26 Proceedings of the Royal Irish Academtj. 



Temperatuke ok Surface "Water. 



1910— Feby. 23 ; March 5, 4-4° ; March 17, 5-5° ; March 30, 7-2= ; 



April 13, 7-7° ; April 27, 8-6° ; May 11, 9-5° ; . May 25, 14-3° ; 



Jirne 10, 15-0° ; July 13, 20-0° ; July 27, 16-5' ; Aug., 17-3° ; 



Sept. 8, 17-5° ; Sept. 29, 14-5° ; Oct. 21, 12=^ ; ^ox. 11, 7-5° ; 



Dec. 1, 5-0= ; Jan. 12, 4-6°. 1911— Feby. 3, 5-0°. 



Probably the summer temperature always exceeds 18° C. The high 

 summer temperature is probably caused in the same way as that of the 

 Danish lakes, by the broad littoral zone, where the water is heated up, and 

 then distributed by currents. 



The colour of the water of lakes is afl'ected by four factors. First, we have 

 the natiural colour of pure water, due to selective absorption of the colours 

 towards the red end of the spectrum and the reflection of blue rays. Secondly, 

 there is the reflection of other rays of the spectrum by the bottom in shallow 

 waters, and by mineral particles in suspension. Thirdly, it is affected by the 

 plankton; and fourthly, bj' matter in solution. The colour is, to a large 

 extent, therefore bound up with transparency. We were not able to make any 

 determinations of the traiisparency of Lough Neagh, with either white screens 

 or photographic methods. The water is, however, but slightly transparent ; 

 and the white plankton net disappeared at a depth of a few feet. The colour 

 of the water is most marked. This is due to the large quantity of plankton 

 present : and in this respect Lough Xeagh presents the same contrast to the 

 Scottish lakes as do those of Denmark. 



The colour of the Scottish lakes appears to vary but little with the 

 seasons, and never to show the turbid yeUowish-green colour characteristic of 

 the Danish lakes between May and Xovember. Fmthermore, they are but 

 rarely covered with the " water-bloom " produced by the blue-green algae. 



Lough ISTeagh, like the Danish lakes, is characterized by the yeUowish-green 

 colour due to the plankton. In the spring of 1910, the colour of the water 

 was a dark olive green in March, but very much more blue in April ; and the 

 plankton catches in April were deep blue in colour. In Llay the colour of 

 the plankton attained the yellowish-green hue, and became almost yclloic in 

 July. 



Water-bloom was observed in Lough Neagh on March 30th, 1910, on an 

 extremely calm day after several days' brilliant sunshine. This is, however, 

 an early appearance. The greatest formation of water-bloom occurred in 

 September. It was observed by one of us then on September Sth, but had 

 been present for three days before this ^isit. On this occasion the surface of 



