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Page:Popular Science Monthly Volume 90.djvu/282

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��Popular Science Monthly

��the span is reached which corresponds to a pressure as great as the stones can safely bear, and accord- ingly we thus find the limiting span over which a single arch of masonry can be extended. Apply these principles to the stupendous arch formed by the ring of Saturn. It can be shown that the pressure on the materials of the arch capable of spanning an abyss of such awful magnitude would be something so enormous that no materials we know of would be capable of bearing it. Were the ring formed of the toughest steel that was ever made, the pressure would be so great that the metal would be squeezed like liquid and the light structure would collapse and fall down on the surface of the planet."

What materials can sustain a stress so stupendous? We must look not for ma- terials, but for force which opposes the attraction of Saturn. This force is the so-called centrifugal force. Sir Robert Ball says: "If we imagine the ring to rotate, the centrifugal force at all points is in an op- posite direction to the attractive force, and hence the enormous stress on the ring can be abated and one difficulty can be over come."

What are the Strange Rings?

Mathematicians have studied this prob- lem of Saturn's rings. It is one of the most difficult, most fascinating in all astronomic- al science. And what is the conclusion of the mathematicians? Simply that each ring is an enormous shoal of extremely minute bodies. "Each of these little , bodies pursues an orbit of its own around the planet and is in fact merely asatellite. These bodies are so numerous and so close to- getherthat they seem to us to be continu- ous, and they may be very minute — per- haps not larger than the glob- ules of water found in an or- dinary ■ cloud above the sur- face of the earth, which even at a short distance, seems like a continu- ous body." In- struments have been devised which show that the mathe- maticians are

��right in thus conceiving the structure of Saturn's rings.

Ten Moons Revolve Around Saturn Saturn has ten moons. The largest of these is Titan which has a diameter of 2,484 miles. Their names in the order in which they are to be found outwardly from the planet are Mimas, Enzeladus, Tethys, Dione, Thea, Titan, Themis, Hyperion, Japetus and Phoebe. Two of the moons are so small that they cannot be seen through the telescope at all. Only on the photographic plate are they visible. Our cover picture, based on a drawing by Morell, the astronomical artist, shows the wonderful ring system as it would appear in the most powerful telescopes. For the sake of effect, he has, as it were, taken his stand upon Japetus, the only one of Saturn's ten moons from which this view could be obtained. The other satellites keep strictly to the plane of the ring. Japetus itself — which is apparently the size of our moon but is ten times farther from the parent body — is remarkable for the variability of its light, which suggests the presence of an atmosphere and the formation of clouds. Proctor and other astronomers even be- lieved that it might be the abode of living creatures, but that view is not shared by astronomers of our time.

���Caissons built on a pontoon on which they are car- ried to the desired location and floated into position

��Floating a River Caisson Into Position for Sinking

RIVER caissons which act as bridge or . pier supports are usually built either on the shore and skidded or launched into the river, or else they are built on a pontoon, made of two or more parts bolted together, as the accom- panying photo- graph shows. When the cais- son has been placed in the correct position for sinking, the pontoon is un- bolted and flood- ed , which allows the caisson to float into posi- tion.

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