10 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



and the threads simultaneously arranged themselves alongside of 

 each other on the spindle from which they were drawn always in a 

 horizontal direction. Underneath the roller I connected both threads 

 by means of a light rod, and on this hung the scale-pans for weights ; 

 I also fastened thereon a pointer which slid close to the graduated scale 

 and served to measure the velocity. 



Notwithstanding the great convenience of this change it introduced 

 the troublesome consideration that the friction became disproportion- 

 ately great and varied so much during the observation that its magni 

 tude and its influence on the measured velocity could not be determined 

 with the necessary accuracy. This great evil I removed in that I al- 

 lowed a steel point to work in the conical depression already formed 

 by the turning lathe at the lower end of the axis, which point exerted 

 an upward pressure equal to the weight of the arms, the discs, and the 

 axis. The axis is therefore completely supported by the steel point, 

 and the screw serves only as a guide in order to raise and lower the 

 spindle corresponding to the windings of the thread. This steel point 

 forms the upper end of a stout wire 12 inches high, whose lower end, 

 ground to a wedge shape, stands in a metallic groove that is fastened at 

 the end of a lever whose equal arms are 19 inches long. This lever, 

 whose center of gravity lies in its axis of rotation, was so formed that 

 its axis lay in a straight line with the metal groove and the point of 

 suspension of the scale-pan, and was equally distant from both. This 

 pan, with the counterpoise, corresponded exactly to the pressure of the 

 axis on the wire when no resisting discs were placed upon the arms, 

 but as soon as the latter occurred the counterpoise was always increased 

 in a corresponding degree by an appended light cup with shot. Before 

 attaching the discs these were laid upon a balance and the cup was 

 partly filled with shot until brought into equilibrium with it. 



Since the lever changes its position during the rotation of the axis 

 the steel wire deviates somewhat from the vertical position, but, as will 

 be shown in the following, so slightly that this may be overlooked. 

 The result of these changes in the apparatus proved to be very favora- 

 ble, for whereas before at least 3 Prussian loths had to be placed in 

 each scale in order to set the arms in permanent motion, now, the weight 

 of the rod and the scale-pan, which together weighed 3.3 loths, sufficed 

 without any additional weight to produce a uniform motion. 



At the ends of the arms pieces of perforated cork are glued, and in 

 these the stems of the various discs find their support. The discs were 

 always pushed so far on that they closely touched the ends of the arms. 

 The disiance of the disc from the axis of rotation is found from the 

 known lengths of the arras ; the stems of the discs did not extend 

 through the corks, therefore the resistance of the air against the arms 

 was only increased by that which the discs themselves experienced. 

 Therefore, after the resistance which the arms experienced at each 

 velocity had been determined by observation of the rotation of the 



