Why does the author of federalist #51 call the u. s. a 'compound republic,' and why does he argue that this creates a 'double security' for the 'rights of the people'?

James Madison, the author of the paper "Federalist #51" call the US a "compound republic" to describe the complexity of the government system comprised of several parts that form a substance. The consequence of having a government system that is complex and divided is that the federals check on the states, and the states check on the federals. That is what he refers to as "double security". This generates more insecurity about how the rights of the people are handled, and the feeling that the power of the government is independent of them.

the mending wall was written by robert frost

  this riddles' answer is library.

  the "leaves" are a reference to the books' pages that the library contains; the branches are a network of libraries that connect multiple libraries' collections, resources, and items in a system of a common database, integrating them; the branches' organization are a useful management tool for coordinating the multiple collections many libraries have.