Undoubtedly the sensible advice to someone planning to see the British Museum in a day is: Don't even try! C'est impossible! However, as tourism is an art of the impossible, this article is for those who (sensibly enough) would rather eat some of that gigantic cake than none of it.
So what, for a start, should you absolutely not miss?
The Elgin Marbles, obviously. These are the great 5th-century BC Parthenon sculptures brought from the Acropolis in Athens to London in the early 19th century by Lord Elgin. They consist of the frieze, the metopes, and the pediments of the temple and are considered by many the finest accomplishment of Greek art. Particularly fine is the carved drapery of the female figures in the pediments, and the heroic reclining male nude - the only sculpture not to have lost its head. My own favorite piece is the horse's head - one of the horses of Selene, goddess of the moon, descending into the waves. (ground floor, Room 8, the Duveen gallery.)
The Rosetta Stone. This surviving chunk of a large basalt monument provided the key for scholars to decipher the hieroglyphs of ancient Egypt - and consequent access to Egyptian culture as a whole. Inscribed on its black surface is a priestly decree in three scripts. The first two are Egyptian - hieroglyphic and demotic. The third is Greek, dated to 196 BC. (ground floor, Room 25, the Egyptian sculpture gallery.)
Assyrian Reliefs from the Palace at Nineveh. These dramatic carved reliefs depicting the royal lion hunt show a practice that was rather more brutal than sporting. But their display of energy and elegance and evident sympathy for the hunted animals somehow allow the beautiful to outweigh the cruel. (ground floor , Room 17, the Lachish room and Assyrian saloon.)
Our visitor-in-a-hurry will have been strong minded indeed if he or she has managed to look exclusively at these three outstanding British Museum possessions on the ground floor. The distractions are legion.
He or she has overlooked no fewer than two of the ''seven wonders of the ancient world'' (in Room 12, the Mausoleum room): huge Sculptures of Mausolus and his Wife are part of one wonder, and also the gigantic head of a horse. They were part of the monument that crowned the enormous tomb - the first of all mausoleums - of this Prince of Caria in Asia Minor. The other wonder, in the same gallery, is represented by a Sculpted Drum from a Column of the Temple of Diana of the Ephesians (mentioned in the Acts of the Apostles). The column would have been almost 60 feet high. This fragment is superbly carved with three life-size figures.
The one-day tourist must give five or 10 minutes to the splendors both large and small of the Egyptian Sculpture Gallery (Room 25). The most notable sculpture includes the three black granite Statues of King Sesostris III, the colossal red granite Head of an Eighteenth Dynasty King, the less intimidating and serene limestone Funerary Statue of a Noble and his wife, seated hand in hand. Among the many small pleasures is the seated cat obligatory to most Egyptian collections.
Rooms 68 and 69 continue the story of Greek and Roman art and life encountered initially downstairs. Room 68 takes one into the charming world of (mainly) small bronzes and terra cottas. Here are truthful and engaging Tanagra Figures, and, of great appeal, a number of little Bronze Animals of the 6th century BC from Cesme in Asia Minor: a mouse, a snake, a panther, and birds.
Room 69 is even more attractively set out, and it provides intriguing glimpses of Greek and Roman life as illustrated by objects of supreme quality. Of outstanding interest are cases devoted to women; to children; to dance, drama , and music. Further subjects range from religion and plumbing and toys to lighting, weaving, transport, and dress. If Egyptian Mummies are to the taste of the by-now-galloping visitor, then Rooms 60 and 61 are a must. These galleries are popular and crowded. Personally, I find Room 63 more inspiring, devoted rather busily to every conceivable detail of daily life in ancient Egypt.
However, we have promises to keep and ''miles to go before we sleep.'' Room 54 - the Sumerian and Babylonian room - should be the next port of call, and here not to be missed are the Silver Lyre found in excavations at Ur of the Chaldees and elaborate He Goat standing on its hind legs, its forelegs supported by the branches of a tree. And (for fun) there are also numerous Babylonian Stone Weights in the form of sleeping ducks of various sizes, asleep with their heads over their shoulders (if ducks have shoulders).
The department of Prehistoric and Romano-British Antiquities occupies Rooms 35 to 40. In Room 39 are masterpieces of Celtic art of the British Isles. Essential to see is the Battersea Shield (mid-1st century BC), and the Bronze Disc from Ireland appeals for its vigorous and bold design. On no account should one be prevented from seeing the 4th-century Roman silverware, gorgeously decorated, of the Mildenhall Treasure. Thirty-four pieces were ploughed up in Suffolk during World War II. The Great Dish is the finest item of all, with its pagan excitements watched over by a staring and rather wild face in the center, possibly belonging to Neptune.
Rooms 41 to 47, filled with the collections of the Medieval and Later Antiquities department, are also, until the end of 1983, in upheaval due to redecoration. Some objects are inaccessible until then; others are in unexpected places. The splendid and happily arranged Clocks and Watches display, however, in Room 44 is intact. The major developments in the European history of mechanical timekeeping are traced from the Middle Ages to the start of the 20th century. Many of the movements are working. Perhaps the most spectacular piece is Emperor Rudolph II's Ship Clock of the late 16th century.
Decidedly to be lingered over, though, is the richly various material of the Sutton Hoo Treasure (normally in Room 41), a 7th-century royal ship burial. An extraordinary helmet is very popular. The Franks Casket, of whalebone, made for a person of rank in early 8th-century Northumbria, is a fascinating mix of runic inscriptions and figures acting out rather primitively carved and oddly juxtaposed scenes of Christian history and folklore.
If I had to choose only one thing in Room 42 (which covers the period of the Middle Ages from AD 800 to 1500) it would have to be those tough little characters the Lewis Chessmen: walrus-ivory carvings a few inches high. From the 12th century, probably Scandinavian, they combine the bold fact and vital fantasy of the Romanesque. A Fifteenth-Century Shield paraded at tournaments, and decorated with a lady and a knight, seems to epitomize the romance and idealism of medieval chivalry. The Royal Gold Cup of the Kings of France and England - solid gold, decorated with translucent enameling - is a combination of opulence and simplicity, luxury and superbly controlled craftsmanship.
I calculate that (with a break for lunch and time to buy post cards) it is unlikely that real enjoyment could be derived from much more than the items already listed. But it is a sobering thought that one would still have missed the entire Oriental collection, and the astounding display of manuscripts and autographs in the British Library next door. Included here are the Magna Carta, the Lindisfarne Gospels, and other ''light'' classics. . . .
Nor would one have gazed in delight at the lucid glories of the Wilding Bequest of Huguenot Silver made in England (Room 46 upstairs), or glimpsed Pisanello's Portrait Medal of John VIII Palaeologus: the first medal by the Renaissance pioneer of medallic art. The breathless tourist would probably not have even noticed, in the same corridor gallery showing objects from the ''Renaissance and Later'' the Terra Cotta Model by Michelangelo for an unfinished statue of a bearded slave. But there it is, neatly placed in among an overwhelming wealth of objects, just one more small item of superlative originality and touch, in one of the world's most amazing museums.
The British Museum is open Monday to Saturday 10 to 5, and Sunday 2:30 to 6. It is closed on national holidays, including Christmas, Boxing Day, New Year's Day, and Good Friday. There is a recorded message (telephone 01-580-1788) that also gives information about when the museum is open.
To get to the museum by subway, Tottenham Court Road, Russell Square, and Holborn are the closest underground stations.