On A Mixed Constitution

I've been trying to track down whether the American Constitution was inspired by the theory of the mixed constitution. I've read claims stating that the separation of powers results from accepting Newtonian mechanics as the model for government, but never quite fully believed them. I always thought the original American Constitution functioned more like a mixture of monarchical, aristocratic, and democratic elements, each serving to offset the other. Now I have found some hard evidence to back up my intuition.

First, a paper by a student who examines the influence of Polybius on the separation of powers doctrine found in Madison's works, like Federalist 10.

Second, a paper titled The Mixed Constitution and the Common Law which is even more detailed than the aforementioned text, tracing the influence of the mixed constitution from the decline of absolute monarchy through the American War of Independence. It indicates that Montesquieu was indeed of a mechanistic bent. However, he was undeniably influenced by Polybius, the originator of the idea of the mixed regime, as was Madison.

So now let's consider the American system of government as a mixed constitution. The president obviously corresponds to a monarch, the House of Representatives and Senate corresponds to the democratic elements. But what of the aristocracy? Perhaps before the 17th Amendment, which mandated the popular election of senators, rather than indirect election by the state legislatures, the Senate could indeed have been considered the aristocratic portion of the government. Now, it seems, the judiciary branch is the closest we have to an aristocratic order in the government. Of course, the independent judiciary is a development based not on the classical theory of the mixed regime, but on English attempts to develop that tradition further and provide additional checks on government for the securing of liberty.

But wait--that word "liberty"--isn't that a change from classical mixed government, which sought to secure the common good? I suppose it is, unless we are astute enough so as to define liberty as a common good. Which, all things considered, is a very good move.