120 SCIENCE IN AFRICA 



been felt for some index of the amount of water available for 

 plants at different times and places. Various indices have been 

 suggested for this purpose. R. Lang in 1915 proposed a 'rain 

 factor' depending on precipitation and temperature; in 1926 de 

 Martonne proposed and used as an 'indice d'aridite', precipita- 

 tion (in millimetres) divided by temperature (in degrees Centi- 

 grade plus 10) ; both of these may be criticized in that temperature 

 is not the sole factor controlling evaporation, since humidity and 

 wind-rate are also very important. A. Meyer in 1926 suggested 

 precipitation divided by saturation-deficit, but this could only be 

 used in places where records of humidity are taken frequently, or 

 are obtainable from recording instruments. A better index, where 

 the evaporation rate is obtainable, is precipitation divided by 

 evaporation, a ratio which is considered fully by Livingston and 

 Shreve (192 1). A similar index is proposed and used for French 

 West Africa by R. Porteres (1934). He terms this the 'indice de 

 secheresse' which is 'hauteur d'eau en mm. au moment ou nous 

 percevrons sur A (i.e. the plant in question) le seuil de secheresse'. 

 Rates of evaporation are also relevant to hydrological studies, 

 particularly in the region of the great lakes, on most of which 

 evaporation from the surface is nearly equal to or even greater than 

 the sum of rainfall on their surfaces and the water received from 

 affluent rivers. Some uniform system for recording evaporation is 

 therefore a matter of importance. 



METEOROLOGY AND MEDICINE 



Meteorological studies are relevant to medical work from two 

 points of view. The first is concerned with the effects of climate on 

 such animals as mosquitoes, tsetse flies, and ticks, which convey 

 disease. These are the effects which are normally implied in the 

 expressions 'good climate' and 'bad climate' in the tropics. In 

 this branch of the subject, known as insect ecology, numerous 

 special studies are being made, showing how closely the carriers of 

 disease depend on relatively minute changes in their meteorological 

 environment, especially perhaps on humidity. 



Secondly, there are the direct effects of meteorological conditions 

 on the human system; about these relatively little is known, even 



