Why is the needle in the New York City smooth today? Why were the obstacles protected in Egypt?

I was pleased with Robert Whiteford's affection for our obelisk (which was a gift to New York City, by the way, not to the nation), but disturbed over the canard about ''the damage done in less than 100 years'' (''Disappearing Needle,'' letter, Dec. 16).

This has often been asserted by thoughtless geologists and journalists (a writer in Smithsonian magazine declared that the pair of obelisks left Egypt, one for New York and its mate for London, ''in mint condition''). In 1965, however, a temperate writer stated, ''the disastrous disintegration rate of granite in city atmosphere, as exemplified by Cleopatra's Needle, is a myth,'' adding, ''It is therefore hoped that the obelisk will be eliminated from textbooks of physical geology as a 'good example of weathering in cities.' ''

Historically, the New York obelisk, which Napoleon had intended for France when he saw it, was subsequently refused because of poor condition, in favor of the well-preserved Luxor shaft removed to Paris in 1836. The London twin to New York was rejected in 1849 for the same reason. In the 19th century there were observations of native vendors peddling granite chips from the obelisk at the site in Alexandria. In New York, in 1891, it was observed of the Needle's west face that ''the abrasion caused by the sand of the Libyan desert has effaced almost every hieroglyph on this side of the obelisk.''

Conservation began early. The tip was said to have been repaired in Alexandria circa 1880. During re-erection in New York in 1881, some structural repairs were made (note replacements in the limestone foundation steps; the bronze crabs were replacements forged at the Brooklyn Navy Yard). By 1885, 780 pounds of granite scale were removed from the shaft, which was then treated with paraffin. As for the resistance of such granite in New York's climate, there are miles of curbstone regularly polluted by exhausts and urine, and exterior building walls (e.g., the Museum of Natural History), in good condition.

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Meanwhile, the obelisk, in Central Park, on its knoll, amid the trees, behind the sheltering wall of the museum, away from the motor traffic, appears to be ideally sited; when conditions become severe, it will also be time to put Trinity Church, the Public Library and City Hall under glass.

Incidentally, the literature, considerable and thorough (woodcut illustrations of the hieroglyphs were published in the 1650's), is available in the public library. FRED BRAUEN Jackson Heights, Queens, Dec. 16, 1986

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