. The principles of physics. isplaces a weight of the fluidequal to its oivn weight, or until it reaches a depth where the upward pressure of the fluid is equal to its own weight. Experiment S. — Place a baroscope (Fig. 150), consisting of a scale-beam, a small weight, and a hollow brass sphere, under the receiver of anair-pump, and exhaust the air. In the air the weight and sphere balanceeach other; but when the air is removed, thesphere sinks, showing that in reality it is heavierthan the weight. In the air each is buoyed up bythe weight of the air it displaces; but as thesphere displaces mo
. The principles of physics. isplaces a weight of the fluidequal to its oivn weight, or until it reaches a depth where the upward pressure of the fluid is equal to its own weight. Experiment S. — Place a baroscope (Fig. 150), consisting of a scale-beam, a small weight, and a hollow brass sphere, under the receiver of anair-pump, and exhaust the air. In the air the weight and sphere balanceeach other; but when the air is removed, thesphere sinks, showing that in reality it is heavierthan the weight. In the air each is buoyed up bythe weight of the air it displaces; but as thesphere displaces more air, it is buoyed up , when the buoyant force is with-drawn from both, their equilibrium is destroyed. Ordinary weighing conducted in the airconsists, therefore, in a comparison ofeffective weights in that fluid. The abso-lute weight is, evidently, the effectiveweight plus the weight of the excess of airdisplaced by the body over that displaced by the weights, orit is the weight of the body in a Fig. 150. DENSITY AND SPECIFIC DENSITY. 177 The density of the atmosphere is greatest at the surface ofthe earth. A body free to move cannot displace more thanits own weight of a fluid; therefore a balloon, which is alarge bag filled with a gas about fourteen times lighter thanair at the sea-level, will rise till the weight of the balloon,together with its car and cargo, equals the weight of the airdisplaced. Section VII. DENSITY AND SPECIFIC DENSITY. 147. Meaning of the terms and their relation to each other.—The density of a substance at any temperature is the mass ofa unit volume of the substance at that temperature. Thus, thedensity of water at 4° C. is one gram per cubic centimeter, andthe density of cast iron at the same temperature is about per cubic centimeter. The mean density of a body isfound by dividing its mass by its volume. Thus if the massof a body be 30 grams and its volume be 6 cubic centimeters,its mean density is (30 4- 6 =
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysics, bookyear1895
