1881.] of some Experiments with Whirled Anemometers. 183 



situated in a plane through CD, which will pass of course through 

 the centre of the ball. 



If the ball rolls without rubbing at any one of the four points F, I, 

 K, M as the anemometer turns round, its instantaneous axis must be 

 the line joining the points of contact, F, I, with the fixed box. But 

 as at M and K likewise there is nothing but rolling, the instantaneous 

 motion of the ball may be thought of as one in which it moves as if it 

 were rigidly connected with the shaft and its appendage, combined 

 with a rotation over LNAB supposed fixed. For the two latter 

 motions the instantaneous axes are CD, MK, respectively. Let MK 

 produced cut CD in 0. Then since the instantaneous motion is com- 

 pounded of rotations round two axes passing through 0, the instan- 

 taneous axis must pass through 0. But this axis is FI. Therefore, 

 FI must pass through 0. Hence the two lines FI, MK, must 

 intersect the axis of the shaft in the same point, which is the con- 

 dition to be satisfied in order that the ball may roll without rubbing, 

 even though impelled laterally by a force sufficient to cause the side 

 of the shaft to bear on it. The size of the balls and the inclinations 

 of the surfaces admit of considerable latitude subject to the above 

 condition. The arrangement might suitably be chosen something 

 like that in the figure. It seems to me that a ring of balls con- 

 structed on the above principle would form a very effective upper 

 support for an anemometer whirled with its axis vertical. Possibly 

 the balls might get crowded together on the outer side by the effect 

 of centrifugal force. This objection, should it be practically found 

 to be an objection, would not of course apply to the proposed system 

 of mounting in the case ©f a fixed anemometer. Below, the shaft 

 would only require to be protected from lateral motion, which could 

 be done either by friction wheels or by a ring of balls constructed in 

 the usual manner, as there would be only three points of contact. 



2. Influence on the Anemometer of its own Wake. — By this I do not 

 mean the influence which one cup experiences from the wake of its 

 predecessor, for this occurs in the whirling in almost exactly the same 

 way as in the normal use of the instrument, but the motion of the 

 air which remains at any point of the course of the anemometer in 

 consequence of the disturbance of the air by the anemometer when it 

 was in that neighbourhood in the next preceding and the still earlier 

 revolutions of the whirling instrument. 



It seems to me that in the open air where the air impelled by the 

 cups is free to move into the expanse of the atmosphere, instead of 

 being confined by the walls of a building, this must be but small, more 

 especially as the wake would tend to be carried away by what little 

 wind there might be at the time. On making some enquiries from 

 Mr. Whipple as to a possible vorticose movement created in the air 

 through which the anemometer passed, he wrote as follows : — " I feel 



VOL. xxxii. 



