1881.] of some Experiments icith Whirled Anemometers. 185 



which will be considerably larger than that depending on W 2 . Thus 

 suppose the velocity of whirling to be 30 miles an hour, and the 

 average velocity of the wind 3 miles an hour ; the correction for 

 the wind supposed uniform, or if variable, then with not very rapid 

 variations, will be comparable with 1 per cent, of the whole ; whereas, 

 with rapid variations, the effect in any one revolution may be com- 

 parable with 10 per cent. There is, however, this important difference 

 between the two : that whereas the correction depending on the square 

 leaves a positive residue, however many experiments be made, the 

 correction depending on the first power tends ultimately to disappear, 

 unless there be some cause tending to make the average velocity of the 

 wind different for one azimuth of the whirling instrument from what 

 it is for another. This leads to the consideration of the following con- 

 ceivable source of error. 



5. Influence of Partial Shelter of the Whirling Instrument. — On 

 visiting the merry-go-round at the Crystal Palace, I found it mostly 

 surrounded by trees coming pretty near it, but in one direction it was 

 approached by a broad open walk. The consequence is, that the 

 anemometer may have been unequally sheltered in different parts of 

 its circular course, and the circumstances of partial shelter may have 

 varied according to the direction of the wind. This would be liable 

 to leave an uncompensated effect depending on the first power of W. 

 I do not think it probable that any large error was thus introduced, 

 but it seemed necessary to point out that an error of the kind may 

 have existed. 



The effect in question would be eliminated in the long run if the 

 whirling instrument were capable of reversion, and the experiments 

 were made alternately with the revolution in one direction, and the 

 reverse. For then, at any particular point of the course at which the 

 anemometer was more exposed to wind than on the average, the wind 

 would tend to increase the velocity of rotation of the anemometer for 

 one direction of revolution of the whirling instrument just as much, 

 ultimately, as to diminish it for the other. Mere reversion of the 

 cups has no tendency to eliminate the error arising from unequal ex- 

 posure in different parts of the course. And even when the whirling 

 instrument is capable of reversion, it is only very slowly that the 

 error arising from partial shelter is eliminated compared with that of 

 irregularities in the wind ; of those irregularities, that is to say, which 

 depend on the first power of W. For these irregularities go through 

 their changes a very great number of times in the course of an ex- 

 periment lasting perhaps half an hour ; whereas, the effect of partial 

 shelter acts the same way all through one experiment. It is very 

 desirable therefore, that in any whirling experiments carried on in the 

 open air, the condition of the whirling instrument as to exposure or 

 shelter should be the same all round. 



o 2 



