MALARIA. 



419 



perature, 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 ex- 

 ceedingly 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 re- 

 ceptacle 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 cov- 

 ered 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 in- 

 stances 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 tli at the sandy soil of the Lancles 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 exam- 

 ples 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 con- 

 dition of things was long sought. A proposition had been made in 

 1714 by the engineer Rondelli to attempt the exclusion of the sea. 



