472 SEGREGATION. 



When towards the end of September, the trees are gain- 

 ing their autumn colours, and we are hoping shortly to see a 

 further change increasing still more the beauty of the land- 

 scape, we are not uncommonly disappointed by the occur- 

 rence of an equinoxial gale. Out of the mixed mass of 

 foliage on each branch, the strong current of air carries 

 away the decaying and brightly-tinted leaves, but fails to 

 detach those which are still green. And while these last, 

 frayed and seared by long-continued beatings against each 

 other, and the twigs around them, give a sombre colour to 

 the woods, the red and yellow and orange leaves are collected 

 together in ditches and behind walls and in corners where 

 eddies allow them to settle. That is to say, by the action of 

 that uniform force which the wind exerts on both kinds, the 

 dying leaves are picked out from among their still living 

 companions and gathered in places by themselves. Again, 

 the separation of particles of different sizes, as dust and sand 

 from pebbles, may be similarly effected ; as we see on every 

 road in March. And from the days of Homer downwards, 

 the power of currents of air, natural and artificial, to part 

 from one another units of unlike specific gravities, has 

 been habitually utilized in the winnowing of chaff from 

 wheat. In every river we see how the mixed ma- 



terials carried down, are separately deposited how in rap- 

 ids the bottom gives rest to nothing but boulders and peb- 

 bles ; how where the current is not so strong, sand is let fall ; 

 and how, in still places, there is a sediment of mud. This 

 selective action of moving water, is commonly applied in the 

 arts to obtain masses of particles of different degrees of fine- 

 ness. Emery, for example, after being ground, is carried by 

 a slow current through successive compartments ; in the first 

 of which the largest grains subside; in the second of which 

 the grains that reach the bottom before the water has es- 

 caped, are somewhat smaller; in the third smaller still; until 

 in the last there are deposited only those finest particles 

 which fall so slowly through the water, that they have 



