92 THE FAIRY-LAND OF SCIENCE. 
the liquid water, and when the star melts, this 
space reveals itself to us in the bright spot of the 
centre. 
We have now seen our drop of water under all its 
various forms of invisible gas, visible steam, cloud, 
dew, hoar-frost, snow, and ice, and we have only time 
shortly to see it on its travels, not merely up and 
down, as hitherto, but round the world. 
We must first go to the sea as the distillery, or the 
place from which water is drawn up invisibly, in its 
purest state, into the air ; and we must go chiefly to 
the seas of the tropics, because here the sun shines most 
directly all the year round, sending heat-waves to shake 
the water-particles asunder. It has been found by 
experiment that, in order to turn I Ib. of water into 
vapour, as much heat must be used as is required to 
melt 5 Ibs. of iron ; and if you consider for a moment 
how difficult iron is to melt, and how we can keep an 
iron poker in a hot fire and yet it remains solid, this 
will help you to realize how much heat the sun must 
pour down in order to carry off such a constant supply 
of vapour from the tropical seas. 
Now, when all this vapour is drawn up into the 
air, we know that some of it will form into clouds 
as it gets chilled high up in the sky, and then it will 
pour down again in those tremendous floods of rain 
which occur in the tropics. 
But the sun and air will not let it all fall down at 
once, and the winds which are blowing from the 
equator to the poles carry large masses of it away 
with them. Then, as you know, it will depend on 
