648 THE POPULAR SCIENCE MONTHLY. 



ging ; and, 2. Making excavations. Which of these two is the more 

 fruitful in producing malaria is not stated, nor is the modus operandi 

 of either suggested. In Hong-Kong, an island consisting of little more 

 than bare and barren rocks of weather-beaten granite, and whose soil 

 contains but two per cent of organic matter, malarial disease was 

 formerly unknown, and only became prevalent, as it is at present, after 

 excavations had been made in digging granite for building purposes. 

 So, again, tanks and pools of water cess-pools, mill-ponds, reservoirs, 

 and bilge-water on shipboard appear to be specially productive of 

 malaria. In Ceylon, the tanks of Candelay and Minery the one 

 twenty miles in circumference, the other twelve have been consid- 

 ered the cause of malaria in that region (see Davy on " Diseases of 

 the Army," p. 51, etc.). It is easy to comprehend how such pools, 

 tanks, and excavations containing water may constitute mosquito- 

 nurseries, where the female may deposit her eggs and propagate, 

 which would probably have been prevented in the absence of such 

 water accumulations. How simply digging up the soil may contrib- 

 ute to the formation of malaria, or to the development of mosquitoes, 

 tcithout excavations, I am not able to explain. 



9. " In certain countries it " (malaria) " seems to be attracted and 

 absorbed by bodies of water lying in the course of such winds as waft 

 it from the miasmatic source." That the malaria itself is absorbed by 

 water is pure hypothesis. The known fact embodied in this ninth 

 statement is really this : A body of water intervening between a healthy 

 locality and a fever district will, j:>rovided it be sufficiently wide (three 

 fourths of a mile or more) prevent the transmission of fever from the 

 infected to the healthy locality, also provided, of course, that the pre- 

 vailing wind does not blow the fever-generating element from one side 

 to the other, as we have already seen it may do over a much wider 

 space probably five miles. This, again, is not difficult of possible ex- 

 planation by the mosquito theory. All depends upon the answer to 

 this question : Over how wide a sheet of water will the mosquito, in 

 the absence of irresistible winds, attempt to fly ? I am unable to 

 answer this question positively. It may depend upon the degree to 

 which the insect possesses far-sightedness, for, if it can not see land 

 across a body of water three fourths of a mile wide, such a width 

 would appear to its vision boundless as an ocean, and under those 

 circumstances it might not voluntarily attempt to cross. Further- 

 more, the flight of the insect being mostly nocturnal, long vision would 

 be all the more difficult. These suggestions need confirmation : they 

 are tentative, but still sufficient to suggest the possibility of the pro- 

 tective influence of wide bodies of water being explicable on the 

 mosquital basis. 



10. " In proportion as countries, previously malarious, are cleared 

 up and thickly settled, periodical fevers disappear." Here, too, we 

 may remark that in such countries the land is cultivated, and' its 



