224 PROGEESS OF METEOROLOGY IN 1889. 



iicss the direct dew-point determination gave the vapor pressnre 5.2"""', 

 while Regiiault's formula gave 3.9"'™, a value considerably too small.* 



Anemometry. — Prof. C. F. Marvin, in presenting the results of the 

 recent experiments of the U. S. Signal OfiQce for determining anemom- 

 eter factors, points out that an important consideration has hitherto 

 been overlooked in applying the factors obtained in whirling-machine 

 experiments to anemometers in actual use. In the experiments the 

 velocity is uniform, in practice the wind is extremely variable. The 

 revolving parts of an anemometer exposed to a variable wind will, in 

 consequence of tlieir inertia, fall short of their proper rate of revolu- 

 tion when the velocity of the wind is increasing, and then catch up and 

 run too fast when the wind velocity diminishes, and it has been gener- 

 ally assumed that the amount lost in the one case is balanced by that 

 gained in the other, so that the mean rate is not aifected. But such is 

 not the case, from the fact that the resultant force acting on the cups 

 when the wind is increasing and tending to increase the velocity of the 

 cups is much greater than the corresponding force in action when the 

 wind is diminishing in intensity and tending to retard the velocity of 

 the cups. In one case it is a question of the resistance of the air to the 

 concave side of the cups, and in the other that to the convex surface; 

 the latter being smaller, the cups will always continue to move more 

 rapidly and longer after the wind has diminished in velocity, than they 

 lagged behind when the wind was increasing, so that the mean velocity 

 of the cups exposed to a variable wind is appreciably higher than it 

 would be in a uniform wind having the same mean velocity. Conse- 

 quently, if the equation of an anemometer whose constants have been 

 determined upon a whirling machine be used to reduce ordinary obser- 

 vations, the computed wind velocities will be too high by an amount 

 depending upon the moment of inertia of the cups and the extent of 

 the wind variations. Since these latter are too complicated to be de- 

 termined, it is impossible with anemometers of the Robinson type to 

 obtain accurate measures of the mean velocity of a variable wind unless 

 the moment of inertia of the cups is very small in relation to the wind 

 pressures, and even then the result is still aifected by another error. 



For accurate results a formula involving the square as well as the 

 first power of the velocity of the cups must be used, as in the form 

 V=a-\-hv-\-cv^ where r=velocity of wind and i'=mean velocity of the 

 cup centers. Now, when the wind is variable the third term should not 

 hQ cv'^hut -{Vi^-\-r-/-\-V3^-\-Vi'^-\- .... v^^)- even in ordinary cir- 

 cumstances this difference between the square of the mean velocity 

 and the mean of the squares is appreciable. 



The constants for a small anemometer of extreme lightness were de- 

 termined with great precision on a whirling machine and comparisons 

 in open air made between it and a Signal Service anemometer of the 



* Ferrel's tables would have made the vapor pressure 6.0"™, which is a fair approx- 

 imation to the value given by the direct hygrometer. — G. E. C. 



