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XX. Intelligence and Miscellaneous Articles. 



CONTRIBUTIONS TO THE KNOWLEDGE OF TROPICAL RAIN. 



BY PROF. T. WIESENER. 



A N incentive to this series of researches carried out in Buiten- 



-V-*- zorg, Java, in the winter 1893-94, was the question as to the 



direct mechanical effect of tropical rain on plants, on which subject 



entirely erroneous views are current. 



The author first determined the rainfall per second, and found 

 the highest value to be 0*04 millhn. If a rain of such intensity 

 had continued, within a day it would almost have equalled a year's 

 fall at Buitenzorg. 



The masses of rain in tropics even in the heaviest falls are very 

 small in comparison with those from the rose of an ordinary 

 watering-pot. The former are to the latter as 1 : 25 up to 100. 



From the heaviest rainfalls and the smallest number of drops 

 falling on a surface of 100 sq. cm. in a second, the weight of the 

 greatest possible raindrop was calculated at 0*4 gramme. This 

 number is far too great, for the largest drops which can be made 

 (of 0*25-0*26 gramme) break up at a height of fall of over 5 metres 

 into a larger one weighing 0*2 gramme aud into several smaller 

 drops. The weight of the largest raindrop in Buitenzorg, mea- 

 sured by the method of absorption, is still smaller, amounting 

 namely to only 0*16 gr. 



Experiments by the author on free fall have shown that drops 

 of water of 0*01-0*26 gramme, at heights of more than 5-10 

 metres fall with approximately uniform velocity of about 7 metres 

 in a second. The acceleration is therefore very soon after the 

 commencement of the fall almost entirely compensated by the 

 resistance of the air. 



The vis viva of the heaviest drops of rain calculated by the 

 formula 



¥ 



amounts to 0*004 kgmetre. In heavy falls no doubt several drops 

 fall in rapid succession on a leaf — 2-6 large drops per square centi- 

 metre per second, — but the impact of each falling drop is diminished 

 by the elastic attachment of the leaf to the stem. 



It follows from the experiments, that the force which tropical 

 rain falling in a still atmosphere can exert is far too small to 

 cause injury to vegetation. The mechanical action of the strongest 

 tropical rain on plants is seen in a violent agitation of the leaves 

 and twigs. Injuries only occur in isolated cases in the more 

 tender parts of the plants which cannot give way to the impact, 

 for instance on the tender shoots of tobacco when they lie on 

 a coarse soil consisting of angular particles of earth and sand. 



