I would like to report that the following individuals submitted correct solutions to the problem:
Holly N. Likely
Hugh Lyon Sachs
Lotta B. Essen
Payne N. Dias
C. Colin Backslash
Ralph Oliver d'Fleur
However, I am unable to do so. Because nobody with a really funny name managed to solve the problem. In fact, only one reader was able to do so at all. "Steve Davidson" (if that is his real name) is a member of the Inventor's Association of Western Australia, obviously highly intelligent, of average weight and height for an Australian, and an excellent driver. Congratulations, Steve!
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The solution? The following series of images explains how little more than rocks and string can be used to make an accurate water tank depth indicator.
Three rocks are connected by two pieces of string. The bars, tree branches or pulleys over which the strings are looped are incidental to the way this depth gauge works, rather than fundamental. Less friction there improves performance, but they aren't central to how this device works.
Two of the rocks are of roughly equal weight. The third rock is much smaller than the other two. Because the the two outer rocks together weigh more than the middle rock, they rest on the ground while the middle rock is suspended.
The right-hand rock is positioned inside a water tank. The middle rock is often concealed inside a tube or pipe. The smaller rock must be visible from the outside.
When water is placed in the tank, the right-hand rock gets submerged. Under water, things weigh less. The two outer rocks combined no longer weigh more than the middle rock. It therefore falls.
It is so simple that most of you are thinking, "Why didn't I think of that?" While the rest of you are thinking, "I KNEW it was something like that!"
The first guy to really understand this business about things weighing less under water was so excited by his new understanding that in his eagerness to get back to his laboratory he forgot to put his pants on. He ran all the way there in the nutty. This is why today we have the saying, "he is too smart for his britches."
Archie of Syracuse (b. 287 BC) realized that for anything to be under water, it must push out of that space an equal of volume of water. Therefore its weight is reduced by the equivalent weight of the displaced water.
Rocks take up space and are heavy; string keeps things connected. Everything built by Human can be reduced to the four Blokian Elements: Rocks, Sticks, String and Glue. If you understand the way these four concepts work, you can build anything.