NEWTON S PRINCIPIA. 361 



a fluid round a vertical axis, perpendicular to its plane, 

 and passing through its centre. The force by which this 

 was effected was the force of Torsion. His published 

 account is in the Mem. de Tlnstitut, vol. iii. The mode 

 of experimenting was adapted for the measurement of the 

 decrements of the arcs, but not suited to the accurate 

 determination of the effect of the fluid on the time of 

 vibration. 



The results of these experiments may be arranged as 

 follows : 



1. That the effect of the resistance of the air is not 

 insensible. Du Buat, Sabine, Baily. 



2. The value of n does not depend on the density of 

 the body, but only on the extent and form of surface. 

 The body might even be hollow. Baily. 



3. Spheres. The value of n is a little greater for 

 small than large spheres. Du Buat, Baily. But on the 

 whole, x. is not subject to very great variations, and is 

 nearly equal to 7 or *8. Baily. 



4. Cylinders. The value of n varies according to the 

 face opposed to the resistance, being greatest when the flat 

 side is in front. In this case the height or thickness had 

 considerable effect ; the less this is, the greater is n. 

 Baily. 



5. Pendulum rod. The thinner the rod the greater is 

 n. Du Buat, Bessel, Baily. 



6. Discs. Flat sides being opposed to the motion. 

 The quantity of air dragged was found to be *149 d 3 , 

 while that by a sphere was 123 d 3 grains troy, where d 

 is the diameter in inches. A disc, therefore, drags more 

 than a sphere of equal diameter. When placed edgeways 

 thev dragored some air. Bailv. 



J J 



7. Nature of the surface. On covering Coulomb s disc 

 with a thin coating of tallow, the resistance was found to 

 be the same as before, and even when the tallow was 

 sprinkled with powdered sandstone by means of a sieve, 

 the augmentation of resistance was &quot; a peine sensible.&quot; 



