THE FRENCH GARDEN is a fact and a concept of Western civilization, more than it is a "garden" as we know it.

Someday, beyond doubt, the highway from Washington to New York or, for that matter, to Chicago, will be called a garden, though we do not think of such roads as gardens.

We do not have in our language a convenient word for "managed landscape" and "park" does not quite do, so we say "garden," when we mean those tremendous landscapes of Vaux-le-Vicomte or Versailles.

Because of our own (deplorable) economics, our gardens are small, often smaller than the house itself.

Therefore, to us the word "garden" means the small bit of land about the house, usually with a paved place to sit and few agonized-over bushes and herbs.

We are far more at home with the gardens of ancient Egypt, or of Europe in the Middle Ages, or of Persia and Spain, than we are with the great gardens that began appearing in France in the the 1500s and reached their height of opulence in the 17th century.

Because we have no use for such gardens in our own designs, we too easily forget that the things we know best and use most - the small stone terrace, the small pool with one water-lily, the four chairs, the arbor for one vine - are hardly suitable for surrounding a Renaissance palace.

And just as Versailes looks absurd in imitation in a small town house, so our own gardens look absurd blown up to cover a square mile.

Now a new book, "The French Garden" by William Howard Adams, does a great deal to correct the prejudice we are likely to feel for those gardens we can learn nothing from, in a practical way.

He takes an old garden like the Chateau d'Amboise and shows the plans for it. This was of course a garden for a Renaissance palace, yet the garden itself was laid out in neat little squares with a fountain basin, exactly as if it were in some medieval fortress or cloister.

But the French began learning from Italy in the late 1400s, and learning not least from the new-fangled Italian gardens. Adams shows how the Italian vision of spaciousness and leaping the boundaries startled the French, with their little squares inside walls, and led at last to places like Versailles where the avenues stretch for a mile.

Adams is so much a man of his (current) century that he halfway apologizes for Versailles.

We see it now, he says, bereft of its crowds of bright courtiers and music and theaters and declaimed poetry.

Well. My own view of Versailles was, and is, that all the brocade pants in the world and all the music the French ever made is not going to convert Versailles into our notion of what a garden should be.

There were, after all, residents of Versailes who so hungered for the intimate garden with perhaps a bit of thatch here and a lomp of hollyhocks there, that they turned aside to an intimate sort of garden within the vast spread of Le Notre's grand design, and found more pleasure in their cottage garden than in the great one.

But of course the point of Versailles and the other great places was not ot provide the pleasures of cottages and the flowers of milkmaids.

It was to extend the splendor of the palace as far as the eye could see, with masses of trees, noble and enormous fountain basins, and rhythmic patterns of open space and neat forest, round points and goose feet (those intersections of great avenues) and it must be said the garden of Versailles is ample, noble, serene, full of dignity and clarity.

It is one way to manage a landscape. How these gardens came to be, and what virtues they reflect, is Adams' theme. He does not say they are gardens to inspire the designer of small town lots.

Pyramids and cathedrals, come to think of it, also are foreign to our practical building today, yet every civilized person is interested in them.

I found Adams' book readable and beautifully organized, the result of many years of thought.

Especially valuable are the ground plans for the gardens - in many cases there is nothing else remaining of the original gardens.

The concept of the "garden as theater" is valuable and the chapter on that topic is impressive.

The book covers French gardens - as design - from 1500 to 1800, but of course one thinks of the 17th century as the apex of French style.

Adams, a citizen of Washington with a place in the country, was long with the National Gallery of Art. The book with results from his own fondness for gardens and his longtome admiration of things French, and I cannot think of another book that could replace it. (Published by George Braziller, New York, 1979, paper cover, 160 pages). Adams is also the editor of the new Braziller series of garden books. CAPTION: Picture 1, "Bassin d'Apollon" by P.D. Martin, courtesy Musee de Versailles/Lauros-Giraudon. "The French Garden"; Illustration 1, "Fontainebleau" from the Royal Library, Stockholm; Picture 2, Eugene Atget's photo of "Cascades de St. Cloud," not Meridian Park, from Caisse National des Monuments Historiques; Illustration 2, "Treilliage," from Dumbarton Oaks; Picture 3, "French Garden of Love," from the British Museum from "The French Garden"; Illustration 3, the d'Amboise shateau, "Gaillon," from Cabinet des Estampes, Bibliotheque Nationale; Illustration 4, Versailles' garden theater, "Plaisirs de l'Isle Enchantee," from the Metropolitan Museum of Art.