■44 On Malaria. 



England, there is perhaps not one acre in a hundred thousand 

 where there is danger from night air, or from malaria in any 

 mode ; so that to distinguish where that lies, is to have relieved 

 from useless fears all those who may learn to make the distinc- 

 tions under review. 



To pass from what relates to climate and season, and to pro- 

 ceed to the propagation, simply, of malaria, it is almost super- 

 fluous to say, that its influence, as to the production of disease, 

 is much regulated by proximity, which implies a state of con- 

 centration or accumulation. Hence the danger arising from 

 vicinity; while, as I formerly remarked, where the generating 

 source is small, this becomes necessary to its effect, since dilu- 

 tion may be expected to destroy the power of the poison. 



For analogous reasons, its effect in the production of disease 

 is increased by concentration or condensation ; and such a 

 state of things takes place in narrow and confined valleys, or 

 in places surrounded by woods, or in woods themselves; in 

 any situation, in short, where the poison is produced, and is 

 so sheltered from winds that ventilation becomes difficult. And 

 if it is probable that this is one chief reason of the peculiarly 

 insalubrious nature of woods and jungles in hot climates, so is 

 it an universal remark in Italy, that the short valleys in which 

 the air cannot circulate are among the most pestilential spots. 

 And if this explains, also, in some measure, the bad effects of 

 calm weather, so does it account for the unusually pestiferous 

 nature of rivers and lakes confined within wood, as are those 

 of the tropical climates, and as there are many also in different 

 parts of Europe. That we ourselves are not exempt from 

 these additional causes of the influence of malaria, would be 

 easily shown by many references, were it not for the reason 

 which has caused me to exclude them. 



It is another important question for practice, how far and in 

 what manner malaria can be conveyed by the winds to places 

 where it is not produced, so as to act in exciting disease. 

 That it is conveyed to certain distances by winds is amply 

 proved by an abundant experience, and I may first detail a 

 few of the most useful particulars as to this fact. In Italy and 

 Greece, it is obser\^ed, that where long valleys terminate on sea 

 shores, on which the exits of the rivers are swampy, it is an 



