186 Prof. G. (1. Stokes. Discussion of the Results [May 12, 



The trees, though taller than the merry when I visited the place 

 last year, were but young, and must have been a good deal lower at 

 the time that the experiments were made. Mr. "Whipple does not 

 think that any serious error is to be apprehended from exposure of the 

 anemometer during one part of its course and shelter during another. \ 



From a discussion of the foregoing experiments, it seems to me that 

 the following conclusions may be drawn : — 



1. That, at least for high winds, the method of obtaining the factor 

 for an anemometer, which consists in whirling the instrument in the 

 open air is capable, with proper precautions, of yielding very good 

 results. 



2. That the factor varies materially with the pattern of the anemo- 

 meter. Among those tried, the anemometers with the larger cups 

 registered the most wind, or in other words required the lowest factors 

 to give a correct result. 



3. That with the large Kew pattern, which is the one adopted by 

 the Meteorological Office, the register gives about 120 per cent, of the 

 truth, requiring a factor of about 2*5, instead of 3. Even 2'5 is pro- 

 bably a little too high, as friction would be introduced by the centri- 

 fugal force, beyond what occurs in the normal use of the instrument. 



4. That the factor is probably higher for moderate than for high 

 velocities ; but whether this is solely due to friction, the experiments 

 do not allow us to decide. 



Qualitatively considered, these results agree well with those of other 

 experimentalists. As the factor depends so much on the pattern of the 

 anemometer, it is not easy to find other results with which to compare 

 the actual numbers obtained, except in the case of the Kew standard. 

 The results obtained by Dr. Robinson, by rotating an anemometer of 

 this pattern without friction purposely applied, are given at pp. 797 

 and 799 of the " Phil. Trans." for 1878. The mean of a few taken 

 with velocities of about 27 miles an hour in still air gave a factor 

 2*36, instead of 2*50, as deduced from Mr. Jeffery's experiments. As 

 special antifriction appliances were used by Dr. Robinson, the friction 

 in Mr. Jeffery's experiments was probably a little higher. If such 

 were the case, the factor ought to come out a little higher than in Dr. 

 Robinson's experiments, which is just what it does. As the circum- 

 stances of the experiments were widely different with respect to the 

 vorticose motion of the air produced, by the action of the anemo- 

 meter in it, we may I think conclude that no very serious error is to 

 be apprehended on this account. 



In a later paper (" Phil. Trans." for 1880, p. 1055), Dr. Robinson 

 has determined the factor for an anemometer (among others) of the 

 Kew pattern by a totally different method, and has obtained values 

 considerably larger than those given by the former method. Thus 

 the limiting value of the factor m corresponding to very high 



