Page:Popular Science Monthly Volume 92.djvu/918

 902

��Popular Science Monihhj

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���A passenger ferryboat rammed this mammoth floating drydock in a dense fog. The damaged sec- tion promptly separated from the others, turned sidewise and floated inside its former companions

��Nothing Troubles This Drydock. It Can Repair Even Itself

THE big floating drydock in the harbor of Tacoma, Washington, was rammed in a dense fog by a passenger ferryboat, and one of its four sections driven in. The ferryboat was of the old-fash ned river type with a draft of only three feet. In the winds and currents of Tacoma's open harbor it was notoriously unmanage- able.

But the drydock was a match for the ferryboat. The section which the ferry- bor.t had damaged was promptly sepa- r»jl:ed from the others, turned sidewise, and floated inside its former companions. Men thereupon went to work with a will and elevated the damaged brother above the water's surface so that repairs could be made. And by this time all four sec- tions are back in the water again, joined together, and doing each day their daily work as if nothing at all had happened.

Incidentally it may be worth while to note that do?toring up the damaged section was the drydock's first job, and it accord- ingly started a life of repairing by first re- pairing itself.

In the illustration above, the entire dry- dock is shown at thfe left, while at the right the damaged section is shown inside the two submerged sections of its former self, undergoing repairs. The diagram below shows how collision between drydock and ferryboat occurred.

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��Dry dock

��Ferry

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��Does a Locomotive Wheel Travel Slower or Faster than the Train ?

T is an interesting point to consider that on a locomotive wheel, the cir- cumference is continually traveling at different speeds. First a point on the circumference of the wheel will go faster than the rest of the locomotive; then that same point will go slower; at still other times, the point will travel at a speed equal to that of the locomotive cabin.

This paradox is explained by con- sidering first the point on the circum- ference farthest to the rear of the wheel's center. When the center of the wheel moves forward with the same speed as the rest of the locomotive, that point will move around and in a short time it will get ahead of the center. Obviously, to do this, this noint has to travel faster than the locomotive.

As the train moves on from this position, however, the average speed of that same point will become less than that of the loco- motive. This is evi- dent, since the point will soon change from a position directly in front of the wheel's center to another point directly in the rear. This apparent para- dox is not related to the old saw con- cerning the relative speeds of a kangaroo's hind legs and front legs when jumping Australian sand hills.

���cvjrrervt forced ferry agairvst dry dock '

How ferryboat hit the" dock that repaired itself

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