324 On Malaria. 



soil to the condition of upland. Ditching, clearing ditches, and mow- 

 ing willows, stubble, and brush for burning, should be done when the 

 weather is frosty or immediately after frost j nor should reclaimed lands 

 be tilled, in summer, until the marshy character has been entirely sub- 

 dued, and the surface has become firm and dry; but when that is 

 once achieved, no assiduity or labor should be omitted to prevent 

 the slightest approach to the former condition. Ponds with grassy 

 margins, and little coves indenting the edges, or having shallow ba- 

 sins and pools with grass, weeds and bushes growing around them 

 and over their bottoms, are abundant sources of malaria, if exposed 

 to the due degree of heat. When these do not admit of draining, 

 there is no alternative but to fill them up, or to deepen them, so that 

 their edges may not be infested with weeds. No insalubrity will arise 

 from ponds if they are surrounded by a clean margin of earth, with 

 an outlet sufficient to prevent the waters from rising to the surround- 

 ing grass, and if the bottoms are kept submerged or free from aquatic 

 plants. 



Canals and ditches, whether for agricultural or commercial pur- 

 poses, should be kept clear from vegetation, by walls or clean banks of 

 earth, nor can I omit to notice those little harbors or basins near the 

 feeders, where the waters spread abroad to some adjacent elevation, 

 making a grassy shallow on its verge. 



Drowned lands, when free from trees, are more dangerous than 

 when shaded by forests ; whenever therefore, they are found to be 

 productive of ill health, in any of the forms which have been stated, 

 measures should be adopted to prevent their encroaching on the dry 

 land ; and if they cannot be effectually drained, whether they are 

 salt marsh, bog meadows, or wet pastures, groves of trees should be 

 planted on their margins, to attract their moisture, and to prevent the 

 poison from being carried by the winds to adjacent roads, or fields, 

 or dwellings. The Romans ordered trees to be planted on the 

 shores of Latium, to check the currents from the Pontine marshes. 

 They rendered those groves sacred, and enacted heavy penalties, 

 and other laws for their protection. The entrance of malaria upon 

 the southern side of Rome, has been attributed to the cutting down 

 of those forest trees, which, for a succession of ages, had occupied 

 the declivities of the hills, between the marshes and tlie city. If 

 to plant trees on the principle of intercepting the insalubrious exhala- 

 tions blowing from the marshes, or other malarious tracts, is useful, so 

 to cut them off on the sea board, when on the seaward side of a 



