The first English daily newspaper, The Daily Courant, began publication is 1702. This might be called the beginning of the mass media, at least in England. And with the rise of mass media, “the public” was invented. The public is basically the reading public, or the public that follows the news. The following Google Books Ngram shows that the usage of the term “the public” began to sharp rise in 1736.
Closely associated with the public is public opinion. Here is an Ngram for public opinion.
The rise begins in the 1750s and hits it peak in 1917, during World War I.
Two common phrases that start with the public are “the public peace” and “the public welfare.” Here is an Ngram for the two phrases:
They begin to rise together in about 1740, and they track closely for about 100 years.
Lei Zhang and I discuss the rise of journalism as a function system in our 2017 article “Grass-mud horse: Luhmannian systems theory and internet censorship in China.”