348 Mr. F. Guthrie on Approach caused by Vibration. 



At a distance of about 0*005 metre the magnesia begins to 

 be disturbed. It is dragged from a distance of about 0*008 

 metre on both sides of the fork towards the fork. It collects in 

 three ridges beneath the fork — namely, in two very fine ridges 

 exactly beneath the edges of the prong, and a very much thicker 

 ridge beneath the centre of the prong. Seen in section parallel 

 to c, the accumulations appear as in fig. 3 (1). The general ap- 

 pearance when the fork is withdrawn is shown in fig. 3 (2). 



§ 12. It appears from the ultimate distribution of the powder, 

 as well as from the motion of the particles during the commence- 

 ment of the disturbance, that four cylindrical whirlwinds are 

 here concerned, the position and direction of which are shown in 

 section in fig. 4. As the air moving in the whirl m and the air 

 moving in the whirl n approach one another horizontally, they 

 drag the powder with them towards s. When they meet and 

 turn vertically upwards, there is necessarily about s a region of 

 comparative calm. For the same reason there is a region of 

 comparative calm about t. Similarly the air which descends at 

 v is divided as it approaches the rigid surface, some of it entering 

 the whirl n and some of it entering the whirl o. At the exact 

 centre of v there is calm, where the air goes neither to the right 

 nor left. That there are upward currents above s and t is shown 

 by the projection of particles in the directions q and r when the 

 vibration is very violent and the plate is very near the fork. 



§ 13. That there is a region of calm where the axis of the 

 descending current would, if produced, cut the horizontal sur- 

 face, and where the axis of the vertically ascending current, 

 formed by the clashing of two horizontal currents, would meet 

 the surface, is shown by blowing dry air upon the strewn plate, 

 (1) through a single vertical tube, and (2) through two neigh- 

 bouring ones. In the first case, fig. 5 (1), the powder appears 

 undisturbed immediately below the centre of the tube ; and in 

 the second case the powder remains undisturbed, not only below 

 each tube, but also midway between the two, fig. 5 (2) . 



It is clear that the occurrence of the powder in the places de- 

 scribed may be partly attributed to the equilibrium between 

 equal and opposite forces. This would account for the repose of 

 the particles in the places specified, but not for their accumula- 

 tion there. 



§ 14. Experiment 5. — An arrangement was employed pre- 

 cisely similar to that described in § 11, with the exception that 

 (H 6 ) the face b was horizontal. On stroking the fork across 

 Y and bringing the magnesia near it, no disturbance was 

 shown until the interval between the fork and powder was about 

 0*001 metre. Then the powder moved slightly en masse towards 

 that portion of the plate which was accidentally a little the fur- 



