40 On Malaria. 



it is in these countries especially, that the diseases from this 

 source constitute nearly the entire mortality of the human race. 

 And thus, for Europe, it is in Spain, Italy, and Greece, and 

 chiefly on their Mediterranean shores, that the activity of mal- 

 aria scarcely yields to that of the intertropical climates ; while 

 in France, Holland, Germany, Hungary, and with us, in a far 

 less degree, the production will be found regulated by the heat 

 of the summers, all other circumstances being the same. 



And if we thus account for the variations in the quantity and 

 virulence of diseases in any given country, for noted seasons of 

 epidemic in the countries which I have just named, and for the 

 great prevalence of fevers among ourselves during the last few 

 years, and particularly in the last summer, there is another 

 point of scarcely inferior importance to be taken into the con- 

 sideration, independently of that which relates to peculiar 

 winds as connected with the propagation of this poison ; — and 

 this is, moisture. 



I need not repeat that water in some form is necessary to 

 the production of that peculiar vegetable decomposition which 

 is the source of this poison ; and so true is this, that even in 

 the tropical regions, the diseases from this cause are nearly 

 miknown]in districts of peculiar dryness, as they are in the 

 drier seasons of those countries. Thus, for example, Egypt is 

 free from such fevers, except at the period of the subsidence of 

 the Nile, unless where, as at Damietta, the cultivation of rice 

 is pursued ; and the same is true of Mesopotamia very remark- 

 ably: and if I dare not extend these illustrations, I must 

 remark that in all these cases, the action of moisture is two- 

 fold, inasmuch as it not only accelerates vegetable decompo- 

 sition, but renders the atmosphere a fitter conductor of this 

 poison. 



Taking these two causes of the increase in the quantity and 

 in the action of malaria, we can explain many particulars which 

 relate to its power in producing diseases ; and as the knowledge 

 of these is important as far as relates to the main object of this 

 paper, prevention, it becomes necessary to explain them at a 

 little more length. 



As to season, the simplest case is that of the intertropical 

 climates; and Africa offers the plainest instance among the 



