GENERAL PRINCIPLES 463 



scope substage, so that the two bandings are separated over glass plates below a 

 travelling microscope which enables the plankton to be counted section by section. The 

 details of this process are described later on p. 491. 



Weight alone is not sufficient to take the machine far down below the surface when 

 towed at speeds over 6 knots. The machine is therefore provided with a system of 

 planes like a paravane or water kite ; these, when it is towed, make it dive below the 

 surface and swim horizontally at the desired depth. It is also furnished with a vertical 

 tail fin. The exact arrangement of these planes will be described in the detailed account 

 of the machine which follows : their size, angle of incidence, and position in relation to 

 the centre of gravity of the machine must be exactly right to ensure stability at high 

 speeds. As will be acknowledged later (p. 502) I have had invaluable assistance in this 

 matter from the Admiralty. Between 8 and 16 knots differences in speed make little 

 difference to the depth at which the machine swims. Its depth, within limits governed 

 by the size of the diving planes, is determined by the length of the towing line employed. 

 The machine takes up a position which is the resultant of the downward thrust of the 

 diving planes and the backward thrust caused by the resistance to the water of the 

 machine and its towing cable. With increased speed, within the limits above mentioned, 

 both the downward and the backward thrusts increase together in approximately the 

 same proportion. The depth to which the machine will descend for a given size of 

 diving plane is not unlimited; a point is reached when so much cable is out that the 

 backward thrust greatly exceeds the downward thrust and the machine will descend no 

 further. If more cable is let out the machine will merely lie further astern ; if greater 

 depths are desired they must be attained by fitting larger diving planes (and employing 

 a stronger cable to withstand the increased downward pull). 1 



In this general account of the principles employed the achievement of the desired 

 result will appear simple, but in actual practice there have been many difficulties to be 

 overcome. For this reason a detailed description of the latest machine is given later. 

 So much depends upon the slits and plates in the water tunnel being exactly of the right 

 dimensions, likewise on the position of the driving rollers, the right tension on the gauze 

 bandings and devices for preventing the fabric adhering to the driving rollers. The 

 mechanism of the later machines may appear at first sight to be unnecessarily elaborate, 

 but each modification has been designed to overcome some cause of failure. 



The method of towing and avoiding vibrational fatigue which may result in the 

 fracture of the towing cable is described on p. 489. But an important principle in towing 

 may be noted here. The towing head is placed above the body of the machine over the 

 centre of pressure of the diving planes. The towing cable, which makes an inverted 

 catenary curve in the water, descends to the towing head at a steep angle. This being so 

 there is no disturbance in the water in front of the opening to the water tunnel which 

 is at the extreme front of the streamlined body of the machine ; the entrance to the water 

 tunnel therefore cuts the virgin water cleanly so that the organisms are swallowed in 



1 This matter is referred to again on p. 489 where the methods of towing are described, and in Fig. 18 

 is given a graph from which the depth per length of towing rope may be determined for the Type II machine. 



