and albumine. Annales du Museum, T.xv. p. 77.
square metres*; for Vera Cruz forms a semicircle of which the radius is not 600 metres. As the greatest part of the houses have only one story above the ground floor, it follows that among the lower orders the number of persons inhabiting the same apartment is very considerable. The streets are broad, straight, the longest in a direction from the north west to the south east, and the shorter or cross streets from south west to north east; but as the town is surrounded with a high wall, there is little or no circulation of air. The breeze which blows feebly during summer from the south east and east south east, is only felt on the terraces of the houses; and the inhabitants, whom the north wind frequently prevents in winter from crossing the streets, breathe nothing in the hot season but a stagnant and burning air.
The strangers who frequent Vera Cruz have greatly exaggerated the dirtiness of the inhabitants. For some time the police has taken measures for the preservation of the salubrity of the air; and Vera Cruz is at present not so dirty as many of the towns of
* 5,361,988 square feet. Trans.
+ Thorne in the American Med. Repos. T. xxx. p. 46. Luzuriaga de la calentura biliosa, T. i. p. 65. (Translation of the work of Benjamin Rush, enriched with the obser. vations of M. Luzuriaga.)
the south of Europe ; but as it is frequented by thousands of Europeans not seasoned to the climate, and situated under a burning sky, and surrounded by small marshes from whose emanations the air is infected, the fatal effects of the epidemics will not diminish: till the police shall have continued to display its activity for a long succession of years. . .
An intimate connection, is observed on, the coast of Mexico between the march of diseases, and the variations of the, temperature of the atmosphere. Two seasons are only known at Vera Cruz, that of the tempests of the north (los Nortes) from the autumnal, to the spring equinox, and that of the breezes or south · winds (brizas) which blow with considerable regularity between March and September. The month of January is the coldest in the year, because it is farthest from the two periods in which the sun passes through the zenith of Vera Cruz*. The vomito generally begins first torage in that town when the mean temperature of the months reaches 24° of the centigrade thermometert. In December, Ja. nuary, and February the heat remains below this limit; and accordingly, it seldom happens that the yellow fever does not entirely disap
pear in that season when a very sensible cold is frequently felt. The strong heats begin in the month of March, and the epidemical scourge begins at the same time. Although May is warmer than September and October, it is however, in the two last months that the vomito commits the greatest ravages; for in every epidemic it requires a certain time before the germ of the disease is developed in all its energy; and the rains which last from the month of June to the month of September have an undoubted influence also on the production of the miasmata which are formed in the environs of Vera Cruz.
The entry and termination of the rainy season are dreaded the most under the tropics, because an excessive humidity arrests almost as much as a great drought the progress of putrefaction of the vegetable and animal substances which are accumulated in marshy situations. More than 1870 millimetres of rain water* fall annually at Vera Cruz; and in the month of July, 1803, alone, an accurate observer, M. Costanzo, colonel of the corps of engineers, collected more than 380 millimetrest, which is only one third less than the quantity which falls at London during the whole year.
To the evaporation of this rain water we must account for the caloric not being more accumulated in the air at the second than at the first passage of the sun through the zenith of Vera Cruz. The Europeans who dread falling victims to the vomito, consider those years as very fortunate in which the north wind continues to blow with violence till the month of March, and when it begins to be felt in the month of September. To ascertain the influence of temperature on the progress of the yellow fever, I examined carefully during my stay at Vera Cruz, tables of more than 21,000 observations which Don Bernardo de Orta, captain of the port, made there during the fourteen years preceding 1803. The therinometers of that indefatigable observer were compared with those used by me in the course of my expedition.
In the following table I exhibit the mean temperature of the months deduced from the meteorological tables of M. Orta; and I have added the number of patients who died of the yellow fever in 1803 at the hospital of Saint Sebastian. I could have wished to know the state of the other hospitals, and especially that of the monks of San Juan de Dios. The task which I have merely sketched will be finished at some future time by some of the intelligent persons who reside in Vera Cruz. I have