METHODS OF PLANKTON RESEARCH. 511 



the rope shows that it has not descended vertically, the 

 following simple method used by Apstein is of great 

 service. It can also be used to determine the depth at 

 which a dredge or any other piece of apparatus is trailing. 

 The apparatus consists of a piece of plate glass 

 about a foot square and ruled at regular intervals 

 of one centimetre with horizontal lines. A flat strip 

 or ruler of aluminium is fixed at one corner by a 

 pivot passing through the plate glass, so that the 

 ruler can be made to describe an arc over the glass plate. 

 The rule is divided into centimetres and perforated along 

 the middle line at each centimetre mark, so that when it 

 is standing vertically, the perforations for each centimetre 

 mark lie exactly over the corresponding horizontal lines 

 on the glass. Both the lines on the glass and on the rule 

 are numbered 1-2, 3..., similarly, but each space can be 

 used to equal 10, 20 or 100 metres, as the case requires. 

 If the net has been drawn somewhat out of the vertical, it 

 is only necessary to hold the glass plate up so that its 

 upper and lower edges AB, CD are perfectly horizontal 

 and so that the net rope from the pulley to the water lies 

 between the observer and the light. Supposing now that 

 80 metres of rope have been paid out, let each division 

 on rule and glass equal 10 metres, then the rule is moved 

 until it is parallel with the net rope as seen through the 

 glass plate, and the line on the glass plate intersected by 

 the 80 metre mark on the rule will be the true depth of 

 the net. Let us suppose a certain station has been reached 

 where it is desired to make a quantitative catch. The 

 ship should be anchored so that it remains in the same 

 position during the work, and all fear of towing the net 

 in a horizontal position will be done away with, unless a 

 strong current is present. This is most important where 

 it is most difficult to perform, namely, where the depth is 



