CONDITIONS INFLUENCING PREVALENCE 199 



larger scale. Every road and railway has, on either side of 

 it, a continuous chain of " borrow pits," which, in rainy 

 weather, form simply ideal nurseries for mosquitoes of all 

 sorts. 



It is obvious that, in many cases, these pits might easily 

 be converted into excellent surface drains, but unfortunately, 

 partly to facilitate the measurement of the work done by 

 the excavating gangs, and partly to prevent the scouring 

 effect that might be exercised by a continuous channel, the 

 engineers carefully avoid doing so, and leave them as a chain 

 of pools, which remain continuously full of water for months 

 together. The results, especially where a road or line passes 

 close to habitations, are so serious that, at very least in 

 such situations, the making of such undrained hollows should 

 be prohibited by departmental regulation. 



The results even of avowedly sanitary works are unfortu- 

 nately too often no better. The untoward effects of pure 

 municipal water supplies, on modern lines, have already 

 been adverted to, and very often those of attempts at 

 surface drainage are no more fortunate. 



On the next page are four photographs of pools in 

 the course of the local surface-drainage system, all taken 

 within a few hundred yards of my bungalow, and it would 

 have been perfectly easy to fill a large scrap-book with 

 similar prints. In hill stations, such as Naini Tal, the small 

 masonry tanks, such as are shown in the two upper illustra- 

 tions, form almost the only nurseries for Mosquitoes to be 

 found in such places, as the precipitous lay of the ground is 

 very unfavourable to the formation of natural collections of 

 water. 



Celli (C. M., p. 147), also emphasises these undesired 

 effects of otherwise invaluable public works, and is particu- 

 larly emphatic on those of railways. Railways, and neces- 

 sarily also ordinary roads, when embanked, may also often 

 increase the malariousness of a locality in another manner ; 

 for when, as is often the case, they chance to be carried 

 across the natural lines of drainage, they inevitably bank 

 up the drainage of the land lying above them ; and this, too, 

 in spite of an apparently liberal allowance of culverts. 



