522 THE POPULAR SCIENCE MONTHLY. 



avidity, and the next trial shows the value of the discipline that has 

 been undergone. 



One remark more will close the view of the conditions of plas- 

 ticity. It is, that discrimination and retentiveuess have a common 

 support in rapidity and sharpness of transition. A sharp and sudden 

 change is commonly said to make a strong impression : the fact im- 

 plied concerns discrimination and retention alike. Vague, shadowy, 

 ill-defined boundaries fail to be discriminated, and the subjects of 

 them are not remembered. The educator finds great scope for his art 

 in this consideration also. 



FORMATION OF RAINDROPS AND HAILSTONES. 1 



WHEN" the particles of water or ice which constitute a cloud or 

 fog are all of the same size, and the air in which they are sus- 

 tained is at rest or is moving uniformly in one direction, then these 

 particles can have no motion relatively to each other. The weight 

 of the particles will cause them to descend through the air with 

 velocities which depend on their diameters, and, since they are all of 

 the same size, they will all move with the same velocity. 



Fig. 1. Perfect Hailstone. 



Under these circumstances,. therefore, the particles will not trav- 

 erse the spaces which separate them, and there can be no aggrega- 

 tion so as to form raindrops or hailstones. 



If, however, from circumstances to be presently considered, some 

 of the particles of the cloud or fog attain a larger size than others, 

 these will descend faster than the others, and will consequently over- 

 take those immediately beneath them ; with these they may combine 



i Abstract of pnper " On the Manner in which Raindrops and Hailstones are formed," 

 by Prof. Osborne Reynolds, M. A., read at the Literary and Philosophical Society, Man- 

 chester. 



