1881.] of some Experiments with Whirled Anemometers. 187 



velocities, is given at p. 1063 as 2'826, whereas the limiting value ob- 

 tained by the former method was only 2*286. Dr. Robinson has 

 expressed a preference for the later results. I confess I have always 

 been disposed to place greater reliance on the results of the Dublin 

 experiments, which were carried out by a far more direct method, in 

 which I cannot see any flaw likely to account for so great a difference. 

 It would be interesting to try the second method in a more favour- 

 able locality. 



I take this opportunity of putting out some considerations respect- 

 ing the general formula of the anemometer, which may perhaps not 

 be devoid of interest. 



The problem of the anemometer may be stated to be as follows: — 

 Let a uniform wind with velocity V act on a cup anemometer of given 

 pattern, causing the cups to revolve with a velocity v, referred to the 

 centre of the cups, the motion of the cups being retarded by a force of 

 friction F ; it is required to determine v as a function of V and F, 

 F having any value from 0, corresponding to the ideal case of a friction- 

 less anemometer, to some limit F 1? which is just sufficient to keep the 

 cups from turning. I will refer to my appendix to the former of 

 Dr. Robinson's papers ("Phil. Trans." for 1878, p. 818), for the 

 reasons for concluding that F is equal to V 2 multiplied by a function 

 of Y/v. Let 



then if we regard £ and rj as rectangular co-ordinates, we have to 

 determine the form of the curve, lying within the positive quadrant 

 i-Or), which is defined by those co-ordinates. 



We may regard the problem as included in the more general pro- 



