It's at its best at night. When you see it from across the dark expanse of City Hall Plaza, the six pale glowing towers resemble the masts or smokestacks of a ghost ship that is passing silently through the streets of Boston. The effect is magical, a mirage, something that seems not so much present as remembered or dreamed.

But at other times, the New England Holocaust Memorial is a mixed bag. Good causes are no guarantee of good architecture. Now that the memorial has been open for a few weeks and the emotion of the dedication has faded, we can experience it in its ordinary state. There is no surprise -- though there is disappointment -- in learning that seen by day, or close up, it is less compelling than that first breathtaking faraway vision.

The memorial was designed by Stanley Saitowitz, an architect born to a Jewish family in South Africa and now based in Los Angeles. Saitowitz got the job by winning an architectural competition in 1991. Designers from all over the country, and a few from overseas, sent in 520 different proposals. A jury chaired by the noted architect Frank Gehry picked Saitowitz's.

The good news is that the memorial is pretty successful urban design. Saitowitz realized that the site was a huge problem, and he solved it well. He had no choice about the location. Stephen Coyle, Boston's chief planner at the time, offered it, and the sponsors took it. It's a difficult site, a relic of the urban renewal of the 1960s, intended for a motor hotel that never got built. The location is handy to the Freedom Trail, but as a space it's no more than a glorified traffic island, cluttered with junk, including two prominent and bulky traffic-signal vaults. And it's caught between a rock and a hard place: the huge Boston City Hall on one side, and the delicate old Blackstone Block -- Boston's last surviving chunk of 18th-century streetscape -- on the other.

Saitowitz realized that if his memorial was going to have any presence in such a place it would have to be very bold. And it is: The towers are as tall as a six-story building. Even forgetting their purpose, they're a net gain for Boston. They've given a useless scrap of leftover turf a unique presence. They make the streets on either side feel more like streets and less like the fringes of some kind of lunatic traffic rotary.

For the visitor, the experience of the memorial is one of moving along a path through a series of small events. The towers are arranged in a gentle curve, straddling a path paved in black granite. The visitor walks in either direction along this path, passing through the towers at their bases.

Here's where the problems begin. The memorial is a try-this, try-that anthology of too many motifs. It's more like a set of unrelated notes than a finished, integrated work. There are the towers, which are made of thick tempered glass etched with 6 million numbers in random sequence, symbolizing the Jewish victims of Nazi murder. The towers are meant to suggest many things, among them the chimneys of the death camps and the seven candles of the menorah (with one symbolically missing). There are fiery pits underfoot, beneath grills, labeled with the names of six of the camps; the pits symbolize the terrible crematoria furnaces. There are words, inscribed on various surfaces of black granite or glass: bits of history, exhortations, statistics, quotations.

There is even, somewhat incredibly, a wall carved with the names of the sponsors and donors, as if the memorial were some ordinary building -- just another hospital wing, say. There are trees, grass, beds of ivy, benches. Saitowitz's original concept was to hack and stunt the trees, which were growing in good health on the site, to suggest both destruction and survival. But this idea was abandoned and the trees have merely been cut back in size to allow the towers to rise above them.

Much of all this is moving, especially some of the quotations, and much is informative. Although the memorial is dedicated to the 6 million Jews destroyed in those years, it is scrupulous in calling attention to other victims of the Nazis: Polish Catholics, Balkan Slavs, gays and Gypsies among them.

But things just don't cohere. The memorial is a rebus of unrelated parts. And some aspects are so literal-minded that they lack gravity, almost like the features of a model railroad, notably the furnaces made from optical fiber and steam. What is missing is the one thing a monument must have: a single commanding, memorable idea that governs every part. I suppose it's unfair to compare any monument with the great Vietnam Veterans Memorial in Washington, but the comparison does suggest what's lacking here. The Vietnam memorial is far richer in symbolic meanings than this one, yet it remains one simple, single, unforgettable thing.

Many features are puzzling. Why, among so many words, are we not told about the numbers etched in the towers -- how many there are and what they represent? Why from the path do we see these numbers backward? More important, why are the towers made in the particular way they are? They are constructed like high-tech commercial storefronts -- an assemblage of plate glass, brushed metal framing and shiny clips that looks as if it could be disassembled with a screwdriver. The details are elegant and suave, but are those the qualities we want in a memorial to the Holocaust?

Other details raise questions. The night lighting, set into the grass, is glary and distracting (perhaps it can be adjusted). Some of the lettering is difficult to read. The path is narrow, and even a small crowd creates congestion at the towers. And there's the issue of maintenance, which is to be taken over by the National Park Service. Those grills in the paving and those innumerable glass connections will be hard to keep clean.

Oddest of all is the relation to the twin bronze statues of Mayor James Michael Curley, which share the site. As you come to the end of the Holocaust walk, the paving changes from black to brick, and suddenly you're confronted by the genial politician. It's like a serious drama breaking for a commercial.

You have to feel sympathy for Saitowitz. It's next to impossible to memorialize anything as vast as the Holocaust. It's hard enough to agree on a way to honor even as specific a subject as President Franklin D. Roosevelt, whose memorial in Washington is now being built after more than 30 years of controversy and two rejected designs. In Berlin, a competition for a Holocaust memorial was held last year, and the government has just decided it won't build the winning proposal.

Creating a convincing memorial is tough in an age like ours, because we lack a common visual language of public symbols. You can't do a hero on horseback anymore; if you did, nobody would notice. How many of us were aware that the recent Million Man March in Washington took place at the Grant Memorial, which is a vast, 252-foot-long heap of bronze horses and cannons and lions with, as a climax, Gen. Grant himself embodied in the form of the world's second-largest equestrian statue? If we don't notice that, what do we notice?

Given all these next-to-impossible problems, the New England Holocaust Memorial comes off pretty well. But it certainly isn't great art or great architecture, and that's too bad. The purpose of a memorial isn't really memory; memory we can get from books. It's emotional catharsis, the kind that comes only from art -- as it does at the Vietnam Memorial, where every visitor I've ever seen is more or less crying.

One design did do this subject justice: The national Holocaust Museum in Washington, by James Ingo Freed, is a masterpiece of its kind. Perhaps once was the right number of times to make the attempt.