Page:Popular Science Monthly Volume 91.djvu/454

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��Popular Science Monthly

��the train in the manner explained in the illustration. Should the block ahead be clear, electric current comes off of the ramp at the same time that the latter is causing the air valve to open. This current works the electro- magnet shown, and sets a catch or bolt so that the shoe in descending auto- matically closes the valve again, thus letting the train proceed unhindered. If the block ahead is not clear, there is no electric current at the ramp, the electromagnet therefore can not set the bolt, and the result is that the shoe and its plunger descend off the ramp without closing the valve in the process. The latter

therefore remains open where it has been pushed, the brakes become set, and the train comes to a halt.

How the electric cur- rent gets into the ramp to perform this setting of the bolt feat is too long a story to tell here. The current comes from a roadside battery which is a part of, or at least under the control of, the regular block-signaling system and its signal arms. This establishes the needed co- operation between the signals and the auto- matic stop, and causes the stop to enforce the signal's mandates, which is the end desired. Be- sides opening the air valve at least momen- tarily at every ramp, whether it is needed or not,. Mr. Webb's appa- ratus sounds a whistle in the cab at the same time, confirming the other indi- cations. A governor also comes into action, pre- venting the engineer from going beyond a certain speed in a danger zone.

���The Webb device. When the engine should stop, the shoe rides up an inclined rail and actuates the brake mechanism

���The shoe is here shown sliding downward and off the ramp, closing the air valve so the train can proceed in safety

��New Answers to the Query, "Why Does a Cat Have a Tail?"

THE question having arisen, "Why does the cat have a tail ?" the scientists seem willing to answer it with another question, "How would the cat look without a tail?" No one can gain- say the fact that the tail is a valuable ornament. The cat without one is a sorry sight. But there are those who maintain that the tail serves the cat as a sort of gyroscope, balancing the body in leaping. This cannot be wholly true, for Manx cats get along very well without tails, and rabbits have no use for them at all. Yet both the Manx cats and rabbits do a lot of leaping. After all it looks as if the tail is only an ornament, unless it is a kind of safety valve for expression in exciting times.

Of course there are in- stances where the tail serves some purpose other than that of art. The monkey finds his useful as a sort of fifth leg; the horse uses his as a fly-swatter, as does the lion; the crocodile uses his for swimming, as do the seal and the turtle and other aquatic creatures; and the rattle- snake uses his for warn- ing enemies. According to W. D. Matthews, of the American Museum of Natural History, the tail was a necessary organ for the aquatic and am- phibious ancestors from which the higher animals are descended. When they took to terrestrial life and to walking on all fours, the tail became more or less superfluous.

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