438 



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. 



