There is only one gift we must have before we can learn to know them — we must have imagination.  I do not mean mere fancy, which creates unreal images and impossible monsters, but imagination, the power of making pictures or images in our mind, of that which is, though it is invisible to us.  Most children have this glorious gift, and love to picture to themselves all that is told them, and to hear the same tale over and over again till they see every bit of it as if it were real.  This is why they are sure to love science it its tales are told them aright; and I, for one, hope the day may never come when we may lose that childish clearness of vision, which enables us through the temporal things which are seen, to realize those eternal truths which are unseen.

If you have this gift of imagination come with me, and in these lectures we will look for the invisible fairies of nature.

Watch a shower of rain.  Where do the drops come from? and why are they round, or rather slightly oval?  In our fourth lecture we shall se that the little particles of water of which the raindrops are made, were held apart and invisible in the air by heat, one of the most wonderful of our forces* or fairies, till the cold wind passed by and chilled the air.  Then, when there was no longer so much heat, another invisible force, cohesion, which is always ready and waiting, seized on the tiny particles at once, and locked them together in a drop, the closest form in which they could lie.  Then as the drops became larger and larger they fell into the grasp of another invisible force, gravitation, which dragged them down to the earth, drop by drop, till they made a shower of rain.  Pause for a moment and think.  You have surely heard of gravitation, by which the sun holds the earth and the planets, and keeps them moving round him in regular order?  Well, it is this same gravitation which is a t work also whenever a shower of rain falls to the earth.  Who can say that he is not a great invisible giant, always silently and invisibly toiling in great things and small whether we wake or sleep?

(I am quite aware of the danger incurred by using this word “force”, especially in the plural; and how even the most modest little book may suffer at the hands of scientific purists by employing it rashly.  As, however, the better term “energy” would not serve here, I hope I may be forgiven for retaining the much-abused term, especially as I sin in very good company.)

Now the shower is over, the sun comes out and the ground is soon as dry as though no rain had fallen.  Tell me; what has become of the rain-drops?  Part no doubt have sunk into the ground, and as for the rest, why you will say the sun has dried them up.  Yes, but how?  The sun is more than ninety-one millions of miles away; how has he touched the rain-drops?  Have you ever heard that invisible waves are travelling every second over the space between the sun and us?  We shall see in the next lecture how these waves are the sun’s messengers to the earth, and how they tear asunder the rain-drops on the ground, scattering them in tiny particles too small for us to see, and bearing them away to the clouds.  Here are more invisible fairies working every moment around you, and you cannot even look out of the window without seeing the work they are doing.