182 Prof. G. G. Stokes. Discussion of the Results [May 12, 



the anemometer is supported near the top on a set of spheres of gun- 

 metal contained in a box with a horizontal bottom and vertical side 

 which supports and confines them. For vertical support, this seems 

 to leave nothing to be desired, but when a strong lateral pressure has 

 to be supported as well as the weight of the instrument, it seems to 

 me that a slight modification of the mode of support of the balls might 

 be adopted with advantage. When a ball presses on the bottom and 

 vertical side of its box, and is at the same time pressed down by the 

 horizontal disk attached to the shaft of the anemometer which rests 

 on the balls, it revolves so that the instantaneous axis is the line 

 joining the points of contact with the fixed box. But if the lateral 

 force of the wind presses the shaft against the ball, the ball cannot 

 simply roll as the anemometer turns round, but there is a slight 

 amount of rubbing. 



This, however, may be obviated by giving the surfaces where the 

 ball is in contact other than a vertical or horizontal direction. 



Let AB be a portion of the cylindrical shaft of an anemometer ; 

 CD, the axis of the shaft ; EFGrHI, a section of the fixed box or cup 

 containing the balls ; LMN, a section of a conical surface fixed to the 

 shaft, by which the anemometer rests on its balls ; FIKM, a section of 

 one of the balls ; F, I, the points of contact of the ball with the box ; 

 M, the point of contact with the supporting cone ; K, the point of 



contact or all but contact of the ball with the shaft. The ball is sup- 

 posed to be of such size that when the anemometer simply rests on the 

 balls by its own weight, being turned perhaps by a gentle wind, there 

 are contacts at the points M, F, I, while at K the ball and shaft are 

 separated by a space which may be deemed infinitesimal. Lateral 

 pressure from a stronger wind will now bring the shaft into contact 

 with the ball at the point K also, so that the box on the one hand and 

 the shaft with its appendage on the other, will bear on the ball at four 

 points. The surface of the box as well as that on the cone UN being 

 supposed to be one of revolution round CD, those four points will be 



