VII 



The Atmosphere 109 



mosphere throughout its whole depth. Twenty million 

 meteorites are calculated to reach the Earth every day, and 

 most of these are broken up by the friction of the air, 

 furnishing a supply of Cosmic dust ( 134), which being 

 excessively fine, and even invisible, settles down very slowly. 

 Terrestrial dust is carried into the atmosphere by ascending 

 currents of air and is of many kinds, resulting from the 

 wearing down of rocks, from volcanic explosions ( 297), 

 from flowers in the form of pollen, from minute organisms 

 either plants or animals ( 401), from burning fuel, from 

 factories, mines, flour-mills, and from the spray of the sea. 

 The number of motes is almost incredible. Every puff of 

 smoke from a cigarette contains about 4000 million separate 

 granules of dust. Dust appears to float in the atmosphere, 

 and the motes of a sunbeam seem to be rising as often as 

 falling. This is, however, a result of currents of air. In 

 still air, dust always falls, but the large motes fall most 

 rapidly under the pull of gravitation, and against the 

 resistance of the friction of the air. When a cube of 

 stone one inch in the side is falling, its mass drags it 

 down, and the friction of the air on its six square inches of 

 surface resists the fall. If the cube were cut into ten slices 

 y 1 ^ of an inch thick, each of these into ten bars, and 

 each of these into ten cubes T ^ of an inch in the side, there 

 would result 1000 little cubes drawn down by the same 

 force as had acted on the one ; but the atmosphere would 

 now have sixty square inches of surface to act on. If each 

 of these little cubes were cut into 1000, the downward 

 attraction of the Earth on the whole million would be the 

 same as for the one-inch cube, but the air-brake would be 

 applied to no less than 600 square inches of surface, so 

 that their fall must be very slow indeed. The average dust- 

 motes of the air are much smaller than the^e, hence it is 

 not surprising that even the stillest air is never free from 

 dust. 



162. Quantity of Dust in Air. Mr. John Aitken, the 

 discoverer of the importance of dust in Nature, invented an 

 ingenious piece of apparatus by which he was able to 

 count the number of invisible dust-motes in any sample of 



