TEMPERATURE. 127 



operating is complex ; but in order to account for the identical 

 effects produced by the atmosphere of swampy places, and from 

 the consideration of the processes of vegeto-animal decay which 

 are uninterruptedly going on in such localities, we are led to 

 admit that they give rise to certain noxious principles, which we 

 call effluvia, miasma, and malaria. Now, are these substances 

 the elements of the noxious influence of swamps ? This would 

 appear to be highly probable. 



The specific gravity of effluvia seems to be greater than that 

 of pure air, for they do not spread beyond a certain height. It 

 would seem, also, that they are soluble in the globular moisture, 

 actually saturating the air, so that they become condensated by 

 the coolness of the night, and fall with the dew — the popular 

 dread of dew-damps being proved thereby to be well founded. 



Local circumstances may greatly influence the condensation 

 of effluvia; for instance, whenever a swamp exists in a deep 

 valley, abuts on a hill or mountain, or is skirted by forests, the 

 miasmas evolving therefrom are likely to accumulate in the 

 locality. The direction of variable or prevailing winds, also, 

 aids materially in the spreading of malaria, and may thus 

 modify the salubrity of a whole district. 



Hills, mountains, and forests may therefore act as a protec- 

 tion against, or, on the contrary, as an aggravation of the 

 noxious influence of a swamp, according as it is situated to the 

 leeward or windward of the tract. In fact, it often happens 

 that the miasmatic influence is almost null in the locality of the 

 swamp itself, whilst it becomes very noxious at a certain distance, 

 particularly whenever any barrier obtrudes, in the shape of hills 

 and mountains to the leeward. Localities to the windward are 

 comparatively exempt and safe. 



The formation of effluvia is more abundant when a larger 

 section of a marsh remains uncovered, or exposed to the direct 

 action of the solar rays ; so that, in our island, the deleterious 

 influence of most of our swamps is materially tempered during 

 the rainy season, when their ground surface is overflowed by 

 pluvial waters. 



We are naturally induced to admit the existence of telluric 

 effluvia, as manifested by the prevalence of remittent and 

 intermittent fever, when forest and brush lands are brought 

 under tillage, and deeply furrowed by the plough. 



