396 



SCIENTIFIC RECORD FOR 1882. 



where V is the uukuovrn velocity of the wind, a and x two constants 

 which are to be determined. Each observation gives two equations in 

 which there are four unknown quantities, for it is found that the value 

 of V changes from one instrument to another ; this is partly owing to 

 eddies caused by tbe buildings, but also in great measure to the irregu- 

 larity of the wind itself. It is, howcNer, also found that these wind-differ- 

 ences are as likely to have -f as — signs, and therefore it may be ex- 

 pected that their sum will vanish in a large number of observations. 

 The ordinary methods of elimination fail here even to determine with 

 precision a single constant, and Dr. Robiuson therefore proceeded by 

 approximations. He tried five different types of anemometers, and ob- 

 tained very unexpected results, for he found that the x varied as some 

 inverse function of the diameter of the cups and the arms. He gives its 

 values : 



No. 6 is similar to No. 2, and it might be expected that their constants 

 would be equal. The cause of these differeuces is partly the eddies 

 caused by the cups which are more powerful when the arms are short, 

 but still more the presence of high powers of the radii and diameter 

 occurring in the expressions of the mean pressured on the concave and 

 convex surfaces of the hemispheres. In the present state of hydro- 

 dynamics we cannot ass'gn these expressions, but we know enough to 

 see that such powers may be i)resent. 



As each type of anemometer has its own constants, the author would 

 suggest to meteorologists the propriety of coniining themselves to one 

 or two forms. For fixed instruments he considers the Kew as good as 

 any, and would wish to see it generally adopted. For portable ones 

 he has no experience except with Oasella's 3-inch cups, 6-inch arms, 

 which he found very convenient; he has not, however, determined its 

 constants. Some selection of the sort seems necessar^^ if it is wished to 

 have a uniform system of wind-measures. {Nature, xxii, p. 404.) 



An unpublished investigation of the accuracy of the Kew anemometers 

 was made by Messrs. S. Jeffoy and G. M. \^' hippie about 1873 at Kew and 

 the Crystal Palace. These observations have been discussed by Prof. 

 G. G. Stokes, who concludes as follows : 



" 1. That at least for high winds the method for obtaining a factor for 

 an anemometer wliich 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 anemom- 



