100 Transactions of the South African Philosophical Society. 



obliged to Mr. Stewart for this calculation, for it enables those who 

 are not familiar with the internal condition and state of a south-east 

 cloud to recognise at a glance what an enormous surface a bunch of 

 reeds only one foot high exposes to the passing cloud. When, how- 

 ever, Mr. Stewart goes a step further, and divides the total quantity 

 of moisture collected in the gauge by the number of vertical square 

 inches found by him, and gives the factor obtained in this way as 

 the true deposit of moisture per square inch of the gauge, I fail to 

 comprehend his reasoning. 



My 5-inch gauge carried twenty-four vertical wires and reeds, 

 corresponding to 176 reeds per square foot of ground. If we find 

 that a layer of water 1 inch thick has been deposited on a piece of 

 ground 1 square foot in extent, we must conclude that the plants 

 growing on that patch have this quantity of water at their disposal, 

 the usual losses, of course, being left out of account. What could it 

 matter, as far as the recording of a quantity of water per square inch 

 is concerned, whether this water was collected directly from rain by 

 the horizontal surface of the ground or indirectly from the clouds 

 by means of the 176 vertical reeds standing on this area ? 



There are several other misunderstandings in Mr. Stewart's paper 

 which would require adjustment, but I fear that this w T ould take up 

 too much of the time of the Society. I must, however, refer to a 

 source of error which has not been mentioned as yet. 



During my earlier experiments I had assumed that the difference 

 between the records of the two gauges, viz., the ordinary rain-gauge 

 and the reed-gauge, represented only moisture captured by the reeds, 

 exclusive of all rain. From the daily readings of the gauges at the 

 Woodhead reservoir it is, however, apparent, that during rain the 

 gauge with the reeds collects much more water than the open one. 

 There is no uniform ratio between the two, the gauge with the 

 reeds showing during ordinary rain from three to four times as much 

 as the other one; but during "misty" rain, when the open gauge 

 records sometimes only -j-J^ or -^ of an inch, the capture of the 

 reeds was ten or even twelve times as much. 



This variation of the ratio shows how effective the vegetation is in 

 comparison with the bare ground or rocks in capturing moisture 

 apart from the real rain, but it will be impossible to ascertain exactly 

 how much be due to the one source and how much to the other. As 

 these experiments were never intended to give accurate quantitative 

 returns which could be entered alongside the rainfall records, but 

 merely to demonstrate that much more water was deposited on the 

 mountains than the ordinary rain-gauges indicated, especially in 

 summer, it will not matter if we exclude all the periods during which 



