1878.] the Constants of the Cup Anemometer. 289 



dots are so straggling that nothing can be made of them. In No. II, and 

 still more in No. IV, they show that the frictions were considerably 

 astray. Guided by these indications, and assuming for a nine- tenths 

 of his measures of that constant, he deduced for Nos. I and III values 

 of /3 and 7 so nearly equal as to make it probable that their means 

 would satisfy both. This would give »■= 1*1282 ; x 2 + ij = z = V 34*0. 



The positive root of (I) gives Y=v lx + a/z + Z- j • (HI). 



L aV 2 J 



Computing V from this, we find AV=obs. — calc, of which tables 

 are given for the tive anemometers. As might be expected from the 

 plottings, they are not very close, but show no systematic deviation 

 from the law denoted by (I). So it may be assumed exact for all 

 practical purposes through a range of V from 5 to 42 miles, and of F 

 from 113 to 3277 grains. For No. Ill the probable error = + 045. In 

 both the errors are less on the hypothesis 7=0. In No. II these mean 

 constants fail, but others deduced for it represent the series, though 

 not so well as in the preceding; here also 7=0 is not inferior. In 

 No. IY the frictions seem to have been deranged so much that the entire 

 series cannot be well represented by any constants. Circumstances 

 detailed in the paper account for this. No. V, cylinder cups, is the 

 best of all. If, as seems probable, x and z are the same for all hemi- 

 spherical anemometers, the difference between their indications will 

 F 



depend solely on - , and using the values given above, the limiting 



value of m=2"286, instead of 3. Though if these experiments were 

 repeated with Dr. Robinson's present experience, and in an undisturbed 

 locality, better results might be obtained, yet the errors of the vortex 

 current would still cause uncertainty ; and he intends to try another 

 plan. 



The anemometer No. I, with its apparatus duly altered, is now 

 erected on the roof of the dwelling-house 22 feet from the Kew one 

 also there, to which it is exactly similar. Denoting the latter as S (the 

 standard one), the other, E, is to be loaded with a brake friction, which 

 will make its v less than that of S ; when this has gone on long enough 

 to ensure that an equal amount of wind has passed each instrument, a 

 larger brake friction is applied to E. We shall thus have three equa- 

 tions (1), but four unknown quantities, a, x, V, y. a, however, is 

 certainly known nearly by the measures already made. F also can 

 now be measured with far greater precision. The chief difficulty 

 to be feared is the unsteadiness of the wind during each experi- 

 ment ; but as the time of each revolution of the two anemometers is 

 recorded on the chronograph, it will be possible to eliminate this ele- 

 ment of doubt by selecting those times which have a given ratio. 



