The Origin of the Second Law of Thermodynamics

I remember when I learned about the second law of thermodynamics: that the total entropy of a closed system can only increase where entropy is a measure of the disorder of a system. 

When I learned this I remember thinking: why did anyone make an equation to describe disorder?  How did they make an equation for disorder?  And how and why did scientists start to believe, to quote the singer David Byrne from Talking Heads, “things fall apart, it’s scientific”?  To answer all of those questions...