156 William J. Hodgetts 
Station) 1 . The average temperature-conditions of the air in the 
neighbourhood of the pond probably did not differ in any important 
degree from those which obtained at the Birmingham Observatory. 
The sunshine-data (the daily mean, in hours, of bright sunshine 
for each month) are plotted in the upper part of Fig. 2, while the 
monthly rainfall (in mms.) is represented in the upper part of Fig. 3, 
the records in both cases also being taken from the Monthly Weather 
Reports (Birmingham Station). These curves will frequently be 
referred to in the detailed discussion of the occurrence of the various 
algal species which will follow, but several noteworthy points in 
connection with the weather during the period of observation may 
be mentioned here. The years 1918 and 1919 were fairly normal, but 
1920 was a very abnormal year. The summer of 1920 was characterised 
by a low temperature, and it was deficient in sunshine; while the 
winter of 1919-1920 was very mild. In the case of each of the three 
complete years the sunniest month was May, and the dullest month 
December. 
IV. CONCENTRATION OF THE WATER 
With regard to the variations of the water-level of the pond, 
shown in Fig. 3, the information was obtained from a convenient 
wooden stake, driven into the bottom of the pond at a short distance 
from the margin. When the water was at its maximum height a 
definite length of the stake projected above water, while even at 
the lowest ebb noted during the period of observation the stake was 
never completely out of the water, so that by measuring the length 
of stake above the water, each time the pond was visited, a record 
of the fluctuation of the water-level was obtained. Of course this 
method gives far more exact information than vague terms such as 
“high,” “ rather low,” etc., such as several workers in this connection 
have used. 
It was recognised at the commencement that periodical chemical 
analysis of the water of the pond would have given data probably 
of great value in elucidating the various factors controlling the 
development of the various algal species, but at the time the work 
was commenced the writer was not in a position to carry out complete 
chemical analyses. As a substitute the total amount of dissolved 
matter in the water was determined each month (from June 1918). 
This was obtained by carefully evaporating to dryness, in a porcelain 
evaporating dish of known weight, 100 c.c. of the filtered pond-water 
1 Monthly Weather Report of the Meteorological Office, London. 
