192 GNATS OR MOSQUITOES — CHAPTER VIII 



malaria is that they carry and distribute over the surface of 

 the land an enonuous amount of fine silt, the deposition of 

 which renders the surface of the soil impervious to vi^ater, 

 and so favours the permanence of puddles. The proportion 

 of solid matter suspended in the water at the intake of some 

 inundation canals may reach .^^q of its weight, but so high a 

 proportion indicates a high velocity in the supplying river, 

 and therefore a coarser deposit, most of which will fall in 

 the earlier reaches of the canal before the water is dis- 

 tributed on the land. In spite of this, however, the amount 

 of fine deposit may be ver}^ large, especially in rivers such as 

 the Indus, which flood mainly owing to the melting of the 

 snows supplying the glaciers of the far distant mountains in 

 which they take their source ; and this really dangerously 

 fine, clogging silt, is naturally the most difficult to get rid 

 of. Col. Tremenhern, R.E. (" Roorkee Professional Papers," 

 first series, vol. iii., p. 25), states that "Those inundation 

 canals in Sind which draw their supply from branches 

 separated from the main river by islands covered with 

 brushwood and long grass, contain a comparatively small 

 amount of material in suspension. The brushwood and 

 grass impede the velocity of the water and clarify it," and 

 the selection of a swampy tract of this sort, at a distance 

 from dwellings and cultivation, as the site for the intake of a 

 canal of this sort, could hardly fail to be useful in diminish- 

 ing its malarious tendencies. 



I do not think that the importance of this water- 

 proofing effect of fine silt deposited by floods is sufficiently 

 appreciated as a malaria favouring agency ; but it appears 

 to me to be the true explanation of the way in which a 

 previously healthy place may be rendered permanently 

 malarious by a flood of but short duration. Anyone who 

 has been concerned in the management of large municipal 

 filter beds knows well how rapid and efficient is this 

 " staunching " action of fine silt. In the Calcutta water- 

 works, for example, after about a fortnight, the filters at 

 certain seasons become practically watertight to a head of 

 24 inches or so of water. 



All canals, of course, carry more or less silt, but the 



