December 1, 1905.] 



SCIENCE. 



697 



in concluding that, as a general rule, the 

 number of immigrants into any area of 

 operations must, for practical purposes, be 

 very small or inappreciable a short distance 

 within the boundary line. The following 

 diagram probably represents with accuracy 

 the effects of thorough suppression of 

 propagation within a circular area. 



Diagram III. Effect of drainage of a circular 

 area. 6 = boundary of drained area. Mosquito- 

 density begins to diminish at the circle a; be- 

 comes one half at the boundary 6; and is small, 

 inappreciable or zero at the circle c. 



At the circle (a) and beyond it the mos- 

 quito density will be the normal density 

 which existed before the operations were 

 commenced. At (&), the circle bounding 

 the drainage operations, the density will 

 always be about half the normal density. 

 At the circle (c) and within it, the density 

 will be small, inappreciable or zero. The 

 distance from (a) to (&) may be taken as 

 being the same as that from (6) to (c) ; and, 

 as the mosquitoes penetrating from (6) to 

 (c) must be drawn from the zone between 

 (a) and (6), the average result will be the 

 same as if no immigration at all takes 

 place. We do not possess sufficient data 

 to enable us to calculate the actual distance 

 between (a), (6) and (c)— this will depend 



in a certain measure on the activity of the 

 species of insect concerned and on the ex- 

 istence or absence of special local attrac- 

 tions; but this fact does not discredit the 

 general principles involved. 



One case has not yet been considered, 

 namely, that in which there exists only a 

 single feeding place in the whole tract of 

 country— such, for instance, as a single, 

 house or group of houses situated in the 

 midst of deserted swamps. In such a case 

 the insects may be compelled to come from 

 considerable distances— from as far as their 

 senses are capable of guiding them— in 

 search of food; and drainage operations 

 carried on with a view to relieving such a 

 house may, for all we know, have to be 

 extended over miles. But such cases are 

 not of great consequence, because drainage 

 is seldom the appropriate measure for iso- 

 lated dwellings, which can generally be 

 protected at far less cost by means of gauze 

 screens. Moreover, it is very doubtful 

 whether feeding places for mosquitoes are 

 ever so solitary as the case assumes. "Where 

 there is one dwelling there are generally 

 many, scattered at various distances over 

 the country; and the insects are known to 

 feed on cattle, birds and other animals. 

 For towns, where anti-mosquito measures 

 are most demanded, our first assumed con- 

 dition of uniform attractiveness must, as a 

 rule, be the one in force ; and in such cases 

 the centripetal law will hold. 



The effect of wind required examination. 

 Theoretically, if the insects are supposed 

 always to remain on the wing, wind blow- 

 ing on a generating pool will merely have 

 the effect of drifting the whole brood to a 

 certain extent in one direction without 

 changing the relative positions of the in- 

 sects to each other. The result would be 

 the same as indicated in Diagram I., except 

 that the generating pool would now be ec- 

 centric. If a proportion of the insects 



