146 SIR W. A. HERDMAN : RESULTS OF CONTINUOUS 
On analysing the annual curve of the total plankton into its three chief 
factors, the diatoms, the dinoflagellates, and the copepods are found to succeed 
one another in that order. For example, the diatom vernal maximum was in 
March in 1907, in April in 1909, and in May in 1908, the dinoflagellate 
maximum was about a month later in each case, and the copepod maximum 
is usually about a month (sometimes more) after that of the dinoflagellates. 
The autumnal maxima are less definite than the vernal. The copepod rise in 
September or October is the most marked, the diatom increase is usually 
much less evident than in spring and is less regular in its appearance, while 
the dinoflagellate elevation is still less constant. 
The cause of all these seasonal changes is still very obscure, and they may 
be due to the interaction of several factors. In addition to the normal series 
of stages in the life-histories of the organisms throughout the year, one 
naturally turns to the meteorological conditions prevailing at the various 
seasons as being a cause of the increase or the diminution in numbers. 
Series of hydrographic observations were taken from the yacht on many 
occasions by several fellow-workers, who kindly helped me in different years— 
especially W. J. Dakin, W. Riddell, G. H. Drew, and H. G. Jackson. 
The water of the Irish Sea west of Port Erin may, on the whole, be regarded 
as a homothermal and homosaline mass. The temperatures in many vertical 
series showed a slight gradual cooling from the surface to the bottom at 
depths down to 60 fathoms, the difference on nearly all occasions being less 
than half a degree centigrade, and the few exceptions may be due simply 
to surface-heating by the sun. Such slight differences cannot be used as 
evidence for the inflow of bottom colder currents from outside, and probably 
have little influence upon the vertical distribution of the plankton. The 
surface-temperatures at the inshore stations are lower in winter and early 
spring and higher in summer and autumn than at the stations furthest from 
the shore, and this is probably due to the influence of the land winter and 
summer temperatures upon the adjacent water. 
The salinities (determined by titrations of the chlorine, and conversion by 
means of Knudsen’s tables) show a difference of only 0:05 per mille between 
surface and bottom water, except in the case of the deep water in mid-channel, 
over 60 fathoms, where the bottom water may be as high as 34°38 per mille 
when the surface at the same spot is 34°05 per mille. Probably these small 
differences in vertical salinities do not cause any great changes in the 
distribution of the plankton. Consequently the observed differences between 
surface and deeper gatherings must be due to other causes. The influence of 
heavy rain, of sunshine, and of the alternation of night and day probably 
produce greater changes in the vertical distribution of the plankton than the 
slight hydrographic differences we have recorded. 
The oxygen determinations varied from 6°5 to 6*8 c.c. per litre of water at 
