11 THE MECHANICS OF THE EARTHS ATMOSPHERE. 



In order to determine with tbe greatest aceuraey the resistance of 

 the air against each separate pair of disks it certainly would have been 

 advantageous to employ very different weights and thereby attain very 

 different velocities. This intention, however, could not be carried out 

 by reason of the moderate length of tbe arms, which was limited by the 

 dimensions of the room. If I loaded each scale pan with more than 1 

 pfund then the whole mass of air in the room, especially when using 

 larger disks, assumed a rotatory motion, in which case the resistance dur- 

 ing the individual observation is always less or the velocity is always 

 greater. Even with a load of 1 pfund the light paper vanes that floated 

 at the tips of the needles already showed a feeble continuous rotation, 

 although the flame of a caudle did not allow of its recognition. In all 

 the following observations therefore in the extreme cases only 28 loth 

 was placed in each scale pan. To this it is to be added also that the 

 measurements for very large velocities lose in accuracy on account of 

 the relative maguitude of the unavoidable error. According to this 

 the index should not move faster than an inch in 1.8 seconds. On the 

 other hand, however, on account of the excessive influence of the very 

 variable friction, the movement became highly irregular, when more 

 than 8 secouds elapsed while the index described 1 inch. Within these 

 limits the times in which 10 inches were described did not easily devi- 

 ate more than half a second from the average value. The velocities of 

 the disks were therefore not greater than 66 and not less than 17 inches 

 per second.* 



In order to attain a uniform tension with reference to the axis the 

 weights placed in the two scale pans were always equal and since on 

 each occasion the disks attached to the arms were also always of equal 

 magnitude, therefore each of these weights corresponded to the resist- 

 ance of one disk. To this indeed should still be added one-half of the 

 weight of the rod and the two scale pans but this may be disregarded 

 since for each individual observation the value of the constant term 

 which indicates the friction has to be especially computed. This con- 

 stant term will then be the sum total of these weights less the friction, 

 and presented itself always with the negative sign because the friction 

 remained less than the weight of the rod and the scales. 



In order to simplify the computation I have at first referred not to 

 the velocity of the disks, but only to that of the index, whence as above 

 mentioned the velocity of the rotation can be easily deduced. In this 

 way the opportunity was offered at each observation with disks to take 

 into consideration that resistance which the arms alone experienced for 

 the corresponding velocity of rotation. 



Before and after each series of observations, which generally oc- 

 cupied 3 or 4 hours, the barometer and thermometer were read oft, 

 the latter being at the same altitude above the floor as that at which 

 the arms revolved. The computed coefficients of resistance were re- 



* Between 3.6 miles and O.'J mile per hour. 



