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We are so familiar with the restlessness of the sea, and with the havoc which it works on our shipping and our coasts, that we need no demonstration to convince us that waves can carry energy from one place to another. Few of us, however, realise that the energy in the sea is as nothing compared with that in the space around us, yet such is the conclusion to which we are led by an enormous amount of experimental evidence. The sea waves are only near the surface and the effect of the wildest storm penetrates but a few yards below the surface, while the waves which carry light and heat to us from the sun fill the whole space about us and bring to the earth a continuous stream of energy year in year out equal to more than 300 million million horsepower.

The most important part of the study of Radiation of energy is the investigation of the characters of the waves which constitute heat and light, but there is another method of transference of energy included in the term Radiation; the source of the energy behaves like a battery of guns pointing in all directions and pouring out a continuous hail of bullets, which strike against obstacles and so give up the energy due to their motion. This method is relatively unimportant, and is usually treated of separately when considering the subject of Radioactivity. We shall therefore not consider it in this book.