MEASUREMF^.NT OF RAINFALL. 8.? 



relation of rainfall to time — li.:., intensity— the more valuable 

 become such data. 



Jn the Transvaal it is not uncommon to receive a precipita- 

 tion of one-third of an inch in lifteen minutes, without further 

 rain for the twenty-four hours ; hence, by the ordinary rain-jjauge 

 measurement, which is read once a day, such a shower would 

 be credited as a day's rain, and would sug^g'est, without further 

 explanation, a beneficial rainfall, but bv the automatic g'aug'e the 

 ciiart would show the true character of the rainfall. 



The soil obviously takes a much longer time to absorb rain 

 than the latter takes to fall; consequently, one-third of an inch 

 precipitated durinic^ twenty- four liours is much more beneficial to 

 the country than one-third of an inch, in fifteen minutes, which 

 represents an intensity of 1.32 inch per hour. For these reasons 

 the automatic rain gaug'c is much more valuable for a correct 

 study of rainfall than the ordinary gau^e, and when calibrated 

 occasionally with, some standard instrument, gives far more re- 

 liable data, because of the elimination of the personal equation. 



At i^resent time, in the Transvaal Province, there are over 

 fXDO ordinarv gauc^es of the " Tnrlian " pattern in use. Of the 

 automatic rain gauges there are some forty whose charts are 

 divided into two-hour sections, and are changed every week. 

 One automatic instrument (and one only, as far as can be ascer- 

 tained ) of the same type, whose chart is changed every twenty- 

 four hours and whose sections read for single hours, ; and there 

 is also one Halliwell Automatic Rain fiauge. The former are 

 manufactured by Richards, of France. 



The ITalliwell gauge is an extremely useful instrument, for 

 the reason that its charts are large, and. consequentlv, the mark- 

 ing is easilv read to single minutes. Its fine adjustment, too, 

 ensures maximum accuracv in working; but it requires handling' 

 In' trained men. and. as the chart has to be changed everv twelve 

 hours, renders the instrument onl\- suitable for important meteor- 

 oloi2^ical stations. 



The simplicity in constructicn and management of the 8-day 

 and 24-hom' auto gauge by Richards recommend them for more 

 extensive use ; both instruments are identical, except that the 

 clock gearing- is adjusted for one or the other period of charts, 

 rmd the cost is practically the same. 



The only advantage that can be urged for adopting the 

 weekly geared instrument, in preference to the daily one. is the 

 time saved in attendance ; this, however, is a small matter, and is 

 mainly a question of habit. 



On the other hand, the weekl}- chart has an important dis- 

 advantage, and that is, if the pen has to travel more than once 

 ])er hour up the chart — vis., to record a rainfall greater than 

 0.40 inches per hour — then it is almost impossible to obtain even 

 an approximate idea of the character of such rainfall from the 

 chart, because the pen markings overlap and obliterate each 

 other, whereas the markings on the twenty-fcur-hour chart can 

 easily be read by means of a tracing cloth template to every five- 



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