138 Report S.A.A. Advancement of Science. 



rarely give an appreciable rain, though the air is unpleasantly damp. 

 Drizzles, lasting two or three days, and yielding anything from zero 

 to one-tenth of an inch per day is the most they give us. During that 

 time the Hygrometer remains very high, and evaporation below the 

 average even with high Avind. This kind of weather is a noted 

 featuie of the Rhodesian climate; no data are, however, available to 

 decide whether it could be traced right to the Indian Ocean or whether 

 it might not be due to rains further South or South-East. These 

 reasons would make me hesitate to accept on the whole for Rhodesia 

 Mr. Sutton's theory of the Cape rains. * 



It will be of interest to compare the other records with the 

 Anemometer sheets. The Barometer, Thermometer, and Hygrometer 

 show a distinct periodic movement, and a comparison with the winds 

 shows a very close synchronism with the wind periods. 



To demonstrate this roughly, ] have drawn up the following 

 table of means for each wind period : — t 



Mean of Mercurial Barometer reading at 8 a.m. reduced to 32* Fahr. : 



Mean Temperature by formula '- '— 



Mean Black Bulb Thermometer readings : 



Mean Relative Humidity by the formula ... — ^^" ^^' 



2. 

 read off a Richard Hair Hygrometer sheet. 



For all these means I have followed the wind periods except now 

 and then for the hygrometer. I found there that the humidity curves 

 have a different appearance with the different winds, but that the 

 change from one period to another does not always coincide exactly 

 with the changes in the other elements. 



There is sometimes a difference of 24 hours, sometimes, on the 

 contrary, the changes in the Hygrometer follow very closely the wind 

 variations. In cases where I found only one day's difference in the 

 periods, I put the day of transition now in one and now in the other 

 period, according to the shape of the curve. The mean relative 

 humidity, calculated in this way, is, of course, liable to be very 

 erroneous in the rainy season ; but I do not want absolute figures, I 

 onlv want to find the wav in which the quantity varies. In the dry 

 season from the beginning of April to the end of October, the mean 

 relative humidity does not differ very much from the mean of the 

 highest and lowest readings of the day. 



The general result of this tabulation may be stated as follows : 

 A period of easterly winds corresponds to a high barometer, low 

 temperature (both air and sun temperature) and a high Relative 

 Humidity. A period of veering wind to a low barometer a high 

 temperature and a low Relative Humidity. 



Out of 84 periods in which there was a daily veering of the winds 

 round the compass 65 fall between two high barometers, 3 between 

 two low barometers, and the others on a rising or falling barometer. 



* At Boroma, near Tete, on the Portuguese Zambezi, latitude 16" North East 

 of us, I find that half the rainfall of 1891-92 is attributed to S.E. winds- 

 I have not yet seen the later rainfall returns. 



t See Appendix. 



