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THE POPULAR SCIENCE MONTHLY 



it with its side flat against the air. This may be shown by dropping 

 a thin paper disk through the air or by blowing a blast of air against 

 a disk which is pivoted about a diameter as an axis. Fig. 25 repre- 



M 



M 



Fig. 25. 



sents a thin metal disk DD which is thrown in the direction of the 

 arrow V and at the same time set spinning in the direction of the curved 

 arrow S, the thrower standing at MM. Figs. 26 and 27 are top views 



M 



D 



F 



top view 



Fig. 26. 



of the disk. Fig. 26 shows the disk starting to glance to the right 

 (with reference to the thrower at M), and Fig. 27 shows the disk 

 starting to glance to the left (with reference to the thrower at M). 

 This glancing action of the disk causes the air to exert upon the 



,M 



D 



I 

 F' 



D 



F r 



t 



top view 



Fig. 27. 



disk a torque about a vertical axis in Figs. 26 and 27, which torque 

 is represented by the forces FF in Fig. 26 and by the forces F'F' in 

 Fig. 27. This torque would turn the disk flatwise against the air if 

 the disk were not spinning, but the effect of the torque on the spinning 



