“Bert Hall, of Tacoma, sprung something new on the Seattle batters yesterday and he had them walking from the plate to the bench shaking their heads as if puzzled. Bert has practiced pitching what he calls the fork ball until he can actually control it.” Seattle Daily Times, September 19, 1908, p5
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1913. “It does look as if the broken finger which he [Oakland southpaw Harry Ables] sustained is still stiffened up, but we have seen him throw his fork ball and he looked to have as much stuff as ever” (San Francisco Bulletin, May 27; Gerald L. Cohen).
The Dictionary also includes this usage note: “John Thorn and John B. Holway (The Pitcher, 1987, p. 158) claim that Bert Hall of the Tacoma Tigers first employed the forkball on Sept. 8, 1908, and assert that “that day the pitch—and the name forkball—were both born.” The above citation seems to how that they were correct.