286 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



to find the angle to which the cloth must be cut. This is 

 calculated as follows : — 



x : x + y = r : R. 



That is x = nl 



R — r 



Now suppose the conical mantle is unrolled (Fig. 3). 

 Then it follows that the segment of the circle subtended 

 by the angle a bears to the circumference of the circle 

 the ratio of a to 360°. 



n ., 2x* 860 



Consequently — = 



1 J 2r7r a 



Therefore a = — — 

 x 



For the large plankton net used in marine investiga- 

 tions R is 50 cm., r is 10 cm., and y is 144 cm. 



When the net is cut out according to the above pattern, 

 an extra inch of cloth must be allowed on all sides, so that 

 the edges ^4 C and B D (fig. 3) may be sewn together by 

 means of a fine needle. The silk cloth along the line A B 

 is sewn on to the upper ring, and along the line C I) it is 

 fastened between the metal cylinder and a brass ring. 



There are several varieties of the metal cylinder, or 

 bucket, that is attached to the apex of the net. One only 

 is described here (Fig. 4). The upper part of the cylinder 

 is formed of a brass ring, B, which by means of three 

 screws, s, is fastened on to a brass ring of similar circum- 

 ference at the apex of the net. The walls of the upper 

 part of the cylinder are composed of silk bolting cloth, c. 

 The lower part of the cylinder is of brass, which is painted 

 green. By means of a turncock, t, the catch can be run 

 off into the filtrator. 



The filtrator (Fig. 5) consists of four principal parts, a 

 metal base (m), a removable glass plate (g), the filter itself 



