He was on the ball! (actually, he was IN  the ball . . . )  The man standing defiantly in front of the falls (or is it a picture backdrop of the falls?) is Jean Lussier, and that's his original signature penned above his head. The date of July 4, 1928 in the autograph signifies when he successfully went over the falls in a 6-foot-diameter rubber ball, built around a steel frame covered with automobile tire material (Notice that the ball is superimposed onto the photo to recreate the event). Lussier was a machinist who lived in Niagara Falls NY, and he designed and built the sphere himself. Inside were rubber chambers filled with oxygen under 35 pounds of pressure, which provided him with such complete protection that he sustained only a minor bump on his forehead. After the 162 foot drop over the crest, the ball was out of sight under the water for only 65 seconds, and Lussier later said that he had absolute confidence in the safe completion of his journey.
Lussier was mistrustful of promoters, so after his successful plunge, he hesitated at signing any contracts for promotional representation. After lengthy procrastination, though, he reconsidered, but then it was too late - he was old news to the promoters. But he was still popular with the public, and being quite enterprising on his own behalf, he may have actually fared better by representing himself. For many years, he displayed his ball at Niagara Falls, and sold small squares of the inside rubber inner tube padding at fifty cents apiece. He was quite a talker, and one can imagine him autographing this postcard (which is most likely from a batch that he might have had printed for promotional purposes) while at the same time exuberantly recounting, and embellishing on, his daredevil prowess, and then selling this card to an enthralled listener.  (Thanks again to Carla Simmons of Texas for donating this postcard to Edsen Breyer's Postcard Museum.)