EXPANSION SMOKE. 173 



aim, and at the same time throw away the 

 present. 



When the air is heated, its tendency is to spread 

 or expand, equally in all directions, upwards, 

 downwards, and laterally ; but the actual motion 

 is in the direction of the least resistance ; and 

 heated air ascends in the atmosphere on the very 

 same principle that the heated lava of a sub -marine 

 volcano rises through the waters of the ocean, and 

 does not form a flat cake, or bed, at the bottom. 

 The heated air ascends, and as it gets into air, 

 having less resistance to its expansive force, it ex- 

 pands and cools, so that it at last comes to a 

 place where it has no tendency to move unless it 

 is acted on by a fresh cause. We can have a 

 very tolerable notion of it in the ascent of smoke. 

 That is really the ascent of warm air ; and it is 

 hindered, and not promoted, by the particles of 

 charcoal and water, and other matters, which give 

 the colour to the smoke. A chimney often smokes 

 the most vigorously, where is does not appear to 

 smoke at all ; that is, where there is a bright clear 

 fire, and nothing but warm air ascending; and 

 those furnaces which have their smoke so that it is 

 hardly visible, send their currents of air to a much 

 greater height than those which rain soot all over 

 the neighbourhood. 



Still the visible smoke of fires is one means of 

 observation by which we can get some insight 

 into the motions of ascending currents in the air 

 produced by heat. When they blow all in the 

 same direction, and soon melt into the air, it is a 

 sure sign that the lower stratum of the air is dry, 

 and will resist the descent of rain, even though the 

 upper part of the sky may be cloudy. But when, 

 Q 3 



