1142 Proceedings of Royal Society of Edinburgh. [sess. 
on 1st August, but as early as 15th August it has gone down to 8°. 
On 18th September it further falls to 6°, and then rises a little 
(on 1st October it is 7° C.) ; but then it falls rapidly, and on 24th 
October the lake is already icebound. It was frozen over the rest 
of the year, with the exception of a short period from 7th to 
10th November, in which the lake was again open, but at the time 
when these observations were finished (2nd April) it was not once 
open. Hence we learn that the lake, in the year when the 
investigations were carried on, was only free from ice 152 days (£) 
of the year, being icebound 213 days (f). Further, we see that 
the water of the lake follows the variations in the tp. of the air 
extremely quickly, owing to which the lake is able to reach the 
relatively high tp. of 12’ C. in a period of but 45 days. 
Owing to the hot springs, the tp. in the bays of the lake may 
often rise to 15°-20° C. (Thoroddsen) at an air tp. of only 10° C. 
Monthly Mean tp. of the Air at 
Ahureyvi and Modrudalur. 
1903-1904. 
Akureyri. 
Modrudalur. 
i 
March, 
-4*1 
... 
April, 
- 0*8 
- 27 
May, 
47 
3-3 
June, 
9-6 
77 
July, ... 
9-8 
August, . 
5 6 
3-3 
1 September, 
6-9 
4 3 
October, . 
0-5 
-1*1 
November, 
-1-6 
-6-3 
December, 
-0’4 
-3'6 
January, 
-2-3 
-6-3 
February, 
-2-9 
-6-9 
March, 
-2*3 
-6-6 
2. Plankton. 
It is the first time a regular plankton exploration has been* 
established in a lake which is ice-covered for more than seven 
months, and it must, according to my opinion, be of great interest 
to receive some information concerning the plankton of such a 
lake. 
Owing to the nature of the lake in question, the results of the 
