204 VAPOUR IN THE AIR. 



purposes 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 fall- 

 ing 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 individual 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 suppose 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 atmosphere ; 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 va- 

 pour 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 den- 

 sity 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 



