372 Sir W. Thomson on the Influence of Wind 



Observation shows the sea to be ruffled by wind of a much 

 smaller velocity than this. Such ruffling, therefore, is due to 

 viscosity of the air. 



W. T. 

 Postscript to Part IV. [October 17, 1871). 



The influence of viscosity gives rise to a greater pressure on 

 the anterior than on the posterior side of a solid moving uni- 

 formly relatively to a fluid. A symmetrical solid, as for example 

 a globe, moving uniformly through a frictionless fluid, experi- 

 ences augmentation of pressure in front and behind equally; 

 and diminished pressure over an intervening zone. Observation 

 (as for instance in Mr. J. R. Napier's experiments on his " pres- 

 sure log/' for measuring the speed of vessels, and experiments 

 by Joule and myself*, on the pressure at different points of a 

 solid globe exposed to wind) shows that, instead of being increased, 

 the pressure is sometimes actually diminished on the posterior 

 side of a solid moving through a real fluid such as air or water. 

 Wind blowing across ridges and hollows of a fixed solid (such 

 as the furrows of a field) must, because of the viscosity of the 

 air, press with greater force on the slopes facing it than on the 

 sheltered slopes. Hence if a regular series of waves at sea con- 

 sisted of a solid body moving with the actual velocity of the waves, 

 the wind would do work upon it, or it would do work upon the 

 air, according as the velocity of the wind were greater or less than 

 the velocity of the waves. This case does not afford an exact 

 parallel to the influence of wind on waves, because the surface 

 particles of water do not move forward with the velocity of the 

 waves as those of the furrowed solid do. Still it may be expected 

 that when the velocity of the wind exceeds the velocity of pro- 

 pagation of the waves, there will be a greater pressure on the 

 posterior slopes than on the anterior slopes of the waves ; and 

 vice versa, that when the velocity of the waves exceeds the velo- 

 city of the wind, or is in the direction opposite to that of the 

 wind, there will be a greater pressure on the anterior than on 

 the posterior slopes of the waves. In the first case the tendency 

 will be to augment the wave, in the second case to diminish it. 

 The question whether a series of waves of a certain height gra- 

 dually augment with a certain force of wind or gradually sub- 

 side through the wind not being strong enough to sustain them, 

 cannot be decided offhand. Towards answering it Stokes's in- 

 vestigation of the work against viscosity of water required to 

 maintain a wavej% gives a most important and suggestive instal- 



* " Thermal Effects of Fluids in Motion," Royal Society Transactions, 

 I860; and Phil. Mag. 1860, vol. xx. p. 552. 



t Transactions of the Cambridge Philosophical Society, 1851 ("Effect 

 of Internal Friction of Fluids on the Motion of Pendulums/' Section V.). 



