The Rev. T. R. Robinson's Description of an improved Anemometer. 1 65 



from 2-23 to 8-50 feet. The values of i^ were determined by attaching to the 

 centre of a hemisphere a fine thread perpendicular to its diameter, and passing 

 over a delicate pulley (whose friction is knoTO), to which weights were sus- 

 pended, such that the driving weight just moved the apparatus when slightly 

 jarred. These weights, divided by the mean squares of the velocities, give 

 a + b and a' + b'.* The result is 



" + * =2-019. 



a' + b' 

 and assuming a = a' x 4-011, we have 



6' = a' X 0-9866+^x0-4953. 



No means of determining the ratio of b to b' occurs to me ; I could only 

 satisfy myself that it is considerably less, by suspending a hght body two feet 

 outside the circle, and estimating the resultant of its deflection from the vertical 

 in the direction of the radius. This made it evident that comparatively little 

 air is thrown outwards by the concaves, the hollow, I suppose, carrying it round, 

 and preventing its escape. We may, therefore, safely assiune, that it Is between 

 the limits b^b',b^O, and much nearer the latter. These suppositions givinc^ 

 the limits 6' = a' X 1-9866 ; i-'= a' X 0-9866. ° 



If now we substitute these values and that of —, in (2), we obtain 



— =3-S06, if b'^b; — =2-999, if «» = 0. 



It must, from what precedes, be much nearer the second ; and if we also 

 consider that the mean value of|, through the semicircle is a httle greater than 

 that at 90^ we shall be justified in assuming the theoretic value of m = 3-000. 

 It IS in very unexpected (by me) agreement with that given by experiment. 



The most obvious mode of determining this constant— placing the instru- 

 ment on a carriage, and comparing its record with the space actually traversed - 



* It is assumed in this, that a and a' are the same as in a current of air, which, however, may 

 not be the fact. Especially it is possible, that the air of the apartment may be dragged round with 

 the cups, and thus offer less resistance. 



Z 



9 



