1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1105 
was reported me from the Thingvallavatn, even though the mean 
tp. of the air at the nearest coast station in the months January, 
February, and March was — 1*6 to — 2, and at the inland station 
- 1’9 to -0*5. What may be the cause of this Ido not know; 
probably the tp. of the winter months given me from Thing- 
vallavatn have not been fully correct. All in all, we arrive 
through the observations made at the following facts : — 
The highest tp. of the lake during the period of observations is 
11° C., which was reached on 14th August 1902. Then the tp. 
from 14th August to 31st October falls to 5° C. and keeps this tp. 
to 14th December, when the tp. further falls to 1° C. (16th 
March 1903), not rising until 16th April ; already (30th June) it 
has reached 8J- above zero. During the whole year the lake has 
not once been frozen over. Besides, it appears that the tp. of the 
lake is rather slow in following the variations in the tp. of the 
atmosphere ; of course this must be ascribed to the considerable 
depth of the lake. As I wished to know whether or not it was a 
rare case that the lake had not been frozen, I asked Mr 
Soemundsson to inform me as to this point, but as yet no 
reliable information has been obtained. 
PROC. ROY. SOC. EDIN. — YOL. XXV. 
70 
