Such had been the state of things from time immemorial until 1740 1741, when, after the necessity of excluding the sea from these marshes had been insisted upon by many experienced persons, a sluice, with folding gates, competent to give emission to the waters of the marsh, but prevent the sea from entering, was constructed at the mouth of the Burlamacea. The most complete and unexpected success imme diately followed upon and continued with this work. The year after its completion there were no appearances of the terrible maladies which previously appeared every year. The inhabitants soon recovered health, and the land being very fertile, the population rapidly in creased. But the neighbouring parts were long left a prey to the destroying influence of the mixed marsh-waters. The 19th century had been some years entered upon before the inhabitants around other basins were considered ; and it was not until so late a period as 1621, that the guarding with sluices of the remaining outlets from the marshes was complete. "Since that time," says Sig. 0. Giorgini, whose account of these proceedings is here abridged, "the diseases of malaria have ceased so entirely at all points, that no other dangers are now incurred regarding the insalubrity of the atmosphere, than such as may arise from neglect of these eluicee, which the inhabitants of the country should regard as their palladium." 'Ann. do Chita. et de l'hys.,' 1825, voL xxix., pp. 225-240.
The causes of the malarious condition of the atmosphere occasioned by the mingling of the marsh-waters with those of the sea, constitute too extensive a subject of investigation to be entered upon at any length in this place. It may be stated, however, that by the mutual re-action upon each other of the sulphates contained in the sea-water, and the vegetable matter partly suspended and partly dissolved in the water of the marshes, sulphuretted hydrogen gas is copiously evolved, which if not wholly identical with the deadly miasma, certainly forms a considerable and probably the most active part of it. Further in formation on this subject will be found in the article SEA, where its gaseous constituents and its particular nature on certain coasts are described.
Nevertheless all marshes are not equally prejudicial to health ; but independent of their different degrees of insalubrity, they present other distinguishing features. The climate, the nature of the soil, and the vegetation, are all so many circumstances which vary the appearance and character of marshes. The quantity of water is also very different ; in some cases it is hardly visible, while in others, at least in certain seasons, the marsh presents the aspect of a multitude of stagnant pools covered with aquatic birds. One of the five natural orders of birds, the Waders (Grallm of Linnaeus, Grallatores of Illiger and of Vigors) [Bums, in NAT. HIST. DIV.], are the characteristic feathered inhabi tants of swamps and marshes ; in addition to which their waters support their contingent of the order Swimming Birds, Anseres or Natatores, some of which are peculiar to them. This is the case with many of
the Tuscan marshes, which are moreover remarkable for their floating islands, which sometimes unite and cover a large surface : these islands have little solidity, and, eventually sinking, become in time converted into peat, like those in the lakes of Cashmere ; some of these marshes gain in extent, while the soil of others gradually rises, and the marsh disappears. Reeds are particularly abundant in the Tuscan marshes, and they are applied to a great number of useful purposes. The quality of the marsh-water also differs : thus, in some of the marshes of South Carolina, in the United States, it is salt, as likewise at Rochelle, Rochfort, &c., in France. In other places it is sulphurous, as is the case with the marshes of Mesopotamia ; in many it is ferrugi nous, as in Siberia, where the marshes are strongly impregnated with sulphate of iron from the vitriolic springs which flow into them. The trees which are found imbedded in these marshes are so thoroughly impregnated with oxide of iron, that they supply an ore of excellent quality, furnishing a metal free from the defect of brittleness so common to the iron of most other bog-ores. In some cases the water of the marsh exhales an intolerable smell of sulphuretted hydrogen, arising from the decomposition of the sulphate of magnesia, or Epsom salts (which is continually forming on their banks), in conformity with phenomena already noticed. (Siberia, and tho banks of the E u ph rates.) In cold countries marshes freeze, but seldom become dry ; in warm countries, on the contrary, the marshes are often dry, and such can never form peat. As to the vegetation of marshes, it is either com posed of reeds, rushes, algn, graminete, or mosses, of which several species of Sphagnum, or bog-moss, are' the most common in peat-bogs. Brushwood of kinds, and willows and alders, are also common in marshy grounds.
Marshes are found in all kinds of situations, in continents and in islands (Iceland, Anau, ou the margin of the sea, as well as in the interior of the land, on the slopes and even on the summits of moun tains, as well as in the plains. Most countries have them in greater or less abundance, but it has been remarked that they are less common in Asia and in Africa (as far as the latter is known) than in Europe, and that they are more abundant in America than elsewhere. In this latter part of the world almost all the plains are wet and abound in marshes; they are exceedingly common in the northern countries of the globe, particularly in the flat parts bordering on the sea, where the land is low and the subsoil clay. Here the rain and snow-water accumulate, and remain for want of sufficient evaporation to carry them off.