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merely indicated the individuals of whom the nature of the disease was in no respect doubtful on account of the frequent vomiting of black matter. As in 1803 the concourse of strangers was uniform throughout the different parts of the year, the number of patients sufficiently indicates the progress of the vomito. The same table contains the variations of the climates of Mexico and Paris*, of which the mean temperature forms a singular contrast to that of the eastern coast of New Spain. At Rome, Naples, Cadiz, Seville, and Malaga, the mean heat of the month of August exceeds 24°, and consequently differs very little from the heat of Vera Cruz.

* The mean temperature of Mexico is founded on the observations of M. Alzate. (Observaciones meteorologicas de los ultimos nueve meses del año 1769, Mexico 1770). As observations made in the city of Paris indicate a temperature somewhat higher than corresponds to the latitude of 48° 50', we have preferred the numbers of the calendrier de Montmorency, calculated by M. Cotte for the years 1765 1808. (Journal de Physique 1809, p. 382).

Meteorological and nosological table of Vera Cruz (lat. 19° 11' 52") centigrade thermometer.

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Sometimes the North
Oct. 126o. 21 291.3 Wind blows alternately 16o. 4 1120. O

with the Breezes.

These two months arel

so dry that in 1803 the
Nov. 1249,01 9 2 quantity of rain never 140, 4 160, 5

rose to 14 millimetres,
while on the 18th of
August and 15th of Sep-|-

tember more than 70
3 1 0 millimetres fell in 24 130 7 13. 8.

Thours. The mean temperature of Vera Cruz is 25°, 4 that of Mexico 17o, and that of Paris 11°. 3.

. 1 M 2

I should have added to this table, the progress of the thermometer at Philadelphia, and the number of individuals who died there of the yellow fever each month, if I could have procured observations sufficient to give the mean temperature of the different months of the year 1803. In temperate climates, results drawn from the greatest and smallest elevations of the thermometer at certain periods give us no information respecting the mean temperature. This very simple and very old observation appears to have escaped a great number of the Physicians who entered upon the discussion of the question, whether the last epidemical diseases of Spain were occasioned by heats which might be considered as extraordinary in the South of Europe. It has been affirmed in many works, that the year 1790 was two degrees hotter than the years 1799 and 1800, because in the two last years, the thermometer only rose at Cadiz to 28° and 30°. 5* while in 1790 it rose to 32°t. The excellent meteorological observations of the Chevalier Chacon, published by M. Arejula, will throw the greatest light on this important matter, if we take the trouble of deducing the mean temperature of the months

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from it. Medicine can only be aided by natural philosophy, when we adopt accurate methods for examining the influences of heat, humidity, and the electrical tension of the air, on the progess of diseases.

We have traced the progress which the yellow fever of Vera Cruz generally follows; and we have seen, that on an average the epidemic ceases to rage, when, at the commencement of the north winds, the mean temperature of the months falls below 240*. The phenomena of life are no doubt subject to immutable laws; but we know so little of the whole of the conditions under which disease is introduced into the functions of the organs, that the pathological phenomena appear to exhibit to us in their succession the strangest irregularities. . When the vomito commences to rage at Vera Cruz during summer with great violence, we see it prevail during the whole winter; the lowness of the temperature then diminishes the disease, but does not entirely extinguish itt. The year 1803, in which the mortality was very small, affords a striking example of this sort.

* 75° of Fahr. Trans.

+ The feeling of heat, and the influence of temperature on the organs, depending on the degree of habitual excitation, the same air which passes at Vera Cruz for cold, may, under the temperate zone, favour the developement of an epidemic.

We see from the table which I have already given, that every month there were some individuals attacked by the vomito; but that during the wiuter of 1803, Vera Cruz was still suffering from the epidemic, which during the preceding summer had burst forth with such extraordinary force. The vomito not having bern very frequent during the summer of 1803, ceased altogether in the begioning of the year 1804. When M. Bonpland and myself descended in the last days of the month of February, from Xalapa to Vera Cruz, the town contained no person under the yellow fever and a few days afterwards, in a season when the north wind still blew with impetuosity, and when the thermometer never rose to 19°*, we were conducted by M. Commoto to the hospital of Saint Sebastian, to the bed side of a dying man, a very swarthy Mexico mestizo, who was a muleteer, and came from the table land of Perote, and who had been attacked by the comito in crossing the plain which separates la Antigua from Vera Cruz.

Fortunately these cases in which the disease is sporadical is in winter exceedingly rare; and a true epidemic never developes itself at Vera Cruz, but when the heats of

* 66° of Fahr. Trans.

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