SWIMMER. — So is called a little man from hematite, hanging from a glass sphere B full of air, having a small hole o at its lower part (Figure 70). This contraption is inside a jar full of water, closed at its upper end with a tight membrane AA. When the swimmer is set free, he rises, because his weight is less than the weight of the water he displaces. But if we press the tight membrane AA, we observe that he sinks at once, and if also we stop pressing gradually, the swimmer can balance at whatever depth he is in. The reason for this is as follows. When we press the membrane, the water being pressed by it flows inside the glass sphere B by means of the hole o, and the more of it, the stronger we press; so the difference between the whole weight (which becomes greater by the water flowing in), and the buoyancy (that is to say the weight of the displaced water, which always remains the same), becomes all the more greater, so the downwards acting force increases, and the swimmer sinks as long as this difference is greater than zero. If, while the swimmer is at a given depth, we make this difference equal to zero by lessening properly the pressure on the membrane, the swimmer surely will balance at the depth he is in.
Στοιχειώδης Φυσική Πειραματική, 1879
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