PHYSICAL PKOPERTIES OF WATER 



11 



line and in opposite directions, to meet. No splash occurs, but a per- 

 fectly clear and steady circular film of water is produced in a plane 

 perpendicular to that of the jets, while under favourable circumstances 

 the diameter of this may be extended to some feet before it breaks up 

 into a series of detached drops. 



ART. 4. VISCOSITY. 



Every known fluid offers a resistance, analogous to friction, to the 

 relative sliding motion of any two adjacent layers, and the physical 

 property of the fluid to which this is due is termed Viscosity. This 

 property only noticeable when the 

 fluid is in motion is the cause of 

 all so-called fluid friction and gives 

 the fluid the appearance of being able 

 to withstand a shear stress between 

 adjacent layers. 



The magnitude of this shear or 

 distortive stress over any plane is pro- 

 portional to the rate of distortion, 

 and hence to the rate of change of 

 velocity with space perpendicular to 

 the plane. Thus if co-ordinate axes 

 OX, OY, OZ (Fig. 3) be taken at 

 some point of a stream, and if f s be 

 the distortive stress accompanying 



relative motion of adjacent layers in a direction parallel to the axis of 

 x, the velocity in this direction at any point P, distant y from that axis, 

 being denoted by v, and if the velocity in the direction of OZ is zero, we 

 have, at the point P 



/ oc ^ (1) 



If a curve, having ordinates representing values of v, be plotted on a 

 base parallel to OY, and if 6 be the angle which the tangent to the 

 curve at P makes with this base line, the values which f s adopts as y 

 varies will be represented to scale by the corresponding values of 

 tan 6. 



FIG. 3. 



