76 Mr. Thompson on an Alga which colours Bally drain La ke. 



the lake today for the purpose of investigating this Alga, for to 

 some minute tribe of this order of plants I was satisfied that 

 the substance noticed did belong, I found that the whole 

 body of water was tinged with a dull faintly glaucous hue. 

 On going out in a boat to ascertain the cause of this appear- 

 ance, I saw that the water was everywhere filled with ex- 

 tremely minute particles, which might be compared to the 

 motes in a sun-beam. To the unassisted eye they seemed as 

 delicate as the finest human hair and of a spiral form : with 

 the aid of a lens they were seen to be a vegetable production. 

 Around the boat, which was stationary, their motion was not 

 very rapid, but those on the mere surface moved in an oppo- 

 site direction from the particles beneath, and the latter the 

 more quickly. Their present appearance, together with the 

 recollection of the floating masses observed last autumn, at 

 once brought to mind the Oscillatoria (Brugescens discovered 

 in 1837 by my friend Dr. J. L. Drummond in Glaslough 

 (county of Monaghan*), and where, as in the present instance, 

 that plant performs a similar part in giving a colour, &c. to 

 the water. Until the microscope be resorted to, there seems 

 indeed the strongest analogy between them, but this instru- 

 ment proves that the alga under consideration does not be- 

 long to the genus Oscillatoria. 



On inquiry from some relatives, whose demesne is situated 

 on the borders of the lake, I learned that the appearance de- 

 scribed had been observed only for the last four or five years, 

 and for about three months in each year : one of my fi-iends 

 had looked upon its approach with dread, as it interfered so 

 much with his angling that during the period of its continu- 

 ance this sport had to be abandoned. Eels, pike, and perch, 

 especially the latter, are abundant in the lake, but when the 

 water is clouded by this plant, the diminution in the number 

 of perch taken is said to be not less than about one to fifty — 

 the difference is attributed to the fish being unable to see the 

 bait. About the 1st of this month I am informed that the 

 water was perfectly clear. 



July 30. — I visited Ballydrain and found the entire lake 

 tinged ^^ ith this plant, but unequally so ; in some parts, where 



* See Annals Nat. Hist. vol. i. 



