THE FORMATION OF DEW. 249 



Hexham with an acute observer, trained to farming, that, on my 

 remarking that the farmers might to their profit remove the extra- 

 ordinary quantity of small stones from the fields in order to give 

 room for the growth of the grain, he shrewdly said, 'These stones 

 collect moisture from the ground ; the soil is thin, with a gravelly 

 subsoil, and unless the maximum amount of moisture is collected 

 (which can only be done by allowing these stones to remain), there 

 would be a very deficient crop. They must not, therefore, be 

 removed.' 



Dew, then, rises from the ground. But how is the dew formed 

 on bodies high up in the air ? If the dew comes out of the 

 ground, should it not be found on bodies only exposed to the 

 earth ? Now, dew does not rise in particles, as it was once con- 

 sidered to fall in particles like fine rain. It rises in vapour. Some 

 is caught by what is on the surface of the earth, but the rest 

 ascends in vapour form until it comes in contact with a much 

 colder surface, to condense it into moisture. The vapour does not 

 flow upwards in a uniform stream, but is mixed in the air by eddies 

 and wind currents, and carried to bodies far from where it rose. 

 In fact, dew may be deposited, even though the country for many 

 miles all round is dry and incapable of yielding any vapour. In 

 such cases the supply of vapour to form that dew would depend 

 on the evaporation of the dew, and on what was wafted over by 

 the winds. 



But the most practically convincing proof of the rising of dew 

 from the ground is in the form of hoar frost or frozen dew. If it 

 has been a bright, clear, sunny day in January, with no snow on 

 the ground, look over the garden, grass, and walks on the morning 

 after the intense cold of the night ; big leaves may be found scat- 

 tered over the place. You see little or no hoar frost on the upper 

 surface of the leaves, but turn up the surface next the earth, or the 

 road, or the grass, and what will you see ? You have only to 

 handle the leaf in this way to be highly astonished. A thick, 

 white coating of hoar frost, as thick as a layer of snow, is on the 

 under surface. Leaf after leaf will present the same appearance. 

 If a number of leaves have been overlapping each other, then 

 there will be no coating of hoar frost under the top leaves ; but 

 when you reach the lowest layer, next the bare ground, you will 



