172 EXPANSION SMOKE. 



much ease and certainty as we could state the facts 

 before our eyes. But that is an extent of knowledge 

 which no human being can by possibility attain ; and 

 the utmost we can expect as the reward of the most 

 careful observation is to understand what is actually 

 before us, and make a shrewd but silent guess at 

 what may immediately follow. In all things the 

 past is the only mirror in which we can see the 

 future ; and if we search for knowledge of it any- 

 where else, we fail in our 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, down- 

 wards, 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 prin- 

 ciple that the heated lava of a submarine 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 expands 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 pro- 

 moted, 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 

 it 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 pro- 

 duced by heat. When they blow all in the same 



