466 SCIENCE PROGRESS 



covering up the red circle belonging to any particular water-place with a 

 perforated disc or " washer " varying in colour according to the information 

 received. Thus, a white covering washer indicates that the corresponding 

 water-place is dry ; a hlack washer, that the water has been found free from 

 larvcB for no apparent reason ; a yellow washer, that larvcB are absent and 

 that some of their natural enemies, such as larva-eating fish, are present ; a 

 green washer, t):\di.t paraffining operations have been carried out (Fig. 2). 



Whenever a water-place is paraffined, its register number is entered in 

 a special almanac, under a date three weeks ahead. The almanac is inspected 

 every day and the green washers surrounding those register numbers which 

 appear under the current date are removed from the map. The water- 

 place is then shown as " dangerous " again until further information is 

 forthcoming.^ 



Anti-mosquito measures may be summarised under the two headings of 

 " prevention " and " cure," and the methods available were fully described in 

 the general circular. It was first pointed out that the breeding of mosquitoes 

 in the proximity of a dwelling-house could be prevented altogether by abolishing 

 collections of standing water and by covering over water-butts and tanks in 

 which mosquito eggs might otherwise be laid. Attention was then drawn 

 to the ease with which larvae, if found, could be destroyed, either by throwing 

 the water away on the ground or by pouring a spoonful of paraffin on its 

 surface. It was also pointed out that, since the paraffin did not mix with the 

 water, the latter could be drawn off unchanged as required, either by a siphon 

 or by a spigot inserted in the lower part of the butt. 



Turning now to what may be called " outside " operations, these are 

 again separable into the categories of prevention (chiefly exemplified by 

 drainage) and cure, namely, destruction of larvae. In cases like the present, 

 draining to any appreciable extent is out of the question owing to the financial 

 limitations of a newly-formed organisation, and attention must necessarily 

 be limited to the question of larvicides. Here, again, expense is the deciding 

 factor, since the cost of the larvicidal operations must be reduced to the mini- 

 mum figure consistent with success. Whatever be the larvicide selected for 

 the work, it must obviously be used with the greatest possible economy ; in 

 other words, water should never be treated with the larvicide unless the presence 

 of larvcB has been actually observed. The application of this principle at 

 Hayling, by means of the system of " inspecting and recording " described 

 above, has proved very successful, and it is probably not too much to say that 

 it is of fundamental importance in all cases where expense has to be considered. 



As previously mentioned, larvae may either be suffocated by a film of 

 oil or poisoned by diffusing some suitable substance in the water. In the 

 first method, as might be expected, many varieties and grades of oil have 

 been successfully employed, the usual practice being to select some kind which 

 is both cheap and easy to obtain in the district concerned. In England, 



1 The control map actually in use is built up of the requisite sections of 

 the TTTinr scale Ordnance Survey (25-344 inches to the mile) which is published 

 in sheets measuring about 38 by 25 inches. The separate sheets are mounted 

 on millboard |-inch thick, stiffened at the back by horizontal wooden strips 

 which are grooved in such a manner that any number of sections may be 

 rapidly " assembled " upon a light vertical framework attached to the wall. 

 For " covering up," thin steel washers (easily obtainable of the dimensions 

 quoted) are painted and suspended on minute brass " L " hooks screwed 

 into the map at the topmost points of the " register number " circles. The 

 washers are very conveniently picked up and placed in position by means of 

 a magnet. 



