SECTIONAL TRANSACTIONS.— A. 311 



or ' mean ' being reserved for time averages. It is the general rainfall of 

 which a knowledge is required in such studies as that of Capt. McClean on 

 river flow. The procedure adopted by the B.R.O. in estimating general 

 rainfall consists in measuring, on a rainfall map, by means of a planimeter, 

 the areas included between successive isohyetal lines and the boundaries of 

 the drainage area, and then working the space-average of the fall per unit 

 area. Thus if an area a square miles, within the drainage area, is included 

 between the isohyets r + Sr inches and r — Sr inches and A is the whole 

 area, the general rainfall is given by the expression Sflr/A inches. The 

 accuracy of the result depends simply on the accuracy of the isohyetal lines, 

 and this again depends on the number of rain-gauges and their distribution. 

 In regions where gauges are relatively numerous, monthly evaluations of 

 general rainfall over drainage areas of moderate size can be made with fair 

 precision. 



As an illustration of the part that can be played by the B.R.O. in studies 

 on water level and river flow, monthly maps of rainfall over the Tees 

 drainage area (prepared for the Department of Scientific and Industrial 

 Research in connection with the biological and chemical survey of the 

 river Tees) will be shown, with evaluations of general rainfall over the 

 sections of the watershed concerned in the level measurements. 



Mr. C. Clemesha Smith. — Stream-flow measurement. 



Every self-respecting business concern keeps strict account of its assets 

 or resources. Water is a national asset of paramount importance — the 

 available supply is limited and the demand is always increasing. 



In order that the claims of the various users may be considered and met, 

 it is necessary that the quantities available should be known. 



Gauging of streams and rivers is the only accurate method of obtaining 

 the requisite knowledge. 



Gauging must be carefully organised so as to cover the whole country 

 if water problems relating to domestic supply, trade supply, navigation, 

 drainage, flooding, etc., are to be satisfactorily solved. 



The public spirit of Capt. McClean has urged him to initiate an experi- 

 ment which may be the germ of the necessary organisation. 



It should be possible to develop an organisation on the following lines : 

 The division of the British Isles into suitable areas — possibly watersheds : 

 the collection, verification and recording of all existing stream-flow measure- 

 ments. The provision of further stations as circumstances permit or 

 demand. The publication of results in brief. 



The organisation should be elastic, on lines made capable of expansion — 

 use should be made of existing records and observers. The services of 

 rivers boards, drainage boards, navigation companies, corporations and 

 private observers should be used to the fullest extent. 



Such an organisation involves expenditure, but wise expenditure is true 

 economy. 



Mr. W. J. M. Menzies. 



On water flow, of primary importance to salmon, sea trout and trout 

 fisheries, depends the ascent of migratory fish and the health, and indeed 

 very life, of all fish. 



Exact knowledge of actual flow is of great practical importance in deter- 

 mining water available for power or for domestic purposes, in fixing the 

 regulated flow- allowed from diversion schemes and in computing dilution 

 of polluting eflBuents. 



