CONDITIONS INFLUENCING PREVALENCE 



203 



land, and in a few cases much more can be got by cutting 

 a ditch from the new level of the floor of the old tank site 

 to the nearest nullah. 



The following photograph shows the progress of an ex- 

 periment that I made in this direction. At the time of my 

 taking over charge of the local jail, the site of the newly 

 levelled ground in the foreground was occupied by a tank of 

 some size, which had originated in the usual way ; in this 

 case to get earth for the building of the jail. Being within 



Fig. 37. — Levelling up the site of an actual Anopheles tank. 



jail limits, although in a waste corner, the contents were 

 quite clean enough for the local Mosquito larva, and the 

 place formed one of the principal breeding places supplying 

 the jail. Some of the deepest parts must have been eight 

 or ten feet below the level at which the prisoners are seen 

 dumping the spoil gained from the banks ; and, one way and 

 another, I got about a couple feet more of material, with 

 the result that a plot of valuable garden land was substi- 



