Danger: Do Not Look at Welding Arc Or Flame.

We all do anyway, with quick glances meant to satisfy both curiosity and conscience. Down in the site the welders curled around their I-beams ignore us. I have the feeling they couldn't care less about our retinas.

Behind the row of spectators standing at the wire-mesh fence walk office workers returning from lunch. We can feel them peering over our shoulders. Occasionally one of them slows, stutter-steps for a moment, and finally moves over to join the watchers.

Warm days tend to bring nearly as many people to this construction site as to the square up the street. Since the beginning of lunchtime the prime viewing area, here at the corner where you can see almost the entire lot, has been thronged with onlookers.

For those of us who could not heedlessly pass a construction site even if we were 15 minutes late to our own wedding, this warm-weather company is both reassuring and claustrophobic. Dawdling around the edges of a building under construction is a good way to understand the pitched emotions of a voyeur. Large crowds like this one can dull your appreciation of the process, in all its intricacy and grandeur. But they also allay the embarrassment of standing behind a barricade and watching a bunch of grown men work.

Yet very few of the people passing behind me can totally ignore the bustle going on below. By far the majority at least turn their heads, even if it means risking some of their purposeful composure.

This variety of observer merges gradually with the next most serious variety -- the transient watchers. These are the people who start down at the end of the gangway as regular walkers, perhaps more interested than most, turning their heads and wandering in a crooked line along the barrier. Finally they just can't stand it anymore. They face up to the fence, set their gaze on one corner of the site, and in a single methodical movement scan the entire lot. Satisfied that they've absorbed everything of interest, they continue on their way.

The real devotees, the ones who occupy this corner at almost any time of day, are a completely different matter. Take a look at the rag tag assortment of men watching a site the next time you pass: businessman, bum, clothier, courier, retiree, student, night watchman. Only rarely will you see a woman among them, evidently because, as my wife once told me, women get hassled if they stop.

As is most often the case, the main attraction today is the boom. It pirouettes above us like some gigantic utilitarian version of a Calder mobile. I've seen watchers who can suffer the embarrassment of standing in the sidewalk and staring into the sky do it until their necks ache.

Certain phases in the construction of a building also seem to have a special pull. Because a continuous slab of concrete must dry all at one time, major pours often generate an excitement that leaks past the barricades into the street. For sheer spectacle, though, nothing beats the work that must occur even before the construction can begin -- the tearing down of old buildings to make way for the new.

One would think that most of the men standing with me at this fence, given their appreciation of dramatic engineering, would especially relish a major act of demolition. Yet that is not the case. I once asked a man standing at a different site if he'd seen the previous building come down. "Thank God, no," he said with a shudder.

It's a sentiment that I like to think is shared by many of the men standing with me. They are here to watch something being built, not to watch someone else's work being destroyed. Sometimes you can see it in the eyes of your neighbor at the barricade: the innate fascination with the idea that a person can work at a task for a certain period of time and in the end have something to show for it -- a foundation, a concrete column, an electrical outlet. Perhaps that's why so many of us keep coming back, to renew that belief.