THIS CITY full of monuments is about to get another. Modest Albert Einstein is going to be memorialized, at immoderate expense, by a million-dollar statue to be placed beside the Mall. Slouching on his granite bench, he will be 21 feet long. The proposed Einstein statue - a gigantic, ill-advised chunk of public piety - promises to be gross as well as trite.

There is something wrong here. Washington's old statues - of warriors on their horses and sages on their thrones - mock the Einstein effort. Each new statue we erect - the timid Robert Taft, the equally timid Churchill, the bronzes of the Kennedys, and Aurelio Teno's laughable Don Quixote - seems worse than the last.

The Einstein was designed by Robert Berks, whose mediocre, acned bronze heads of John and Robert Kennedy Washington knows well.

The National Academy of Science hopes to raise $1.66 million for Einstein and his bench, his landscaping and lighting. Berks' clay statue, which is now ready for the foundry, is to be unveiled here next April on the grounds of the Academy at 22nd Street and Constitution Avenue NW.

J. Carter Brown, director of the National Gallery of Art, who as chairman of the Federal Commission of Fine Arts approved the Einstein statue, and the Gallery's president, Paul Mellon, who will help to raise the money, both should have known better. The Einstein by the Mall is bound to seem a death-twitch of an old tradition. Huge public portraits made of bronze no longer seem to work.

Perhaps it is because we cannot make them big enough. The statues in the Parthenon as well as those in Rome, when new, seemed colossal. No body then living had seen human figures larger, but the "heroic scale" no longer seems heroic. The scale of the skyscraper has dwarfed most metal statues. Liberty appears more imposing from the sea than she does from an airplane. The Einstein by the Mall, though more than three times life-size, will seem a giant doll.

Perhaps it is because the sculptors we commission simply are not good enough, despite their monstrous fees. (Berks and his assistants will be paid more than $1 million for their portrait bronze). A sculptor with a vast imagination and a jeweler's skill might be able to do justice to a 21-foot Einstein. But if that man exists, we do not know his name.

Before the camera was invented, public sculptures showed us what our heroes looked like. Today, through films and photographs, we already know.

Perhaps most modern portrait statues flop because by trying to seem modern they insult the great tradition they try to extend. While Daniel Chester French in the 1920s could, like the great Praxiteles, make marble look like flesh. Berks, who slaps on globs of clay, does not even try. The great public sculptors of the past fought for verisimilitude. George Segal's plaster surfaces, like those Berks does in clay, make no attempt to look like skin. Few styles are as dreary as watered-down Rodin.

Admittedly, Berks can catch a likeness. When he works in clay directly, at a modest scale, he can make a handsome sketch. In 1953 he spent about five hours in the company of Einstein; and as portraiture, his Einstein is not bad. Charles Atherton of the Commission of Fine Arts, the Federal review board which approved the statue, says it looks like Einstein. Atherton should know.

He was an undergraduate at Princeton when Einstein was a fellow there at the Institute for Advanced Study. "Einstein was a bird watcher," Atherton remembers. "I often saw him wandering, alone or with his friends, looking at the birds. Once I went canoeing there, the day was hot and fine. I put the paddle down, lay back in the canoe, and let the current take me as the clouds moved by. I'll never forget it. I drifted underneath a bridge. Suddenly, above me I saw Einstein, Robert J. Oppenheimer, and George F. Kennedy, the three of them together. They were leaning on the parapet. I was looking up. They were looking down. Berks' Einstein looks like Einstein. Though, of course, the scientist was not 21 feet tall."

But monuments aren't snapshots; they require more than likeness. The past's "larger than life" statues were not only large physically. They idealized their subjects, they implied the superhuman. In Athens and in Florence, as in 19th-century Washington, the statues of the heroes, the rulers and the gods were looked at and believed.

When Athena, Galatea or the angry father who corrected Don Gilovanni stepped down from their pedestals, those who saw them do so were not entirely surprised. Such legends make a point. Viewers who today feel compelled to whisper at the foot of French's Lincoln receive an ancient message. The most impressive statues are superhuman images that somehow strike the viewer as as real as can be.

What is wrong with most new portrait statues here - the Robert Taft by Wheeler Williams, William McVey's Winston Churchill, the two heads of the Kennedys, the Mary McLeod Bethune (another work by Berks) - is not inadequacy of scale, likeness or material, but flabbiness of concept. They are often merely bronze cartoons. The unexpected bees that the great Bernini added to so many of his statues were not decorations, but symbols of the Pope's family and wealth. All who saw those sculptured insects understood their meaning. The public statues of the past were not merely portraits, but complex coded messages of political, spiritual and economic power. While honoring their subjects - the Emperor or the Pope, Liberty or Lincoln - they taught lessons to the viewer. Berks' Einstein will peer down at a floor-map of the stars. His $400,000 Mary McLeod Bethune hands what appears to be a rolled-up newspaper (it's supposed to be her "legacy") to a pair of Negro children. These gestures have small meaning, and the costly statues bear as much relation to the complex monuments they attempt to emulate as, say, the Rayburn Building does to the holy buildings of ancient Greece and Rome.

Einstein deserves better. He was a genius; but Berks' public sculpture here is at best second-rate. Einstein's thought was elegant; Berks' art is not. The faces of his subjects call to mind the underside of movie seats, as if their skin were made of globs of half-chewed gum. If Einstein needs a monument, and if a work of art is called for, another sort of sculpture - say, a weightless, glowing, city-scaled work of laser light by Washington's Rockne Krebs - would seem vastly more appropriate, as well as vastly less expensive, than Berks' bronze effigy.

There are, of course, alternatives to the standard portrait statue. Lyndon Johnson got a standing stone, Robert Taft got bells, admired politicians have had their names applied to buildings, the Marines who died in World War II received a paraphrase, by Felix de Weldon, of an AP wirephoto, but these efforts have within them something slight and sad.

Who applauding at the ballet or screaming for the Redskins recalls the fallen Kennedys? Who motoring beside the Potomac River thinks of Lyndon Johnson when zooming by his rock? The countless flowering shrubs given us by Lady Bird would have been a far more moving memorial to her husband had they been planted in his name. Einstein was a bird watcher, and his spirit flew - why not give him gardens or mockingbirds or nightingales or anything but bronze?

When John Kennedy was president, Stewart L. Udall, his Secretary of the Interior, unsuccessfully proposed a sweeping moratorium on public sculpture here. The proposal for the Einstein demonstrates, belatedly, the wisdom of his effort. Until we learn again how to raise in Washington fitting public statues, why settle for grotesqueries? Enough already, stop! CAPTION: Picture 1, Working on the Einstein statue is sculptor Robert Berks, who also designed the bronze head of President Kennedy at the Kennedy Center. Photo by Richard Meek, courtesy of the National Academy of Science; Picture 2, no caption, by Ken Feil - The Washington Post; Picture 3, Robert Berk's model of the Einstein statue on his bench, Copyright (c) 1976 by Robert Berks