I stepped up to the rock and began to climb. In climbing terms, I was the leader. Before long, the ugly pink, orange, and black rope snaked down thirty feet from my harness, around Mike's hips, to his right hand, where he held it in a power grip.

Types of hand grips have been classified by the English physician John Napier. The two most common grips are the power grip and the precision grip. The power grip squeezes an object strongly between the pads of the fingers and the palm, allowing the full strength of the forearm muscles to be applied. In the precision grip, you use just the tips of your fingers. This grip is used for fine control. You would use the precision grip to start the threads of a peanut butter jar lid turning, and then switch to the power grip when you wanted to seal it tight.

Mike's grip wasn't doing me any good at the moment. If I fell, the rope and I would crash to the ground below. It was time to do something to in-sure my safety. I selected a small wedge of aluminum, which climbers call a chock, from the dozen or so different sizes that I carried with me. Using the precision grip, I wedged the chock into a quarter-inch-wide crack in the mountain. The grip gave me the precise control I needed to make sure that this chock was placed well enough to with-stand shock loads that could be as great as a ton. I used a snap link called a carabiner to attach the rope to the chock and continued climbing, feeling more at ease. Now if I fell, the rope would come tight after I dropped past the chock, and Mike's hand could stop my fall. Once chocks are in place, climbers call them "protection."

If I slipped off the rock when I was fifteen feet above my chock, I would fall thirty feet and would be going thirty miles per hour when the rope came tight. The rope would bring me to a stop rapidly, stretching a few feet in the process and pulling on me with a force several times larger than gravity. Since gravity pulls me down with one hundred and fifty pounds of force, the rope might have to exert as much as half a ton of force on me. Mike would then have to hold the other end of the rope against that half-ton of force.

Wait a minute! How can the grip of a single hand provide the hundreds of pounds of force needed to stop a falling climber? When Mike's hand, in a power grip, squeezes the rope the friction of the rope against the palm of his hand (or his tape on today's climb), exerts over one hundred pounds of force, but that is not enough to stop my fall. However, the rope also wraps around Mike's hips, and the friction of the rope around his hips amplifies the force of his hand several times. Force amplification provided by the friction of a rope running around a cylinder is known as a belay. (The belaying pins in old pirate movies had ropes from the rigging of the ship wrapped around them. Friction of the rope around the pin held the rope in place.)

I climbed on, secure in the knowledge that if I fell I wouldn't hit the ground-as long as the chock didn't pop out of its crack. I climbed using the hook grip, the same grip you use to carry a briefcase. But I was using the hook grip in reverse- to hold me up. In the hook grip, the last two joints of my fingers were bent over and placed on a small ledge of rock less than a quarter-inch wide, the width of a piece of cassette recording tape. Though the hook grip could hold my entire weight, it usually shared the load with my feet, which found small footholds. To John Napier, the hook grip was a secondary grip, but to a climber or a monkey swinging through the trees, there is nothing second class about it. It is the most used grip.

When I found a hold that was not wide enough to allow me to get all of the fingers of my right hand onto it at the same time, I placed my three middle fingers on the hold, then stacked the thumb and little finger on top of them. I was pleased that I had learned a few tricks in my years of climbing, but my pleasure faded when I noticed that I needed the very same hold for my left hand. With the delicacy of a concert pianist (but at about one-hundredth the speed), I removed one finger at a time from the ledge and replaced it with a finger from my left hand. Successful, I reached up for the next hold with my right hand.