APPARATUS USED WITH PLANKTON NETS 197 



depth is the number of metres of wire rope paid out. The depth at which a towed net 

 is working is governed by a considerable number of factors, the most important being 

 the length and size of the rope, the size and pattern of net, the weight of the lead and 

 the speed of the ship, and since there must nearly always be some variation in one or 

 more of these particulars it is hardly possible, even with the most carefully standardized 

 system of hauls, to work out a table of calibration. The only reliable method is to use 

 a depth-gauge with each haul, and unfortunately no perfect instrument for deep-water 

 work has yet been invented. 



In the ' Discovery ' investigations a number of different types of gauge have been 

 used: (i) the Admiralty pattern, recently employed with such good results by Mr F. S. 

 Russell in his studies of vertical movements of plankton in the English Channel ; (ii) the 

 Kelvin tube, commonly used by seamen to determine shallow soundings from a moving 

 ship ; (iii) the Budenberg gauge, made by Schaffer and Budenberg and formerly used 

 by the Prince of Monaco ; and (iv) a pattern in which the depth is determined by the 

 difference in the readings of protected and unprotected reversing thermometers. In 

 addition a new pattern of gauge was tried, but proved unsatisfactory. The vibration 

 of the warp caused the index to shift and the readings were unreliable. 



It was unfortunately only possible to use the Admiralty pattern gauge to a very 

 limited extent, for though it gives excellent results, it cannot be employed below depths 

 of about 50 fathoms. With most of the shallow horizontal nets a Kelvin tube was used, 

 inserted in a brass case lashed to the shaft of the stream-line lead. By this means a 

 depth for the lower net is obtained without difficulty, but the method is, of course, 

 open to the objection that only a single reading is taken and that the level at which 

 the net is fishing may alter while it is being towed. The Kelvin tube was occasionally 

 used to depths of 100 fathoms, but the scale above 75 fathoms becomes very small and 

 cannot be read with any accuracy. 



For work in deeper water the Budenberg gauge^ was used on many occasions. The 

 instrument is large and very heavy, and was attached to the end of the warp either with 

 or without a stream-line lead. It works on the Bourdon principle, and gives a graph 

 which can be read with reasonable accuracy to about 25 m.; but since its limit of 

 depth is 1500 m., it can only be employed in the upper layers of the water. In 

 certain series of hauls, when five nets were being towed simultaneously, this gauge was 

 used at the bottom in conjunction with a Kelvin tube on the second net from the 

 surface. The Budenberg gauge gave excellent results, but it has one serious defect which 

 detracts greatly from its usefulness. The instrument is lenticular in shape, made in 

 two halves with a large circular washer as a joint, and is held together by twelve steel 

 bolts, all of which must be removed in order to open it. In practice it was found almost 

 impossible to make the joint watertight. No matter what means were tried a small 

 leakage occurred, and this, though insufficient to have any appreciable effect on the 

 reading, made it necessary to dismantle the entire mechanism and clean it thoroughly 



1 Described in Forschungsreise S.M.S. 'Planets', 1906-7, cxi, Ozeanographte, Berlin, p. 11, 1909. 



s 



