As liquids and aeriform substances resist solids in motion in proportion to the amount of surface which the solids present to them, so also when they strike against solids they cause motion in them in proportion to the amount of surface acted upon. Thus a violent wind could not move a lump of tin, but could blow along a sheet of it, or tear up a roofing of it if it got beneath. So clouds of sand are raised into the air in the deserts of Africa, although the particles are of the same material as stones, and therefore have the same specific gravity. For the same reason dust, feathers, the down and pollen of flowers, etc., are blown about, although they are heavier than the air. A pebble is moved more easily by a current of water than a stone, because it has a larger surface, in proportion to its weight, to be acted upon by the water. For the same reason sand is moved more easily than pebbles, and fine mud than sand, though stones, pebbles, sand, and mud may all be of the same material. This explains why you will find mud where the current is slow, sand where it is faster, pebbles and stones where it is still faster, and where the current is exceedingly rapid you find nothing but large rocks--sand, pebbles, and stones not being able to resist its force. For the same reason, in the process of winnowing, the chaff is carried away by the wind; while the grain, presenting less surface in proportion to its weight to be acted upon by the air, falls to the floor. In all the above cases the moving water or air may be considered as acting in opposition to the attraction of the earth, the latter pulling the substance down to the earth, and the former pushing it away from the earth. Of course, the more surface the water or air has to push upon the greater is the effect; and it is to be remembered that the attraction of gravity is as the quantity of matter, without any regard to amount of surface in the body attracted. [Illustration: A B C D Fig. 130.]