CONDITIONS INFLUENCING PREVALENCE 191 



understand that the question is not so much as to the 

 desirabihty of taking measures to stop the waste, as of how 

 to do it. At first sight the remedy seems obvious, namely, 

 to "revet" the channels with a layer of masonry or con- 

 crete, but unless the work is of a very solid description it has 

 been found that such a lining soon cracks and becomes 

 pervious. Doubtless, however, the ingenuity of our engineers 

 will ere long find a method of meeting the difficulty, and 

 whoever answers the question will not only have solved an 

 important economic problem, but will have done much to 

 diminish the untoward effect of canals in water-logging the 

 soil, with its inevitable concomitant of intensification of 

 malaria. 



An occasional way in which canals may directly produce 

 breeding places for Mosquitoes is their being left empty for a 

 period long enough to admit of the rearing of a generation of 

 imagines from the egg. Necessarily a temporarily disused 

 canal is really a chain of pools ; and I have met with an 

 instance where a small canal supplying a tract of thin loam 

 lying on boulder alluvium, furnished the only situation in 

 which I could discover any Anopheles larvae, in the form 

 of pools in the beds of canal channels not in use. Moreover 

 the village in question was notoriously malarious. 



It is obvious that the flushing of each empty channel 

 every week or ten days would prevent their serving as 

 Mosquito nurseries. 



The above remarks apply exclusively to the large perennial 

 canals, but in many places another system is in use, viz., 

 irrigation by " inundation canals." There are channels 

 carried from the river bank to any portions of the country 

 which may happen to lie lower than the flood level of the 

 river, and are so graded that they are filled only during times 

 of flood. Their object is in fact, exactly the opposite of the 

 system of protecting low-lying lands by dykes that have 

 been so efficient in diminishing malaria in Holland and in 

 the English fen country, and as neither the time nor 

 quantity of the supply can be controlled, their effect on 

 health can hardly be otherwise than disastrous. 



Another reason why inundation canals specially favour 



