WILLIAM T. HART, the four-masted schooner, was launched July 21, 1883, "amid the hurrahs of thousands, the ringing of bells, etc.," from Jonah Agnew's shipyard in Alexandria. She cost $45,000. It took seven years to build her. She did not sail long.

Sailing ships are living things and since they live, they die. Their hulls spring leaks, their canvas tears, seas buffet and corrode them. The William T. Hart sank in 1885 off the New Jersey coast when her cargo of coal shifted. But she survives in photographs, in paintings and in models. Her portraits are among those of many ships in "Canvas on Canvas" at the Lyceum, 201 S. Washington St., Alexandria.

Most portraits of sailing ships are mediocre works of art. They have the look of pictures efficiently produced, cramped by convention, painted without passion. Their dramatic skies and white-fringed waves always look a little canned, their wheeling gulls look stuffed. Sails never sag or luff, pennants always flutter. Yet such pictures, old or new, are loved and avidly collected. What saves them is their subject.

Though transatlantic passengers travel now by jet, though garbage soils the oceans, the beauty of sailing ships still glows. And paintings that portray them, like paintings that show nudes or glens, bowls of fruit or flowers, somehow always manage to announce themselves as lovely. In almost all the pictures here, the lifeless V's and W's signifying sea birds and the rote lines of the rigging have been painted in by busy hacks working on commission. But that doesn't seem to matter. What seems to matter more is the strange way that the mind is pleased (perhaps especially in August) by speed and grace and moist air, the fullness of full sails and the thought of wind on water.

That image of the flowing wave--even when abstracted into drapery or golden hair, flags above the battle or cloths beneath the still life--somehow serves to carry an aura of the beautiful, the noble, the important, into works of art. The beauty of the sailing ship and the beauty of the goddess share more than one might think. No wonder ships are she.

Ships in this exhibit--America of Salem, Coquette of Baltimore (she turned turtle, drowning five in 1849), and especially the clipper ships, Staghound (launched in 1850, lost by fire in 1861), Westward Ho (launched 1852, lost by fire 1864), Reindeer (launched 1849, wrecked in the Philippines 1859), Strabo, Gypsy--make this show worth seeing. They are without exception more magical and beautiful than the dull and cracked old pictures in which they appear. "Canvas on Canvas" closes Sept. 20. More Boats

Not all boat pictures are British and not all of them are old. Scores of relatively new ones, many of them prints, are currently on sale at the Atlantic Gallery in Georgetown, 1055 Thomas Jefferson St. NW.

The Atlantic is a gallery for buyers who prefer, above all other images, pictures of the sea. There are certain other sorts of art that never lose their market. Duck art is one. Pictures and porcelains of other birds are always in demand. Anything that shows broncos or cowboys on the trail sells easily in Texas. Sharply focused watercolors of dented buckets, fence posts and old weathered barns sell almost everywhere.

The Atlantic is in some ways a vanity gallery. Large-edition lithographs produced by marine painter John Stobart, who owns part of the place, take up the best wall. Stobart's art, like most such art, fails to offend. He often portrays clipper ships in historical settings. He has a skillful illustrator's knack, his art is throughly researched, his details are sharp, and it is clear he loves the sea.

Many other artists--J. Robert Burnell, who paints the craft of Chesapeake Bay; W.E. Norton, who paints oils of Belgian fishing smacks; Ray Cross, who publishes prints of 12-meter racers; and the Englishman Chris D. Watkiss, 72, who blights his well-made watercolors of old American sailing ships with over-bright American flags--are also represented. These painters all avoid risk-taking and doubt, but they all know their boats and seem to paint to serve the sea.

Two oils there are fine. Both are 19th-century and English. One, "H.M.S. Thetis," signed J.E. Buttersworth (1817-1894), is full of curves and drama. The other, "Clearing the Needles" by Henry Moore (1831-1895), is a seascape of the Isle of Wight in which the sails in the distance are only incidental. Both these paintings cool.