DISCOVERY 



101 



Also in the new instrument the pen-arm is counter- 

 poised to obviate the effect of friction. Possible 

 friction between pen and chart is also eliminated by 



Fig. 7.— SECTION.^l, VIEW OF THE HYETOGRAPH. 



the use of the swinging pen-arm. The pen rests 

 lightly by its owti weight on the chart. This ensures 

 great clearness of writing. 



A more expensive form of Self-Recording Rain- 

 gauge is that known as the " Femley " (Fig. 8), designed 

 at the request and under the supervision of Mr. Joseph 

 Baxendell, the Director of the Femley Observatory, 

 Southport. This gauge ma\' be classed with the 

 Hyetograph as a first-class Recording Rain-gauge. 



In this form of self-recording gauge the rain-water 

 passes from the funnel into the float-chamber and 

 causes the float to rise as far as the top of the chart, 

 this representing i inch of rainfall. .\t this point 

 the water siphons away, the float falling to zero in 

 six seconds, ready to commence a fresh trace. The 

 siphon is actuated by means of a tilting bucket. As 

 soon as the float and pen reach their highest point, the 

 bucket overturns and the water pours through a pipe. 

 In this way the reservoir is quickly emptied. 



Having now outlined the principles and mechanism 

 of the two most important forms of self-recording 

 pluviometers, we will proceed to show how they can 

 be utilised in the pursuit of our knowledge of the cause 

 and effect of precipitation. 



On May 15, 1907, Dr. Hugh Robert Mill, then 



President of the Royal Meteorological Society, read a 

 paper on "The Best Form of Rain-gauge."' In this 

 paper he stated that the three best forms of ordinary 

 rain-gauge are the " Snowdon," the " Bradford," and 

 the " Meteorological Office " patterns, and later (in 

 1908) he gave his unqualified approval of the " Hyeto- 

 graph." It will be seen from this that we have 

 now treated of all the most important forms of 

 pluviometer, and it only remains to show the relation 

 existing between observations made with ordinary 

 rain-gauges and self-recording instruments. 



When we measure the rainfall at 9 a.m. in a simple 

 rain-gauge and write down the result against the 

 previous day, as is the usual custom, we know only 

 that a certain amount of rain-water has been caught 

 in the gauge during the past twenty-four hours, but 

 we have no means of discovering the intensity- of the 

 fall at any particular time during the period. 



The self-recording gauge can give us this information. 

 During the whole of the twenty-four hours a continual 

 record is being taken on a revolving clockwork drum, 

 and when we visit the instrument and look at the chart 

 it tells us at once the rate at which rain was falling at 

 any particular time. 



With a gauge such as the Hyetograph it is possible 

 to secure the maximum rate of rainfall during thunder- 

 storms and other heavy downfalls, in a wa}' which 

 would be entirely impossible with an ordinary gauge, 

 and we can also obtain the mean hourly fall for the 

 month or year. 



For some years past in British Rainjall a section has 

 been devoted to heavy falls in short periods, and the 

 discussion of these values would have been impossible 

 without the results obtained by the improved form 

 of self-recording rain-gauge. 



FIO. 6.— FERNIEY SELF-RECORDING RAIXG.WGE- 



In the previous article it was mentioned that a 

 " rain day " consisted of one on which o-oi inch or 



■ Quarterly Journal ol \\\e R. Met. Soc, vol. xxxiii, Xo. 144, 

 p. 265. 



