746 Proceedings of Royal Society of Edinburgh. [sess. 
about half a mile off the Laboratory. There the channel is about 
15 to 20 fathoms deep, and the flood-tide runs about 2 to 3 knots 
an hour. The strength of the tide was sufficient to keep the net 
well expanded and stretched out from the boat at anchor. So far 
as time and weather permitted, the tow-netting was done on the 
flood-tide, between half flood and high water. Although the tide 
runs fast, it alone is not sufficient to carry the pelagic animals up 
the firth from the south. The wind is the most important agent, 
for moving the plankton over long distances, and a secondary 
agent is the slow movement of water due to variation in tempera- 
ture and density. 
A shoal of Pelagia , to reach the Cumbraes after rounding the 
Mull of Cantyre, would have to travel at least forty-five miles, 
taking the nearest route ; or if the shoal were in the neighbour- 
hood of Ailsa Craig, it would have to travel about thirty-five miles 
on a straight course. With this distance to travel, the chances of 
going into the wrong direction or being drifted back again are 
great, so one must not expect to see many oceanic species in the 
neighbourhood of the Cumbraes. 
The surface temperature of the sea and the nets used are 
recorded in the “Notes on the Pelagic Fauna,” pages 781-82. 
The Surface Temperature of the Sea during 1902. 
The temperature of the sea was taken at the surface whilst the 
tow-net was down, and the readings of the thermometer are given 
in the “Notes on the Pelagic Fauna,” p. 781. 
The lowest reading was on 19th February, when the temperature 
went down to 41 - 6° F., and the highest reading, 56° F., occurred 
on 23rd July and 25 th August. 
The surface of the sea was coldest in February, and it did not 
rise above 45° F. until the middle of April. Then there was a 
quick rise to 47° F., where it remained until early in June. 
About the middle of June the temperature reached 50° F., and 
within a week it quickly rose to 53-54° F. During July, August 
and September it oscillated between 53° and 56° F. The fall in 
temperature began in October, when 52° F. was recorded, and 51° 
about the middle of November. Then, within a week, there was a 
