Still browsing IMSLP for Renaissance and Baroque works I came across this,

Theory of Harmonics by John Keeble dated 1784 (if I read my Roman numerals correctly) (Organist at St George's, Hanover Square, Wiki entry below)

http://imslp.org/wiki/The_Theory_of_Harmonics_%28Keeble,_John%29

https://en.wikisource.org/wiki/Keeble,_John_%28DNB00%29