380 REPORTS ON THE STATE OF SCIENCE, ETC. 
represents the integrated effect of the drainage over the whole catchment 
area above the gauge point, we must assume that changes of level in the 
river will be related to the general rainfall over the area, and the scheme of 
operations must be planned in such a way that the general rainfall can 
readily be evaluated as a matter of routine. 
It may be remarked, in passing, that at any given season the fraction 
of the rainfall, incident on a given area, that finds its way into a stream 
depends on the geological characteristics of the area. This is of course 
quite well known, and the object of mentioning it here is to point out 
that in areas where there are wide variations of geological structure it may 
be necessary to make separate evaluations of the rainfall for subdivisions 
of the area based on geological considerations. This would be a point 
to bear in mind when selecting sites for rain gauges. 
The next point that arises is the question as to how often it is necessary 
to read the gauges. For certain purposes, particularly the investigation of 
flood levels, it would be necessary to have continuous records of rainfall 
furnished by recording gauges, at least at a few points within the catchment 
area, but we must assume that the majority of gauges will continue to be 
ordinary gauges of the type in which rain is collected in a receiver and 
read by eye in a graduated measure. Hitherto it has been customary, in 
systematic studies of rainfall and run-off, to deal with calendar monthly 
values. That is, for example, the method adopted by the Thames 
Conservancy, whose reports show in parallel columns the total discharge in 
the month over Teddington Weir, and the general rainfall in that month 
in the Thames Basin above Teddington Weir. If that scheme were 
standardised as the normal procedure under the Inland Water Survey 
Organisation, it would suffice, therefore, if monthly totals of rainfall were 
available for all stations. In practice daily readings of rainfall are made 
by the majority of observers contributing returns to the British Rainfall 
Organization, the monthly totals being arrived at by summation of the 
daily readings. At certain stations, particularly in remote spots on 
mountains or moors, it is impracticable to read the gauge every day and 
the rainfall is, at such stations, normally allowed to accumulate for the 
whole month and then read. ‘There is no evidence to show that a monthly 
total measured directly in this way is not on the whole as reliable as a 
monthly total obtained from separate daily measurements. If the month 
is adhered to as the major unit of time for Inland Water Survey purposes, 
no need arises therefore for any change from existing practice in regard to 
rainfall measurements. 
Captain McClean, however, has on more than one occasion expressed 
the view that a fixed interval such as a month is inappropriate, and that 
for such studies as are contemplated the days should be grouped into 
“rain periods’ and ‘ dry periods.’ In this Memorandum we need only 
consider the pros and cons of that view in so far as they bear on rainfall 
data. From the observing point of view the only stations affected are those 
where gauges have hitherto beenread onlyonceamonth. Captain McClean’s 
proposal involves either reading such gauges daily or making use of some 
form of recorder, the chart of which when removed at the end of the month 
would yield readings foreach day. The latter solution presents considerable 
difficulty, which will be realised when it is remembered that the instrument 
would have to work for a month without attention and would have to be 
fitted with some form of heating device to prevent its being thrown out 
12 At certain stations the gauge is read weekly, fortnightly or at irregular 
intervals, depending on the weather, as well as on the first of each month. % 
