1881.J of some Experiments with Whirled Anemometers. 175 



The object of the experiment was, of course, to compare the mean 

 Telocity of the centres of the cups with the mean velocity of 

 the air relatively to the anemometer. It would have saved some 

 numerical calculation to have compared merely the spaces passed 

 through during the experiment ; but it seemed better to exhibit 

 the velocities in miles per hour, so as to make the experiments 

 more readily comparable with one another, and with those of other 

 experimentalists. In the reductions I employed 4-figure logarithms, 

 so that the last decimal in Y in the tables cannot quite be trusted, but 

 it is retained to match the correction for W, which it seemed desirable 

 to exhibit to O'Ol mile. 



On reducing the experiments with the low velocities, I found the 

 results extremely irregular. I was subsequently informed by Mr. 

 Whipple, that the machine could not be regulated at these low 

 velocities, for which it was never intended, and that it sometimes 

 went round fast, sometimes very slowly. He considered that the 

 experiments in this group were of little, if any, value, and that they 

 ought to be rejected. They were besides barely half as numerous as 

 those of the moderate group. I have accordingly thought it best to 

 omit them altogether. 



In the following tables the first column gives the group, H standing 

 for high velocities, M for moderate ; the subordinate group, — stand- 

 ing for rotation of the anemometer opposite to that of the machine, 

 + for rotations in the same direction, and lastly the reference number 

 of the experiment in each subordinate group. T gives the duration 

 of the experiment in minutes ; n the number of revolutions of the 

 machine ; N the number of apparent revolutions of the anemometer ; 

 S the space passed over by the natural wind, in miles. These form 

 the data. From them are calculated : Y, the velocity- of the 

 anemometer, in miles per hour ; W, the velocity of the wind ; W 2 /2Y 

 the mean of the two corrections to be added to Y on account of the 

 wind, according as we adopt one or other of the extreme hypotheses 

 as to the moment of inertia of the anemometer, namely, that it is 

 very small or very large. The actual correction will be half the 

 number in this column on the first supposition, and once and a-half 

 on the second. Y 1? Y 2 denote the velocity of the anemometer, or, in 

 other words, of the artificial wind, corrected for the natural wind on 

 these two suppositions respectively, so that the last two columns give 

 1 00 times the ratio of the registered velocity to the true velocity, or 

 the registered as a percentage of the true, the registered velocity 

 meaning that deduced from the velocity of the cups on employing the 

 usual factor 3. 



The dials of the first two anemometers read only to 10 revolutions, 

 which is the reason why all the numbers N end with a 0. 



