Introductory 



did not differ very much from that of the water, we may neglect the 

 errors caused by this method.' 



On hoard the Silver Belle we have always employed Miller-Casella 

 thermometers or Knudsen's bulbs, and we have been very fortunate in 

 escaping accidents by breakages. The reversing thermometer is one 

 supplied by Negretti and Zambra, and this and the Miller-Casella 

 were found to vary only two or three tenths of a degree. The air 

 and surface-water temperatures were taken by a thermometer supplied 

 from the Plymouth Laboratory, made by Miiller, Orme and Co., rising 

 from 9° C. to 36° C, and divided in tenths (compared with thermo- 

 meter 303 Richter, and verified at Charlottenburg). All our instruments 

 have been similarly verified. The depth is recorded by a wheelmeter 

 reading fathoms. 



2. For the collection of plankton silk nets (of bolting silk 100 to 

 170 mesh) were employed, and as the desire was to collect the plankton 

 at definite depths, closing-nets have been always employed when 

 working below 100 fathoms. Closing-nets may be made to work 

 vertically, as in Fowler's net, 1 or horizontally, as in Garstang's and 

 the author's nets. Opinions may be divided as to the relative 

 advantages of these two methods of fishing for plankton, but the 

 objection that a horizontally-towing net, which has to be towed at a 

 very gentle pace (with just sufficient way on the ship to keep her 

 barely moving), is never at the depth imagined loses force when it is 

 realized that a vertically-hauled net is so raised through a hundred or 

 more fathoms at each haul between the opening and closing. Besides 

 which it is probably of little importance in working in deep water 

 whether the net is, say, at 1,000 or 900 fathoms, and, moreover, the 

 accuracy of the observations is checked by appending to the net-frame 

 one or more thermometers. A reversing Negretti thermometer is 

 invariably attached to our nets when plankton-fishing, and as the 

 temperatures in the Atlantic at known depths are fairly constant, the 



1 Dr. Fowler's net is described in the Proceedings of the Zoological Society. He 

 was kind enough to superintend the making of one for me, which we used very much 

 in the Faeroe Channel in 190J3. 



