1900-1901.] Prof. Letts and Mr Hawthorne on Ulva latissima. 293 
undoubtedly been increased since the introduction of the main 
drainage scheme, the Lagan river no longer acting as a settling- 
tank in which the bulk of the sewage solids were deposited. 
The tides in the upper reaches of the Lough are sluggish, and 
from float experiments made by the engineer to the Harbour 
Board, it would seem that the greater part of the sewage does not 
make its way out of the Lough on the ebb-tide, but having drifted 
a certain distance seawards, is washed backwards by the flood- 
tide in a bifurcating stream, which distributes it over a wide area. 
In Dublin Bay the conditions under which Ulva latissima 
occurs in quantity are both interesting and significant. 
Broadly speaking, the upper reaches of the Bay are divided 
artificially into two portions by the so-called Pigeon House wall, 
which extends for more than a mile and a half in an easterly 
direction, and terminates in Poolbeg lighthouse. A second 
wall, called the North Bull wall, juts out from the northern shore 
of the Bay at Dollymount, and extends in a S.E. direction to 
within about 1000 feet of Pool Beg lighthouse, terminating in a 
second lighthouse called the Bull. The northern part of the 
Bay thus almost enclosed by the two 'walls forms the harbour. 
On the other hand, the southern portion of the Bay is quite open. 
The harbour receives not only the waters of the Liffey river 
into which the major portion of the city sewage at present flows, 
but also those of the Tolka river, which is polluted by a large 
sewer running into it close to its mouth, while another large 
sewer discharges directly on to the northern shore close to the city, 
as well as a considerable number of smaller sewers the whole way 
thence to Dollymount. 
On the other hand, no sewers of any magnitude (if indeed any 
at all ? ) discharge their contents into the southern portion of the 
Bay until Blackrock and Kingstown are reached, which are quite 
at its mouth. Thus, broadly speaking, the northern portion of 
Dublin Bay consists of a polluted area, while the southern portion 
is unpolluted. Now, plenty of the ulva is found on the northern 
shores of the harbour, and is washed up along the Clontarf fore- 
shore, where, as in Belfast Lough, it rapidly putrefies in warm 
weather, and gives rise to a nuisance. On the other hand, the 
southern portions of the Bay seem to be quite clear of the sea- 
