VAPOUR IN THE AIR. 199 



so many of the phenomena of the weather phe- 

 nomena which are very important, for the triple pur- 

 poses of pleasure, utility, and health, that every one 

 who is to observe nature, so as either to be pleased 

 or profited by it, should understand them thoroughly. 

 Water can be suspended in the air without falling 

 only when it is in very minute drops ; and as the 

 density of the air decreases as its height above the 

 mean surface of the earth becomes greater, the in- 

 dividual portions of water that it can hold without 

 falling, at any given elevation, must be in proportion 

 to its density at that elevation ; and thus, if we sup- 

 pose water to rise by evaporation from any point in 

 perfectly still air, the vapour which arises from that 

 point will form an inverted pyramid in the atmo- 

 sphere ; and however the upper part of that pyramid 

 may be expanded, it cannot contain more water in 

 the highest foot of its height, than it does in the 

 foot next the point from which the vapour rises. If, 

 instead of a point, the vapour rises from a surface 

 say that of a circular lake one mile in diameter, 

 the vapour will, as it ascends, if there is no wind or 

 current to carry it to one side rather than to another, 

 spread out towards all sides ; so that when it comes 

 to air of only half the density of that on the surface 

 of the lake, it will extend nearly a mile all round ; 

 and as it ascends higher, it will spread wider and 

 wider, till, at the upper part of the atmosphere, 

 where we must suppose the density of the air equal 

 to nothing, it will be diffused round the whole globe. 

 If we could see it, it would be a phenomenon of the 

 greatest beauty ; for the slope of it would not be a 

 straight line, but a logarithmic spiral, similar to that 

 chosen by those consummate artists the gothic 

 builders, by means of which the arches that spring 

 from the columns and corbels melt so beautifully 

 and so naturally into the roof, that all notion of one 

 part supporting another is lost to the perception, and 

 the feeling that we have is that the roof is self- 



