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 of water. That is all. It has been made lighter than the total amount of water it rests upon. A ship is just such a hollow vessel, whether made of iron or wood. When there is nothing in it, a ship stands high, almost on the surface of the water. As it is loaded with goods and people, it rides deeper. Load your sheet iron pan with a cargo of toys. Watch it go deeper. Don’t fill it to the top. That would make it as heavy as if it were solid. Then it would sink.

If you live in a lake or sea-port town, you will find that all ships have a water-line painted plainly around the hull. This is the safety loading line. No ship owners are allowed to load a vessel so heavily that that water line sinks below the surface of the water. Air spaces must be left, to keep the ship and its cargo lighter than the water that is beneath them. In the old days overloaded wooden vessels often sank. Today, iron ships ride the ocean safely.

Have you a stereoscope with views? The views show two photographs, almost alike, mounted side by side on the same card. Yet when you look at them through the lens of the stereoscope, you see but one picture. And that picture stands out, or is, as we say, "in relief." In an ordinary photograph everything appears flat. In stereoscopic views the solid appearance of things, with depth, distance, or perspective, is brought out as in life. That is because the ordinary camera has but one eye, or lens. The stereoscopic camera has two lenses, and takes two views, as far apart as a pair of human eyes. You see but one picture, exactly as you would have seen the real view with your two eyes. If you examine them very closely you will see that the two views, are not exactly alike. One shows more detail on the right outer edge, the other on the left. Look at something with both eyes. Close one eye. At that end the object you look at is blurred. Open that eye and close the other. The object is blurred at the other end. When we look at things, we really get two images from two points of view, as we say. The brain focuses these images as the stereoscope lens does, and brings them together into one view. In this way we see solid, or "in relief," so we judge of size, location, distance, solidity, color, and many other things better than if we had but one eye.

Is your house lighted by electricity? Did you ever stop to think that every other kind of light—of a candle, an oil lamp, or of gas,