I’m 40 feet up in the air on a zipline platform at MONKEY C MONKEY DO in Wiscasset, clinging to a post, and scared out of my mind.

What’s a 75-year-old grandfather, pudgy, never-athletic, and desperately afraid of heights, doing up here? Well, I’m thinking back 40 years to when I first discovered this phobia. Friends took me to the top of a 700-foot cliff in Yorkshire, England. I stepped to the edge and almost fell over. I dropped to the ground and crawled backward, away from the precipice. The void pulled at me, whispering, “Jump, jump.” And ever since, confronted with a treetop or a cliff or a roof or an open balcony, I feel that pull again. The void invites me to jump.

My nine-year-old grandson Joshua needed a partner to scale the huge MONKEY C MONKEY DO obstacle course of rope loops, walking wires, swaying platforms, hanging logs and swings. Nobody was available but me, and I couldn’t back out, much less confess my fear of heights. Macho makes guys like me nuts.

So, for an hour, we struggled up through the vertical maze of three levels, each more difficult and scary, and each higher. Joshua led and kept shouting at me. He has a theory and advice about everything, including rope slings and cable angles. My wife Joan hovered on the ground, taking pictures and thinking about her future life as a widow or the bride of a paraplegic.

We finally managed to reach the top platform, two feet wide and six long, 40 feet up. The attendant suggested that I go first, since the other end of the zipline might require someone to catch my short grandson. I knew perfectly well that I was likely to balk at jumping off this platform, and I didn’t want Joshua to witness that. I also didn’t want him criticizing or coaching or pushing me off. So I suggested that the sublimely confident and highly competent lad should go first. The guide agreed, and Joshua snapped onto the zipline and leapt into space.

Now I’m 40 feet up, clinging to a post, and feeling the platform swaying under me. I’m not confronting the zipline; I’m standing on the edge of a 700-foot cliff with the void below me, urging “Jump, Don, you know you’ve always wanted to jump.”

The attendant doesn’t pressure me. I tell him I cannot do this, and he says that many people hesitate. I say, “This is crazy,” and he replies that many people say that. I confess that I’m terrified of heights, and he just smiles. He’s clearly in cahoots with the void.

But I know that I cannot not do this. I cannot climb down and live the rest of my life. “Jump,” the void commands.

There’s only one way down, the zipline. I clutch the safety straps and plunge into the void.

Don Fry owns a summer house in Edgecomb. He evidently survived.

MONKEY C MONKEY DO is located at 698 Bath Road, Route 1 in Wiscasset. It is open May 19 through June 1 on Saturdays and Sundays only, beginning at 9 a.m. with the last full course reservations at 4:30 p.m. From June 23 through September 3 (Labor Day) it is open 7 days a week, beginning at 9 a.m, with the last full course reservations taken at 4 p.m. From September 8 through October 8 (Columbus Day) it is open Saturdays and Sundays only, opening at 9 a.m. with last full course reservations taken at 4 p.m. Weekday reservations for schools and groups available. For more information, call 882-6861 or visit www.monkeycmonkeydo.smugmug.com.