who were diseased. Thefe powers acted with particular force, especially when preceded by fleep. A gentleman was attacked with the difeafe immediately after falling accidentally into the river. The tranfition of the atmosphere from heat to cold, which occurred about the twentieth of September, and continued three or four days, added confiderably to the number of the fick; nor did the cold weather, which occurred after the beginning of October, check the progrefs of the disease, until aided by heavy frofts.

4. SLEEP.

Many perfons awoke in various hours of the night with a chill or fever. Some were not attacked until the dawn of day.

5. MARSH MIASMA.

I am inclined to rank this alfo among the exciting caufes, when the body had been previously expofed to contagion: Nor is it one of the moft inconfiderable. This acted, when thofe already enumerated appeared to have been infufficient to produce difeafe: for a perfon, in whom the feeds of the fever were received, very frequently escaped its powers by expofition to pure air. Hence the disease prevailed more virulently about its primary fource; and feemed more innocuous, when carried to another place, free from the influence of marsh miafma. On Bowley's wharf, the prevalence of a northerly wind during one day, would be fucceeded by the ficknefs of feveral perfons. The wind blew the water out of the dock fo much, as to expofe the mud to the action of the fun. The noifome exhalations combined their powers, with thofe of the contagion already acting on the body, and excited fever. On the Point the gentle showers of rain which frequently fell, were fucceeded by an increase of patients; thofe fources of miafma, which had been dried up by the fun, being again fupplied with fufficient moisture and heat to generate putrid exhalation.

Accidental circumstances fometimes excited the disease. A mate of a veffel, having received a blow on the head from a cable, was immediately attacked with the fever.

Among the preceding exciting caufes, fome of very difcordant properties are enumerated. To account for their effects in producing the fame difeafe, we must recur to the peculiar nature of the fever itself. The debility attending it, like that of pleurify, is of the indirect kind, or produced by excessive action of stimuli upon the body. That it is of this nature may be inferred from the fymptoms of the difeafe, and from that mode of treatment, which proves most fuccessful in its cure. Causes which induce direct debility, are not preceded by a tense pulse, nor can their effects be counteracted by debilitating medicines. It may be inferred alfo from the means, which prevent the contagion, when the body is impregnated with it, from rifing into actual fever. Did it tend to induce direct debility, would not those powers, which abstracted stimulus from the body and gradually weakened all its functions, accelerate the formation of disease? and would not those persons already labouring under direct debility, be most subject to its influence? But the causes of the yellow fever, like the contagion of the small-pox, and like wine, produce debility of the indirect kind. In the latter disease, physicians endeavour to counteract this effect by debilitating remedies. A moderate quantity of wine gives increased action to the heart and arteries, but when drunken in excefs, it is followed by indirect debility. When the body is impregnated with the contagion of the yellow fever, disease must not neceffarily enfue. Its own ftimulus is not always fufficiently powerful by itfelf to produce this effect: but when affifted by ardent spirits, animal food, heat, or violent exercise, fever is excited. Is it not fufficiently obvious that, under the influence of these causes, excess of arterial action must enfue? Dr. Condict, in his inaugural thefis on contagion, published laft fpring, has, in my opinion, fir, treated this subject so ingeniously, that it will be unnecessary to dwell here any longer on this part of our fubject.

But among the occafional caufes are cold and fleep, and to these we may add grief and fear*, which cannot be faid to ftimulate the body. I admit this truth under certain restrictions. So far from thefe powers giving increase of stimulus to the body, they tend to induce direct debility, because they are of a negative nature. But let this principle in philofophy be remembered, that the abstraction of stimuli from the body is followed by increase of its excitability; this being neceffarily augmented, when the means of exhaufting it are diminished in number. Heat--the moft general ftimulus, which heaven has bestowed for the prefervation of life, varies its influence with its intenfity. How much therefore is abftracted from the powers acting on the human frame, when this is diminished! fleep also abftracts an immenfity from the fupport of life, for it is indeed at tendency to death; during its continuance, the faculties, the operations, the paffions of the mind, even the influence of light itfelf, are totally intercepted and fufpended. When so many powers, whose action exhaust the excitability of the body, cease to exert their wonted influence, does it not follow, that the fyftem must become more fenfible to impreffions that remain? We may deduce a proof from a familiar fact. A drop of cold water falling on the face of a perfon fleeping, will not only break. through the most profound sleep, but will sometimes induce convulfions alfo. In this highly excitable fituation is the body placed by cold and fleep; and the miafmata or contagion, whofe ratio to that of the excitability while acted upon by all the ftimuli of health, was before too fmall to produce any morbid confequence, now finds the body infinitely more fenfible of its impreffion and easily thrown into fever. To this accumulation of excitability, I afcribe also the occurrence of disease at the dawn of day. The stimulus of returning light proves in these instances the exciting cause. That this is by no means improbable

* The influence of these two are thus noticed by Hoffman in his directions for prevention of the plague." Guard against violent paffions, endeavouring to preserve a conftant firmness of mind, and shaking off all timorousness and dejection."

will appear from the fact, that the fun, even in a cold morning, darting upon a perfon fleeping, will roufe him from his slumber. There are fome, who are daily refufcitated from fleep by the return of the morning light.

But how, it may be asked, can abftemious living and artificial evacuations obviate the formation of fever, if the abftraction of ftimuli be followed by increafed excitability? By abftemious diet and moderate evacuations, the miafmata or contagion, though actually existing in the body, may be fuccefsfully oppofed for thefe counteract its ftimulating properties, by mitigating their effects. But evacuations, which have been injudiciously immoderate, have occafioned the occurrence of fever on the fame principles which explain its formation after cold or fleep.

In my next, we will afcend a little higher in the grade of caufes, and enlarge our profpect of their general influence. May the Father of heaven, in the mean while, preferve my friend in his efforts to render his country more happy and the world more wife! ADIEU.

LETTER III.

I AGAIN take up my pen to devote a few more minutes to the duties of friendship and the profecution of my promife. I concluded my laft letter with a view of the exciting or occafional caufes. We will now proceed to notice the intervals, which elapfed between the times of receiving the feeds of the disease, and the excitement of fever. Thefe varied greatly according to the coincidence of exciting caufes, and to the previous conftitution of the body.

The contagion appeared to act immediately upon fome, especially if an exciting caufe happened to concur at the fame period. The old man, who led the chair-wheels, which bore the coffins.

to Potter's Field, was under the neceffity of procuring his stepfon to officiate for him during one day. The latter had not been till now expofed to the difeafe, and being at the fame time fubjected to fatigue in the heat of the fun, the fever was immediately formed. A perfon, who had nurfed another during his illness, retired to the country, but was attacked with the disease in three days. It occurred most frequently within feven or eight days; but there are inftances in which it was protracted eleven or fifteen days. Blane and Lind observe of the remitting fever, that thofe men, who had been exposed to marsh miafmata in watering places, would be taken ill several days after the ships had gone to fea; and John Hunter mentions their occurrence three weeks after exposure to their cause.

In many perfons, fever was never completely formed. The miafmata or contagion would act upon their bodies during two and three weeks after expofure to it; but through defect of a proper exciting caufe, would produce only pains in the head and back, and occafional flushings of heat and unusual fweating through the night. In feveral inftances, only the following peculiar mark of the system being impregnated with the feeds of the disease, would occur. Six or feven nights fucceffively would be passed in anxious restlessness. Although the mind was tranquil, and the body free from fatigue, yet it was impoffible to procure refreshing sleep. If, after much toffing in the bed, a lumber would occur, it was quickly broken, fometimes by terrifying dreams; and even after being roused, the mind could not foon detect the cause of its alarm. In this manner the whole night would elapfe, but an hour's grateful fleep might be enjoyed after the dawn of day. The contagion acted in these cafes, like a cup of ftrong tea, or a fmall quantity of wine.

Thofe, who were exposed to the contagion, would be affected with headach, a heat and fenfe of fulness or oppreffion in the ftomach, want of appetite, a disagreeable taste, and costivenefs; a burning of the hands and feet at night; restlessness or drowsiness; heat of the skin alternating with profuse sweat,