gg-g CALCAREOUS MANURES- APPENDIX. 



such extent, that there is no doubting or mistaking the connexion of causes 

 and effrcfs. Such sure and abundant sources of malaria are the following 

 materials: 1st, The putrid and stinking water of stagnant ponds, partially- 

 dried by tlie heat of summer. 2d, The mud bottoms of such ponds, or of 

 streams reduced by drought, rich in decomposed vegetable matter, and lefl 

 bare of water only in summer. Sd, Fresh-water marshes, of vegetable 

 soil, frequently, but not regularly, covered by the tides. 4th, Fresh-water 

 marshes, laid dry by embankments, and thereby permitted to rot away ra- 

 pidly. 5th, The meeting of salt and fresh waters on land full of vegetable 

 matter. Of these several and most important sources of malaria, I deem 

 the third (fresh-water marshes in their natural state) to be the least hurtful ; 

 and that the sources numbered 1st, 2d, 4th, and 5th, increase in virulence 

 in the order in which they are named. The greater evils produced by the 

 last are universally admitted, but still by an erroneous deduction from the 

 premises. The belt of the tide-water region of Virginia, in which the fresh 

 water flowing down the rivers mingles with the refluent salt water from 

 the ocean, is well known to be more subject to autumnal diseases than any 

 other extensive space in the country. The breadth of this belt varies much 

 in different seasons. The parts of the rivers in which the fresh and salt 

 waters meet, and where each altei'nately has possession as the tide ebbs or 

 flows, may be but a few miles wide, and even that space is not stationary. 

 But if the limits of this belt be fixed by the highest points to which the 

 rivers have been known to be brackish in driest summers, and by the 

 lowest points where they are fresh in winter, then this belt may be consi- 

 dered for the time as 40 or 50 miles wide, and, in length, stretching across 

 all the tide-waters of the slate. But in the much narrower space where this 

 mingling of the salt and fresh waters usually takes place during the heat of 

 summer, malaria acts with most intensity. Hence the general opinion, that 

 it is simply the meeting and mingling of the fresh and salt waters which 

 cause disease. This is not so, or but in a very slight degree. It is either 

 the passage of fresh-water over salt-water marshes, or of salt-water over 

 fresh-water marshes, that causes the great production of malaria, and 

 disease. This is an important distinction, and the truth or error of the 

 position deserves the m.ost careful investigation. If the mere mingling of 

 the waters were the cause of sickliness, any relief for this part of the evil 

 would be hopeless, as the waters must meet and mix together somewhere. 

 But if it be as I suppose, the evil may be greatly restrained by works of art, 

 or by simply preventing the unnatural accumulation of vast reservoirs of 

 fresh water in mill-ponds, which when discharged, by breaches in the dams, 

 or by opening the flood-gates, overflow salt-marshes, which the natural or 

 unobstructed stream never could have covered. 



Salt-water marshes, not touched by fresh-water streams, are not un- 

 healthy to any considerable extent. This is susceptible of proof by innumera- 

 ble examples in Virginia on the borders of the ocean, or of the waters of 

 the Chesapeake bay. It is rare, however, to find a large salt-marsh at- 

 tached to extensive high-land, which is not reached by some small stream; 

 and every salt-marsh of course must sometimes be well washed and fresh- 

 ened by the heaviest falls of rain. Therefore all must, slightly and at some 

 times, be prejudicial to health. These, however, are exceptions of but 

 small practical or sensible operation. 



The view here taken of the manner in which malaria is produced most 

 certainly, and acts most injuriously, though not sustained by any known 

 authority in this country, nor by any other precisely as stated here, is not 

 therefore presented as original. I derived it, and thence deduced my ap- 

 plication to this country in a modified form, from the interesting report on 



