INLAND WATER SURVEY 397 
found in the Book of Normals, Section I. Section II contains weekly, 
monthly, quarterly, half-yearly, seasonal and annual normals for Districts. 
Section III contains small-scale maps showing the monthly and annual 
average distribution of rainfall over the British Isles. Section IV includes 
tables showing the range of variation of rainfall and of the number of 
rain-days at twenty-eight stations. 
Mention should also be made of the Rainfall Atlas of the British Isles, 
published by the Royal Meteorological Society. This contains monthly 
and annual maps of average rainfall, two maps showing the distribution of 
rainfall in the wettest year (1872) and the driest year (1887), and a series 
of maps showing the distribution of rainfall as a percentage of the normal 
in each year from 1868 to 1923. ‘The Introduction, by Dr. H. R. Mill, 
gives the history of the British Rainfall Organization and contains a valuable 
discussion of the data represented in the charts. 
AVERAGES. 
The standard period for which averages are calculated is 1881-1915. 
One of the aims of the Organization is to produce a map of Annual Rainfall 
for the whole of the British Isles, on the scale 2 miles = 1 in. The 
importance of completing this map was stressed by the Water Power 
Resources Committee in paragraph 276 of its Final Report. The position 
(December 1931) is now as follows :— 
England and Wales 1881-1915 (Rainfall Survey) maps completed for 
about two-thirds of the area; work now in progress on Cumberland, 
Leicestershire, Rutland, Gloucestershire, Bucks, Berks, Herts, Middlesex, 
London and Cornwall. Remainder of area already mapped by Dr. H. R. 
Mill, but not for standard period. 
Scotland 1881-1915 maps completed for about half the area. Of the 
remainder about half has already been mapped, but not for standard period. 
In drawing these maps all available information is used, averages for 
stations with short records being weighted to the standard period by com- 
parison with nearby stations having standard averages. The averages so 
computed are included in Part III of British Rainfall. Reduced copies 
of specimen maps and a description of the method used in their preparation 
are given in articles published in Part IV of British Rainfall, 1928 and 1929. 
It should be emphasised that the Rainfall Survey maps are not regarded 
as representing the ‘ last word’ on the subject of the rainfall of a specific 
small area. If required to produce a map, e.g. for the purpose of a water 
supply scheme, the British Rainfall Organization would prefer to go over 
the ground again, and in its final form the map might differ slightly from 
the original Survey map, where the lines are naturally somewhat generalised. 
Ad hoc maps for such purposes can usually be prepared at short notice even 
for areas not yet covered by the Survey. 
RUN-OFF. 
The British Rainfall Organization is not, in present circumstances, 
officially concerned with run-off. The volumes of British Rainfall contain, 
however, certain data which are related to this aspect of hydrology. Each 
volume contains a chapter on evaporation and percolation records. British 
Rainfall, 1931, contains values of rainfall, percolation and calculated 
evaporation at four stations, and of evaporation measured from a free 
water surface in a tank at twelve stations. 
