The ice-capped, sub-freezing continent of Antarctica has posed a challenge to explorers, scientists and adventurers for hundreds of years.

As a tribute to four brave men who first explored Antarctica, the United States will issue a new block of four commemoratives. Featured on the block are Nathaniel B. Palmer, Charles Wilkes, Richard E. Byrd and Lincoln Ellsworth.An individual portrait of one member of the heroic quartet appears to the left in each of the horizontal format designs. An icy scene depicting each man's means of travel is shown at the right with a map of the polar region as a backdrop. A black dotted line follows the route of travel.

The four stamps are arranged in chronological order, left to right and top to bottom, starting with Palmer, who, on Nov. 18, 1820, became the first American to sight the continent. His sailing ship Hero is based on a line drawing provided by the Old Lighthouse Museum of Stonington, Conn., the explorer's hometown.

Navy lieutenant Charles Wilkes, who proved during his expedition of 1839-43 that Antarctica is a continent, is on the second adhesive in the block. The portrait of Wilkes is based on an 1870 painting by Samuel Bell Waugh provided by the National Portrait Gallery. Wilkes' ship comes from National Geographic Society reference materials.

Admiral Richard E. Byrd is the best-known American explorer of the Antarctic. He conducted five expeditions to the region, and oversaw extensive scientific study and photographic mapping of the continent by air. It is interesting for collectors to note that stamps were issued in 1933, 1934 and 1935 to honor Byrd's second Antarctic expedition, charting the course he took establishing Little America.

The likeness of Byrd is a composite based on National Portrait Gallery photos, and his plane is based on National Geographic Society photos.

Lincoln Ellsworth, featured on the fourth stamp, was the first to fly over both the Arctic and the Antarctic. He was the leader of four expeditions during the 1930s. In 1935, Ellsworth made a daring four-stop, 22-day flight in a single-engine plane. His portrait also is a composite taken from several photographs from the Library of Congress. The plane is based on National Geographic photos.

Ninety percent of Antarctica is covered with ice, glaciated for about 170,000 years. Antarctica has the world's coldest temperatures - 20 degrees colder than the Arctic - and, at 6,000 feet, has the highest average altitude of any continent.

No wonder this foreboding continent has been a challenge to mankind! The four men duly honored on the block of stamps are truly deserving.

First-day cancellations are available.

You may buy the block of four from your local post office and affix to your own envelope. Send to: Customer-Affixed Envelope, Antarctic Explorers Stamps, Postmaster, Washington, DC 20066-9991. No remittance is required. Requests must be postmarked by Oct. 14.

If you prefer to have the U.S. Postal Service do the job, enclose a check or money order in the amount of $1 for each block of four and 25 cents for each single stamp requested. The Postal Service will select single stamps at random and cannot honor requests for specific designs. Send to: Antarctic Explorer Stamps, Postmaster, Washington, DC 20066-9992. The deadline is Oct. 14.

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A new definitive coil stamp featuring the honeybee is now off the U.S. Postal Service presses. The 25-cent stamp shows a gold-and-black worker honeybee on the pink florets of a clover. "25 USA" appears in the upper left corner against a golden-yellow background.

The designer of the new stamp is Chuck Ripper, who previously designed the spring booklet stamps featuring the ring-necked pheasant, saw-whet owl and rose-breasted grosbeak. Ripper also designed the beautiful North American Wildlife pane of 50 stamps issued last year. Other philatelic designs by Ripper include the Coral Reefs of 1980, the Wildlife Habitat of 1981, the Kitten and Puppy of 1982, the Louisiana World Expo of 1984, and the Fish Booklet of 1986.

First-day cover collectors who wish to buy the stamp from their local post office may affix it to their envelopes and send to: Customer-Affixed Envelopes, Honeybee Stamps, Postmaster, Omaha, NE 68108-9991. They must be postmarked by Oct. 2.

Philatelists who let the Postal Service do the affixing should enclose a money order or personal check for 25 cents per stamp and send to: Honeybee Stamps, Postmaster, Omaha, NE 68108-9992. The deadline is Oct. 2.