1881.] of some Experiments with Whirled Anemometers. 181 



We may accept, therefore, the mean results as not only pretty well 

 freed from casual irregularites which would disappear in the mean of 

 an infinite number of experiments, but also, most probably, from the 

 imperfection of the representation of a rectilinear motion of the 

 anemometer by motion in a circle of the magnitude actually employed 

 in the experiments. 



Before discussing further the conclusions to be drawn from the 

 results obtained, it will be well to consider the possible influence of 

 systematic sources of error. 



1. Friction. — No measure was taken of the amount of friction, nor 

 were any special appliances used to reduce it ; the anemometers were 

 mounted in the merry just as they are used in actual registration. 

 Friction arising from the weight is guarded against as far as may be 

 in the ordinary mounting, and what remains of it would act alike in 

 the ordinary use of the instrument and in the experiments, and as far 

 as this goes, therefore, the experiments would faithfully represent the 

 instrument as it is in actual use. But the bearings of an anemometer 

 have also to sustain the lateral pressure of the wind, which in a high 

 wind is very considerable ; and the construction of the bearing has to 

 be attended to in order that this may not produce too much friction. 

 So far the whirled instrument is in the same condition as the fixed. 

 But besides the friction arising from the pressure of the artificial wind, 

 a pressure which acts in a direction tangential to the circular path of the 

 whirled anemometer, there is the pressure arising from the centrifugal 

 force. The highest velocity in the experiments was about 30 miles 

 an hour, and at this rate the centrifugal force would be about three 

 times the weight of the anemometer. This pressure would consider- 

 ably exceed the former, at right angles to which it acts, and the two 

 would compound into one equal to the square root of the sum of their 

 squares. The resulting friction would exceed a good deal that arising 

 from the pressure of the wind in a fixed anemometer with the same 

 velocity of wind (natural or artificial), and would sensibly reduce the 

 velocity registered, and accordingly raise the coefficient which 

 Dr. Robinson denotes by m, the ratio, namely, of the velocity of the 

 wind to the velocity of the centres of the cups. It may be noticed 

 that the percentages collected in the table on p. 179, are very distinctly 

 lower for the moderate velocities than for the high velocities. Such 

 an effect would be produced by friction ; but how far the result would 

 be modified if the extra friction due to the centrifugal force were got 

 rid of, and the whirled anemometer thus assimilated to a fixed 

 anemometer, I have not the means of judging, nor again how far the 

 percentages would be still further raised if friction were got rid of 

 altogether. 



Perhaps the best way of diminishing friction in the support of an 

 anemometer is that devised and employed by Dr. Robinson, in which 



