266 



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. 



