[14] PROFESSOR STOKES, ON THE EFFECT OF THE INTERNAL FRICTION, &c. 



neglected. The resistance thus determined proves to be proportional, for a given fluid and a 

 given velocity, not to the surface, but to the radius of the sphere ; and therefore the accele- 

 rating force of the resistance increases much more rapidly, as the radius of the sphere 

 decreases, than if the resistance varied as the surface, as would follow from the common 

 theory. Accordingly, the resistance to a minute globule of water falling through the air with 

 its terminal velocity depends almost wholly on the internal friction of air. Since the index of 

 friction of air is known from pendulum experiments, we may easily calculate the terminal 

 velocity of a globule of given size, neglecting the part of the resistance which depends upon 

 the square of the velocity. The terminal velocity thus obtained is so small in the case of 

 small globules such as those of which we may conceive a cloud to be composed, that the 

 apparent suspension of the clouds does not seem to present any difficulty. Had the resistance 

 been determined from the common theory, it would have been necessary to suppose the globules 

 much more minute, in order to account in this way for the phenomenon. Since in the case of 

 minute globules falling with their terminal velocity the part of the resistance depending upon 

 the square of the velocity, as determined by the common theory, is quite insignificant compared 

 with the part which depends on the internal friction of the air, it follows that were the pres- 

 sure equal in all directions in air in the state of motion, the quantity of water which would 

 remain suspended in the state of cloud would be enormously diminished. The pendulum 

 thus, in addition to its other uses, affords us some interesting information relating to the 

 department of meteorology. 



The fifth section of the first part of the present paper contains an investigation of the 

 effect of the internal friction of water in causing a series of oscillatory waves to subside. It 

 appears from the result that in the case of the long swells of the ocean the effect of friction is 

 insignificant, while in the case of the ripples raised by the wind on a small pool, the motion 

 subsides very rapidly when the disturbing force ceases to act. 



