George Travis, Letters to Edward Gibbon, 1785, pp. 319-20 The value of this opposing "evidence from silence" became a part of the verse debate,

Richard Porson responding in his letters
Letters to Mr. Archdeacon Travis, 1790, p 372
There is too much involving Travis, Porson and many follow-up writers that we can go into on this thread.

The "evidence from silence" Joel is referring to has to do the many dozens of writers, mostly in Latin, who referenced the verse over the years, and why the authenticity of the verse was never questioned. That discussion is on the two pages he gave as urls. This is in fact a very minor blip on the radar of the debate about authenticity.

The solecism, eg. (which did not come up in Travis-Porson) in the short Greek text, would be perhaps 1,000 and much more times more significant as an argument for authenticity.

As would noting the 'frivolous' arguments made against the authenticity of the Vulgate Prologue by Jerome. The Prologue an evidence essentially proves the authenticity of the heavenly witnesses.

As a scholarship reference, Porson and Travis are not a problem, not shoddy scholarship like the five or so I mentioned above. Although it does not even scratch the surface, there is no real substance.

Richard Porson (1759-1808), as I mentioned, was a hard-drinking skeptic who died young. He was a superb Greek scholar in the classics so he used that to his advantage. And he knew every sneering, diversion cheap debating trick in the book. He also know how to avoid and evade salient evidences, like the Eugenius Bulgaris writing on the solecism.

Much of the modern contra movement against the heavenly witnesses verse just involves echoes of Porson.