188 GNATS Oil MOSQUITOES — CHAPTER VIII 



rich in vegetable growths, and the stems protect the surface 

 from the wind, so that we have here absolutely ideal con- 

 ditions for the development of the larvae of Anopheletes. 



Personally I have seen but little of rice cultivation, as 

 irrigated rice is not greatly cultivated in the parts of India 

 in which I have served, but Captains James and Cornwall, 

 of the Madras branch of our service tell me that there, the 

 rice fields absolutely swarm with Anopheles larvae, and this 

 in spite of the presence of numbers of small fish, though 

 the latter do not appear to be often seen in the irrigated 

 rice of the Punjab. Eice is therefore the only irrigated 

 crop in which the used water is instrumental in the propa- 

 gation of Mosquitoes. 



Putting aside the quantity lost by evaporation, which 

 must form a considerable proportion of the used water, the 

 whole of the canal intake, whether used or wasted, must 

 ultimately find its way to the subsoil and so raise the level 

 of the subterranean waters. 



This raising of the spring level, with its attendant increase 

 of malaria, does not take place at once, but is gradual and 

 progressive for many years after the introduction of canals, 

 until the subsoil waters are raised to a level at which canal 

 supply and drainage are at an equilibrium, but this may mean 

 the absolute water-logging of the soil and the progress of 

 these changes has been made the subject of careful observa- 

 tion by the irrigation branch of the Indian Public Works 

 Department. As an example I may quote a note by Mr. T. 

 Higham, Chief Engineer for irrigation in the Punjab, on 

 the " Spring Levels in the Dari Doab Circle," dated March 

 19th, 1896. 



Summary of Kesults, Upper Sutley Canals Division. 



" {a) On the upper two-thirds of the Katora canal, the 

 spring level is rising at the rate of 0.81 feet per annum, the 

 average depth from the surface being 16.2 feet in 1890-94. 

 After a lapse of fourteen years, the spring level may be 

 expected to average but five feet. 



" (6) In the lower third of the same canal the spring level 

 is rising at the rate of 6.32 feet per annum, but as it is still 



