Mr. Thompson on an Alga which colours Bally drain Lake. 77 



the water was two feet deep, the bottom could be seen ; in 

 others it was invisible at one fourth of this depth : at the lee- 

 ward and windward sides there was but little difference, ex- 

 cept that at the latter it was occasionally observed to give a 

 pale green tinge to the surface, where the water circling gently 

 in, congregated it together and threw it thence in a broken 

 cloudy form for a moment, when it was again dispersed: 

 since the 25th I should say that it has increased by at least 

 one third. The lake has at first sight the appearance which 

 Dr. Drummond ascribed to Glaslough, of being " greened" by 

 the reflection of trees : from eminences at some little distance 

 the green tinge of the water is most conspicuous, and parti- 

 cularly so at the further side. In some places the colour is of 

 a pale dull green, in others greenish brown; thus imparting to 

 this fine sheet of water, in place of its wonted appearance of 

 coolness and freshness, the dull dead aspect of a Dutch canal. 

 When viewed at the distance of a few paces from the margin 

 the plant is apparent during sunshine, as it Hkewise is at such 

 times in water lifted in the hand*. 



Sept. 16. — I again went to the lake, and found it to be much 

 more densely coloured than on the 30th of July. The day 

 being perfectly calm, the surface of the water was covered to 

 some extent, where the depth appeared to be about five or six 

 feet, with an alga of a pale but rich green hue. When atten- 

 tively observed it was seen moving in currents presenting the 

 form of what is technically called "the feather" in the most 

 admired mahogany, now moving round a centre or " knot," 

 and again diverging from it rapidly and in the most graceful 

 forms, the water appearing through the moving masses of the 

 plant so as to take the place of the darkest hues and knots in 

 the wood. It was perhaps an appearance similar to this, that 

 MM. Engelhardt and Treschel have described the Oscillatoria 

 rubescens, — a minute alga which tinges with a red colour the 

 lake of Morat in Switzerland, — occasionally to assume, when, 



• A quantity of the alga brought home today in a phial of water remahied 

 scattered through it for twelve hours ; in twenty-four it had all risen to the 

 surface : in another instance it on the third day covered the bottom of th» 

 vessel. 



