74 THE FAIRY-LAND OF SCIENCE. 
a small glistening drop out of the body of water 
below, and hold it before you. Tell me, have you 
any idea where this drop has been ? what changes it 
has undergone, and what work it has been doing 
during all the long ages that water has lain on the 
face of the earth ? It is a drop now, but it was not 
so before I lifted it out of the basin ; then it was 
part of a sheet of water, and will be so again if I Jet 
it fall. Again, if I were to put this basin on the stove 
till all the water had boiled away, where would my 
drop be then ? Where would it go ? What forms 
will it take before it reappears in the rain-cloud, the 
river, or the sparkling dew ? 
These are questions we are going to try to answer 
to-day ; and first, before we can in the least under- 
stand how water travels, we must call to mind what 
we have learnt about the sunbeams and the air. We 
must have clearly pictured in our imagination those 
countless sun-waves which are for ever crossing space, 
and especially those larger and slower undulations, the 
dark heat-waves ; for it is these, you will remember, 
which force the air-atoms apart and make the air 
light, and it is also these which are most busy in 
sending water on its travels. But not these alone. 
The sun-waves might shake the water-drops as much 
as they liked, and turn them into invisible vapour, but 
they could not carry them over the earth if it were not 
for the winds and currents of that aerial ocean which 
bears the vapour on its bosom, and wafts it to different 
regions of the world. 
Let us try to understand how these two invisible 
workers, the sun-waves and the air, deal with the drops 
