You may use a kind of soap that is full of invisible little air bubbles; if you do, the soap will float on top of the water.
20. When you drop a dry dishcloth into water, it floats until all the pores are filled with water; then it sinks.
Section 5. How things are kept from toppling over: Stability.
Why is it harder to keep your balance on stilts than on your feet?
Why does a rowboat tip over more easily if you stand up in it?
Fig. 13. The Leaning Tower of Pisa.
In Pisa, Italy, there is a beautiful marble bell tower which leans over as if it were just about to fall to the ground. Yet it has stood in this position for hundreds of years and has never given a sign of toppling. The foundations on which it rested sank down into the ground on one side while the tower was being built (it took over 200 years to build it), and this made it tip. But the men who were building it evidently felt sure that it would not fall over in spite of its tipping. They knew the law of stability.
All architects and engineers and builders have to take this law into consideration or the structures they put up would topple over. And your body learned the law when you were a little over a year old, or you never could have walked. It is worth while for your brain to know it, too, because it is a very practical law that you can use in your everyday life.
If you wish to understand why the Leaning Tower of Pisa does not fall over, why it is hard to walk on stilts, why a boat tips when a person stands up in it, why blocks fall when you build too high with them, and how to keep things from tipping over, do the following experiment and read the explanation that follows it:
Fig. 14.
Experiment 12.2 Unscrew the bell from a doorbell or a telephone. You will not harm it at all, and you can put it back after the experiment. Cut a sheet of heavy wrapping paper or light-weight cardboard about 5 × 9 inches. Roll this so as to make a cylinder about 5 inches high and as big around as the bell. Hold it in shape by pasting it or putting a couple of rubber bands around it. Cut two strips of paper about an inch wide and 8 inches long; lay these crosswise; lay the bell, round side down, on the center of the cross. Push a paper fastener through the hole in the bell (the kind shown in Figure 14) and through the crossed pieces of paper, spreading the fastener out so as to fasten the paper cross to the rounded side of the bell. Bend the arms of the cross up around the bell and paste them to the sides of the paper cylinder so that the bell makes a curved bottom to the cylinder, as shown in Figure 15.
Footnote 2: To the Teacher. If you have a laboratory, it is well to have this cylinder already made for the use of all classes.
Fig. 15. In this cylinder the center of weight is so high that it is not over the bottom if the cylinder is tipped to any extent. So the cylinder falls over easily and lies quietly on its side.
Fig. 16. But in this one the center of weight is so low that it is over the base, no matter what position the cylinder is in.
Fig. 17. So even if the cylinder is laid on its side it immediately comes to an upright position again.
Try to tip the cylinder over. Now stuff some crumpled paper loosely into the cylinder, filling it to the top. Tip the cylinder again. Will it stay on its side now? Force all the crumpled paper to the bottom of the cylinder. Now will it stay on its side? Take out the crumpled paper and lay a flat stone in the bottom of the bell, holding it in place by stuffing some crumpled paper in on top of it. Will the cylinder tip over now? Take the stone out, put the crumpled paper in the bottom of the cylinder, put the stone on top of the paper, and again try to tip the cylinder over. Will it fall?
The center of the cylinder was always in one place, of course. But the center of the weight in that cylinder was usually near the bottom, because the bell weighed so much more than the paper. When you raised the center of weight by putting the stone up high or filling the cylinder with crumpled paper, just a little tipping moved the center of weight so that it was not directly over the bell on which the cylinder was resting. Whenever the center of weight is not over the base of support (the bottom on which the thing is standing), an object will topple over. Moving the center of weight up (Figs. 15 and 16) makes an object less stable.
The two main points to remember about stability are these: the wider the base of an object, the harder it is to tip over; and the lower the center of the weight is, the harder it is to tip over.
If you were out in a rowboat in a storm, would it be better to sit up straight in the seat or to lie in the bottom of the boat?
Why is a flat-bottomed boat safer than a canoe?
Fig. 18. Which vase would be the hardest to upset?
Where do you suppose the center of weight of the Leaning Tower of Pisa is—near the bottom or near the top?
Application 8. If you had a large flower to put into a vase and you did not want it to tip over easily, which of the three vases shown in Figure 18 would you choose?
Application 9. Some boys made themselves a little sail-boat and went sailing in it. A storm came up. The boat rocked badly and was in danger of tipping over. "Throw out all the heavy things, quick!" shouted one. "No, no, don't for the life of you do it!" called another. "Chop down the mast—here, give me the hatchet!" another one said. "Crouch way down—lie on the bottom." "No, keep moving over to the side
