SUMMER DIARRHCEA I 55 



there is no diminution in the number of flies or of deaths. 

 Summing up his observations on rainfall for five years Niven 

 (p. 162) comes to the following conclusions. "Thus heavy rain- 

 falls tend to lower the temperature of the surface, but have less 

 effect on the atmospheric temperature, which may rise in spite 

 of them. They produce a greater effect on flies than they 

 do on temperature, and this effect on flies is reflected on 

 the number of fatal cases commencing, and on the number of 

 deaths in the week but one following. This is not a quite 

 accurate statement for the end of the curve.... There can be 

 no doubt that heavy rainfall exerts on the whole a disastrous 

 influence on the production of flies. This does not occur 

 in rainfalls of 0"8 inches or under, which appear to have 

 the reverse effect, at all events so long as the atmospheric 

 temperature is rising. In the nice balance more or less rainfall 

 over the quarter cannot much matter. In fact, in Manchester 

 no sustained correspondence can be made out between the main 

 rainfall in the third quarter and the number of deaths from 

 diarrhoea. The years of highest rainfall — viz., 1891, 1892, 1893, 

 1895 and 1903 — have all been years of fairly high diarrhoeal 

 fatality; 1907, the year of lowest diarrhoeal fatality, was not 

 a year of exceptionally low rainfall. Even heavy rainfall, it will 

 be seen, does not necessarily exert any unfavourable influence 

 on the development of flies or the extension of diarrhoea. Its 

 doing so will depend entirely whether it is able to lower the 

 surface temperatures below those which are favourable to the 

 development of the larva; or the escape of the imago. If it will 

 fail to effect this, its influence will probably be in the opposite 

 direction, owing to the great need of moisture for the develop- 

 ment of the larvae, and, one may add, for the health of the 

 flies." 



The course of the 4 ft. thermometer corresponds more closely 

 to the curve of diarrhoea deaths than any other ' varying seasonal 

 fact.' Its course corresponds generally to the excess of heat 

 entering over heat leaving the surface the week before. Many 

 of the breeding grounds of the larva; are greatly influenced by 

 the temperature of the soil, and flies in their development and 

 reproduction respond to all the influences which affect the 4 ft. 



