162 SIR W. A. HERDMAN : RESULTS OF CONTINUOUS 
disturbance of the plankton in the wake of the ship, was to attach the two 
surface-nets to otter-boards, which were run out one to starboard and one 
to port well forward so as to tow the nets in untouched water *, 
E 
VARIATION IN SUCCESSIVE VERTICAL HAULS. 
A few experiments have been made in the past, by Hensen and others, in 
hauling comparable nets simultaneously or the same met several times 
in rapid succession, in order to estimate the amount of variation in the results 
or the divergence of each sample from an average. With the view of getting 
further evidence from a new series of data, taken with all possible care under 
favourable conditions, I carried out a number of similar experiments at 
Port Erin during several months in the spring, summer, and autumn of 1920. 
They consisted of seven series of four to six successive (that is, as nearly 
as possible simultaneous) vertical hauls taken with the ** Nansen” net of 
No. 20 silk f. | 
An apparent uniformity in the successive catches of each series was obvious 
at the time of collecting. It seemed to the eye to be the same catch that 
was emptied from the Nansen bucket into the bottle of formaline time after 
time throughout a series. And this apparent uniformity of volume was in 
most cases confirmed by the measurements in the laboratory—for example, 
the six successive hauls from & fathoms on April 3rd all measure 0'2 c.c., 
four out of five of those from 20 fathoms on April 6th are 0°6 c.c., and all 
four on August 7th from 20 fathoms measure 0:5 c.c. The remaining four 
series show some variation, but the percentage deviation from the average of 
each series is in no case great. 
lf, however, we make a microscopie investigation of the catches, we find 
that, even in the same series, similar volumes of the plankton may be made 
up rather differently, and may in some cases show surprising differences in 
the numbers of a species in successive hauls, such as 10 and 100, 40 and 800, 
4000 and 18,000. Notwithstanding, then, some appearance of similarity 
between the hauls of a series, there is a considerable percentage deviation in 
the case of some hauls from the average of their series—not infrequently 
about plus or minus 50 per cent., and in several cases about 70 and in one 
case plus 129. The following table gives the percentage deviations in the 
case of the volumes of the eatches, and also of the counted or estimated 
numbers of the four chief groups of organisms present, viz., Diatoms, 
Dinoflagellates, Copepoda, and the nauplii of Copepoda :— 
* For further details in regard to these and other experiments see the Annual Reports, 
loc. cit. 
T For full details as to the conditions of the experiment and the methods of obtaining the 
results here given, see Trans, Biol. Soc. L’pool, vol. xxxv. p. 161 (1921). 
