My parenthetical remark that Sir William Jones is incorrectly viewed as the discoverer of the Indo-European language family and founder of modern historical linguistics provoked the question in the comments of why I said this.
There are two points to make here. The first is that Jones was not the first to recognize the relationship of some of the languages that comprise the Indo-European Language family. Hypotheses very much like Jones' go back to the "Scythian Hypothesis". Even Sanskrit was brought into the picture as early as 1643 by Claudius Salmasius.
The second and more important point is that Jones cannot be considered the founder of modern historical linguistics because he did not use the comparative method, the crucial innovation that distinguishes modern historical linguistics from its predecessors.
We don't actually know very much about Jones' methods and the reasons underlying his linguistic proposals because he doesn't tell us much. The famous paragraph about Indo-European appears in one of the series of annual lectures he gave to the Asiatic Society of Bengal. This lecture series was not narrowly devoted to linguistics. Rather, it described the project on which he had embarked of reconstructing the migrations of human populations. The links that he saw between one linguistic group and another formed but one part of the evidence he gave, along with similarities in religion, customs, architecture, and various other features, legends, and the writings of ancient historians. In this context, he couldn't go into the details of his linguistic arguments, and unfortunately, he never wrote about them anywhere else.
As far as we can tell, Jones was vaguely aware of some of the methodological problems that arise in classifying languages, such as the need to distinguish inherited words from loans, but did not in practice take these problems seriously enough, as evidenced by his erroneous classification of Pahlavi and Malay as Semitic and Tibetan as Indo-European. Nor did he use phonological correspondences to determine whether perceived similarities between languages were systematic.
I haven't gone into great detail here because the details are complex and the subject is treated at length in the book on Language Classification that Lyle Campbell and I have just published. Chapter 3 is devoted to Jones.
I hasten to add that although Jones' role in the development of historical linguistics has often been over-estimated, he was nonetheless an interesting and sympathetic figure. He was one of the very few European scholars of his time to learn such languages as Arabic, Persian, and Turkish well and to publish careful translations of their literature. He was also a political progressive. A friend of Benjamin Franklin and supporter of the American Revolution, he very nearly immigrated to the United States. Though an official of the British colonial government in India, he was not a colonialist. One of his reasons for learning Sanskrit was so that he could study Indian law.