HEME DIES AGAINST MOSQUITOES 191 



so that the pond is thus automatieally treated over its whole sur- 

 face by the shifting of the wind, so long as there is a surplus of 

 oil present. If the pond is full of grass and weeds and the surface 

 once becomes covered, there is, of course, much less shifting, but 

 the accumulation of oil on the foliage keeps the surface well 

 covered, and I assume that this is largely due to the oil working 

 on and off the foliage more or less, due to tlie disturbance of the 

 wind. In a perfectly sheltered pond ; for instance, in a deep 

 hollow surrounded by large trees, quite protected from wind and 

 containing more or less of weeds and grass, I think the oily scum 

 stays fairly constant, and in thus referring to the surface-coating 

 I mean that which is plainly visible, and it is possible, even prob- 

 able, that there remains for much longer periods a minute film of 

 oil over the whole surface, which is not observable through such 

 means as I have used. 



The twenty-five or more ponds which were treated under my 

 direction had the oil applied at somewhat irregular intervals, 

 averaging perhaps once a month, and at no time did I find any 

 mosquitoes during that whole sunnner in their vicinity. The 

 mechanical difficulties of oil application, however, led to resort- 

 ing to drainage, and therefore my experience with the oil meth- 

 ods ceased. These were hill-top ponds, chiefly in the Avoods, 

 while the large marsh areas Avere so meagrely treated on the 

 edges with oil as scarcely to be considered as treated at all, and 

 therefore with them no real experience was gained. 



I have thought— but had no opportunity to try the experi- 

 ment—of anchoring one or more barrels filled with fuel oil in the 

 pond ; for convenience perhaps placing them there in the winter 

 when frozen over and boring small holes in the barrel while the 

 oil is thick with the chill of cold weather. Then with the sum- 

 mer's heat this oil will ooze out and maintain the supply in 

 places not accessible by other means. If on experiment the oil 

 oozed out too freely, various expedients could be tried— such as 



