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losses. The loss of the water in the day, which we see v i 
like steam from the earth, becomes itself the refresher of rid oa came 
water being restored which the heat had taken away. In what manner this i 
accomplished I will endeavour to explain. 
«© After the sun has shone on the earth during the whole day, it is not difficult 
to believe that everything is rather warm ; that is to say, that rock, meadow 
and atmosphere alike are raised-to some degree of temperature. ” Now the 
atmosphere contains water ; indeed, we have just mentioned that the water 
which rises from the earth rises into the air in the form of steam or vapour. 
Evidently, also, the hotter the day the more water there is in the air, as the 
greater the heat of the sun the more water in the ground is converted into 
vapour, just as the hotter the fire the more steam we obtain from the kettle 
Suppose, then, that the air containing the vapour and the grass on the 
meadow are all left at the temperature of 60° Fahrenheit by the sun. When the 
sun sets, everything begins to part with its heat by radiation into empty space, 
It is easy to prove that a hot body will part with its heat to any surroundin 
bodies that are colder, until all are of the same temperature. In this ae 
fire gives out heat to a room. If then the meadows and the rocks on the surface 
of the earth are 60° Fahrenheit and empty space is—242", it follows that the first 
two must radiate out heat. Now different substances have different capabilities 
for radiating out heat. The metals, as well as stone, &c., are exceedingly bad 
radiators, while grass and painted wood are exceedingly good radiators : water 
also is a very good radiator. 
Hence it follows that the grass which was 60° Fahrenheit’ will cool much 
faster than the rock or stone which was 60° Fahrenheit. Hence in the night- 
time grass will get much colder than rock or stone. And water is a good 
radiator ; therefore the water in the air (before mentioned) would get cold very 
quickly ; but air is an exceedingly bad radiator, in effect no radiator at all ; 
therefore the water contained in, and surrounded by the air, cannot radiate out 
heat, so as to grow cool, since the air all round it will keep it warm. 
So then the grass, the rock, and the air which were 60° Fahrenheit, will become 
changed in temperature ; the grass to 10° Fahrenheit ; the rock to 40° Fahrenheit 
and the air almost the same as before. : 
«‘ Now vapour, including the vapour in the air, consists of particles of water 
separated, or in other words vaporized, by heat. If then the heat be taken 
away, the vapour remains vapour no longer, but becomes water. This is called 
condensation. 
“‘ Now the grass acts in this way on the vapour of the air. Tt collects the 
pe a vapour in the form of little drops, by condensing that of the stratum 
of air just round it. This is Dew: this is the provision made by Nature, for 
refreshing the world in dry seasons. 
** Now, regarding the formation of Dew, we shall find that several points are 
required for its plentiful deposition. 
«*J.—A hot day preceding, to fill the air with vapour from the earth, and 
thus give a plentiful supply of it, from which Dew may be formed. It 
will be noticed, also, that this is the time when Dew would be most 
wanted. } 
‘¢J],—A still and cloudless night. For if the wind blow, the stratum of 
air around the blade of grass would be changing so often that there 
would be no time to condense its vapour. And if the cloud obscured 
the heavens, radiation could not take place towards empty space. 
‘Here also we must see that a still cloudless night gives least promise of rain; 
and so Dew is on these nights most needed. Gardeners who wish to prevent 
their plants growing too cold at night, and hence being killed, can protect them 
either by clouds of smoke or thin gauze, or anything else which prevents 
radiation of heat ; and thus the advances of science help onward the humbler 
pursuit ot gardening. 
