v. 3 EQUILIBRIUM OF FISHES 137 



as it moves through the water, or allow it to become temporarily un- 

 stable and hence to change direction. Instead of attempting to study a 

 living or dead fish moving in water, Harris made models and supported 

 them in a wind-tunnel in an apparatus suitable for measuring the 

 forces at work in the various directions. Such a method, in which no 

 compensating movements of the fins are allowed, makes it possible 

 to investigate the so-called 'static stability' of the fish, that is to say, 



Fig. 93. Diagram of model of the dogfish Mustelus, showing the conventional terms 

 for describing deviations of motion. The longitudinal axis X is that of the wind 

 tunnel and Y (horizontal) and Z (vertical) are at right angles to it. The arrows show 

 the directions known as positive rolling, pitching, and yawing, which occur about the 

 X, Y, and Z axes respectively, a. is the angle of attack between the axis of the model 

 and the X axis. (From Harris, J. exp. Biol. 13.) 



to see whether the body and fins are so shaped as to provide forces 

 that tend to bring the fish back into its previous line of movement after 

 it has deviated in any direction. Any body such as a fish or aeroplane 

 is said to be in stable motion if when it veers slightly from its line of 

 progress the new forces produced upon its planes tend to restore the 

 original direction of motion. 



The forces acting on the fish are measured along three primary axes, 

 longitudinal, horizontal, and vertical. Deviation from the line of 

 motion about the longitudinal axis is known as rolling, about the 

 transverse axis as pitching, and about the vertical axis as yawing 

 (Fig. 93). The forces along these three axes are known as drag, lateral 

 force, and lift. 



In order to discover the effect of the median fins and tail on the 

 stability, these fins were removed, the heterocercal tail being replaced 

 by a cone having the same taper as the actual caudal fin. The model 

 was then placed in the wind-tunnel with a wind at 40 m.p.h., which 



