Radiation in determining the Site of Malaria, 275 



tially, those marshy or undrained spots, or wet woods or moist 

 meadows, which are the sources of this poison, and conse- 

 quently of the various diseases confounded under the vague 

 term of unhealthiness." — Essay, pp. 19 & 21. 



Now, upon this latter passage I may remark, that as water 

 is one of the best radiators of caloric, so all wet, low, and 

 marshy places will be found the most affected by it ; and it will 

 follow that any soil whose mechanical texture is such as to 

 allow the water to permeate through it, or to drain off, at the 

 same time that other circumstances combine to arrest the dissi- 

 pation of heat by radiation, that soil will be found much more 

 salubrious than one retentive of moisture, and particularly if 

 the surface of this latter is covered with low herbage or grass, 

 which is in itself an excellent promoter of terrestrial radiation. 



" That woocls and jungles in hot countries give origin to 

 miasmata of the worst kind is well known to all medical men ; 

 but some doubt may be entertained as to their insalubrity in 

 Europe." — " Dr. Macculloch thinks there is strong reason to 

 believe that close and wet woods generate malaria in this as 

 well as in the warmer countries of Europe. Certain woody 

 districts in Sussex and Kent produce both intermittent and 

 remittent fevers, — at least there is no other assignable cause. 

 The same may be said of some parts of Hampshire and Essex ; 

 as about Epping Forest, for example." — " On the other hand, 

 we have positive testimony that lands which were healthy when 

 covered with wood, have become extremely unhealthy when 

 cleared and cultivated." 



The thick foliage, as I have elsewhere shown, of the trees 

 composing most of the intertropical forests, and even of some 

 of those also in this country, by obstructing the rays of the 

 sun, preserve in their immediate vicinity a greater degree of 

 stillness and a lower temperature than that attained by the 

 atmosphere over the contiguous grounds; whence the heated 

 air coming to slowly circulate among the branches of the trees 

 of these forests, becomes cooled, and its vapours developed; and 

 it is these which occasion the diseases of malaria. — "Yet it 

 requires much circumspection," says the Doctor, "in deciding 

 upon the propriety of clearing these grounds with the view of 

 rendering them more salubrious." — And why ? Because trees 

 naturally tend to obstruct the force of radiation ; and, if planted 

 on a good radiating surface, not so close together as entirely 

 to obstruct the genial influence of moderate warmth from the 

 sun's rays, or to prevent the free circulation of the air, will 

 prove a valuable defence against the appearance of malaria, 

 by counteracting that unequal distribution of temperature 



2 N 2 which, 



