254 
a few instances when conditions of ice or vegetation necessi- 
tated a temporary modification of the direction. 
Collections subsequent to May 20, 1896, were made by 
means of the plankton pump, a known volume of water being 
strained through the silk net. The water strained was taken 
in such a way as to represent a vertical column of equal di- 
mensions from bottom to top. This was accomplished by low- 
ering the inlet of the hose from the intake of the pump to the 
bottom, or as near it as we could go without fouling the water 
by disturbing the unstable deposits, and raising it to the sur- 
face during the progress of the pumping. To secure a _ perfect 
column it is necessary to begin raising the hose with the first 
strokes that deliver water to the net, and to arrive at the sur- 
face long enough before the required amount is pumped to 
allow surface water to reach the net. With a fixed hose 
length and a known capacity of pump, this is easily deter- 
mined by experiment. The volume strained varied with the 
contents of the water. Asarule, one fourth ofa cubic meter 
was strained. When plankton was scanty and silt light the 
quantity was doubled, and occasionally in excessive plankton 
or unusual silt but half this volume was strained. Variations 
of minor importance in the methods here noted will be men- 
tioned in connection with the discussion of the plankton at 
the several stations. These variations are such as were ne- 
cessitated by difficulty of access with collecting apparatus, or 
by the exigencies of flood, ice, and current 
PRESERVATION AND MEASUREMENT. 
During the first three years the plankton was killed and 
preserved in strong alcohol. In subsequent years 70 per cent. 
alcohol to which formalin had been added to the grade of 2 
per cent. was used, and proved to be a better preservative than 
the strong alcohol. 
The quantity of plankton present in the catch was deter- 
mined by compression in a Purdy centrifuge for two minutes 
