Page:Popular Science Monthly Volume 9.djvu/443

Rh, however, that is of account, and the northern limit of this lies between the isotherms of 59° and 59.8° Fahr., giving a prolonged temperature sufficiently high to insure vegetable decomposition.

The alluvial soil along the banks of rivers and at their deltas, as those of the Ganges, Nile, Orinoco, and Mississippi, gives rise to fevers of a very malignant type. Their banks are subject to overflow, and frequently have a clayey subsoil, presenting an obstacle to percolation—thus upon the river's receding into its ordinary channel its banks remain damp below the surface, and disease is generated by the sun's agency. A like process annually takes place in the extensive plains and table-lands 'formed of alluvium washed down from mountain-ranges during the lapse of centuries, and having few actual marshes. Profuse rains, succeeded by dry hot seasons, render such regions exceedingly insalubrious during certain periods of the year. Somewhat similar in character are the oases of the Desert of Sahara, which abound in malaria. Hirsch describes these spots as consisting of trough-like depressions in a rocky or highly-hygroscopic soil, the receptacle of subterranean waters, and covered with a layer of alluvium, the surface of the oasis. In this the fierce heat of the sun causes cracks and deep rifts in the earth, which give free vent to the miasm evolved from beneath.

Sandy plains, especially when at the foot of tropical hills and covered with vegetation, as the "Terai" at the base of the Himalayan range, are often infested with malaria. In other cases sandy plains at a distance from hills, apparently dry and not subject to variations in the ground-water, are equally sources of the poison. Such instances as the latter might seem to militate against the generally-accepted theory, but actually do not. Some sands which appear quite free from organic admixture are really the reverse. Faure has pointed out that the sandy soil of the Landes in Southwestern France contains a large amount of organic ingredient which is constantly decomposing and gives rise to periodic fevers. Under such sands, moreover, there is frequently a subsoil of clay. Here, then, assuming a continued high range of temperature, we find all the conditions necessary for the production of malaria.

Localities subject to the intermixture of salt and fresh water are particularly prone to malaria. The Maremmas of Italy afford examples of this on a large scale. The Maremma of Lucca consists of three basins formerly dotted over with ponds and pools. It had been for centuries frequently overflowed by the sea-tides which intermingled with its fresh ponds. Malarial fevers ravaged it and rendered it almost uninhabitable. To the wayfarer who was so imprudent as to spend a night of August or September within its desolate bounds, the penalty was almost certain death. A remedy for this deplorable condition of things was long sought. A proposition had been made in 1714 by the engineer Rondelli to attempt the exclusion of the sea.