12 University of California Publications in Zoology [ Von. 14 
positions during the later periods of the work were all treated in the 
ship’s records as new stations, and were designated by new numbers. 
Plankton collections made at the same point at different times thus 
bear different numbers. In Appendix II these later hydrographic 
stations may be identified in position with those of the primary series. 
Ordinarily the entire series of observations for one of the periods 
required six days. A separate day was devoted to each of the regions 
above indicated (containing seven or eight stations) at each phase of 
the tide (flood and ebb). It was our endeavor to commence work in 
each region, or group of stations, as soon as possible after the turn 
of the tide, and to finish the group before the next period of slack 
water. This was commonly accomplished, though it was not invari- 
ably possible. The duration of current flow in a given direction could 
not commonly be predicted with any accuracy from the time of high 
or of low water, as stated in the ‘‘Tide Tables.’’ Except on three days 
of the hydrographic work (one in the first, one in the second, and one 
in the fourth period), the vessel invariably moved up stream in passing 
from one station to another. There was thus avoided the possibility 
of our repeating observations upon practically the same mass of water, 
which might have been the case had we steamed down stream and 
anchored for the customary period. 
Before commencing the observations at any given station, the ship 
was anchored and allowed to swing into position according to the 
direction of wind and current. She remained at anchor for ten 
minutes after the commencement of operations. The phase of tide 
could commonly be determined from the direction of the drift past 
the ship. In a few eases, observations which should have been made 
during the ebb period were made during the commencement (or the 
end) of the flood period, or vice versa, and in certain cases the ebb 
(or flood) observation for a given period was omitted altogether. 
Allowance has been made in the computations for these imperfections 
in our data. 
The procedure followed at each hydrographic station involved the 
following operations : 
(1) The depth was taken by one of the seamen from the bow, while 
it was likewise indicated by the sounding machine on the quarter- 
deck, from which the thermometers and water-sample bottles were 
operated. 
(2) A water sample from the surface was obtained by lowering a 
copper vessel on the end of a rope. 
