98 Transactions of the South African Philosophical Society. 



out that I did not look upon them as representing a corresponding 

 rainfall. 



The main objection raised against the method of measurement 

 employed was that an isolated group of reeds might capture such a 

 quantity of moisture, but that others standing behind the first one or 

 the first row would not be able to do so, owing to the front row acting 

 as a screen and straining the moisture out of the passing cloud — 

 that, in fact, there would not have been sufficient moisture left to 

 supply the reeds in the rear. 



The Society having decided that further experiments were desirable, 

 the Council requested me to undertake them, voting at the same time 

 the necessary funds for the acquisition of additional gauges. With 

 the help of some friends and the assistance of the city water-engineer, 

 Mr. Wynne-Eoberts, and a member of his staff, I was enabled to 

 continue the observations for two years ; hence I think that it may 

 interest some members of the Society to hear what has been the 

 result of this work. 



Before stating these results I should like to refer briefly to the 

 main objection mentioned above, and to see what an amount of 

 moisture would be available in a south-east cloud. In order to 

 simplify the calculations, I beg leave to employ metric measures, 

 and I shall put the height of the mountain, roughly, at 1,000 m., 

 the exact figure being 1,082 m. The lower limit of the cloud varies, 

 of course, considerably ; but it may be put at an average of 600 m. 

 above sea-level, which means that each layer of the cloud, from the 

 time it is formed until it reaches the summit, has to ascend 400 m. 



As the air at the lower limit of the cloud is saturated with water 

 vapour, apart from the water in liquid form which is suspended in 

 the air and forms the cloud itself, and as such air with an initial 

 temperature of, say, 20° C. reduces its temperature by 0-44° C. for 

 every 100 m., the total decrease of temperature during the rising 

 from the 600- to the 1,000-m. level would be 1-72° C* 



If the temperature of fully saturated air of 20° C. be reduced by 

 one degree, a condensation of 0*98 grammes of water will take place 

 in every cubic metre of such air; hence in our case T68 grammes of 

 additional water — above that which originally formed the cloud — 

 would be floating in every cubic metre of air on its arrival at the 

 eastern edge of the top of the mountain. 



The velocity of the south-east wind has been found occasionally 

 to exceed 40 miles an hour ; but let us take a velocity of 20 miles 



* These calculations are based upon Hann's "Lehrbuch der Meteorologie," 

 pp. 240, 241. 



