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



Fig. 1. 



This rotation in guiescent air, will cause a resis- 

 tance to the convex surface of each hemisphere 

 = a'v' ; a' being a coefficient depending on its 

 diameter. To this the wind, supposed to act 

 in the direction "WE, adds another resistance on 

 the convex of AIB ; but it also acts on the con- 

 cave of DKH, with a force which tends to in- 

 crease V ; and as its coefficient a is considerably 

 greater than a', v will increase. In consequence 

 of this, the concave surface recedes from the 

 wind, and the convex meets it more rapidly; the impelling force, therefore, 

 diminishes, and the retarding forces increase. To the latter must also be 

 added the centrifugal force expended in producing an outward current in the 

 air that is dragged with the convex surfaces, and the effect of friction. Evi- 

 dently, therefore, a speed will soon be attained, at which these forces balance 

 each other. If 6 = the angle WEH, V the wind's velocity, we have, by the 

 theory of Borda for the undershot wheel, 



Force on DKH — aV'- sin^ 6 — aVv sin 6. 

 Force on AIB = a' V sm- 6 + a' Vv sin 6. 



The force due to the rotation alone = 2a'v^, and the centrifugal force being as 

 v' may be assumed = 26V. Let/ also = the moment of friction at C, then the 

 actual impelling force 



F={a- a') V sin^ e-(a + a') Vv sin - 2v- (a' + L') -/. 

 We must, however, take the mean value of this through the semicircle. It is 



c 



Fde _a-a' „, a + a' 



TT 2 TT 



x2Vv-2v'{a'+b')-Vf.' 



(1) 



* This reasoning supposes a and a' to retain the same value through the semicircle. Experi- 

 ment shows that they vary ; but as the change is greatest when their influence on the velocity is 

 least, the error of this assumption cannot have much influence. The centrifugal force cannot act 

 on the concave, as there is no tendency in the air which it holds to escape in the direction of the 

 arm. 



