A DROP OF WATER. 77 
a small glistening drop out of the body of water be- 
low, 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 let 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 spark- 
ling 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 learned 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 visible 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 
