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Got a question? Ever wondered about something related to geography? Then ask away here. Every two weeks, Joe Blanton, director of our Research Correspondence division, will post answers to three of the most interesting questions received via e-mail. Unfortunately time constraints preclude individual e-mail responses.

So what IS sea level?

I would like to know how exactly the height of sea level is defined.

I like this explanation from the National Geographic book Exploring Your World: The Adventure of Geography

“Because the ocean is one continuous body of water, its surface tends to seek the same level throughout the world. However, winds, ocean currents, river discharges, variations in gravity, and temperatures of the different water masses prevent the sea surface from becoming level. These effects vary from one place to another and over time.

“So that the surface of the ocean can be used as a base for measuring elevations, the concept of local mean sea level has been developed. In the United States and its territories, local mean sea level is determined by taking hourly measurements of sea levels over a period of 19 years at various locations and then averaging all of the measurements.

“The 19-year period is called a Metonic cycle. It enables scientists to account for the long-term variations in the moon’s orbit. It also averages out the effects of local weather and oceanographic conditions.”

The Permanent Service for Mean Sea Level (PSMSL), based at the Proudman Oceanographic Laboratory at Bidston Observatory in the United Kingdom, tracks monthly and annual mean sea level information from over 1,600 tide gauge stations around the world. You can check out their Web site at http://www.nbi.ac.uk/psmsl/psmsl.info.html.

I’m a 17-year-old member of the Society who thoroughly enjoys every article in the magazine. I do have, however, a favorite article: “Koko’s Kitten,” written, I believe, in 1985. I was wondering if you could tell me what has happened to Koko (and her kitten) since the article was published?

That January 1985 article about Koko, the gorilla who communicates with humans via sign language, and her kitten drew enormous reader interest. Sadly, All Ball, as Koko had named her little playmate, was killed by a car around the time of the article’s publication. News stories about the kitten’s death prompted a flood of sympathetic letters and telephone calls from around the world, and Koko soon had a new kitten.

Koko herself is doing fine and enjoyed celebrating her 25th birthday this past July 4. She’s part of a research project at the Gorilla Foundation in Woodside, California, directed by Dr. Penny Patterson. Dr. Patterson and her associates study and document the abilities of Koko and two male gorillas to communicate. The Gorilla Foundation is preparing to move to the Hawaiian island of Maui where the climate will be more comfortable for the gorillas. Click here to visit the Gorilla Foundation’s Web page.

Can you tell me which NATIONAL GEOGRAPHIC issues have information on the Civil War?

You can find articles on Civil War topics in the following issues of NATIONAL GEOGRAPHIC: December 1994, July 1979, January 1975, April 1965, July 1963, April 1961, February 1960, and July 1931. One of the most comprehensive articles was “The Civil War” in the April 1961 issue, which also included the map supplement “Battlefields of the Civil War.” You may be able to see these issues of the magazine in a library collection. Click here to order a copy of the April 1961 issue online.

National Geographic Society books on the Civil War include Civil War Battlefield Guide, Field of Battle: The Civil War Letters of Major Thomas J. Halsey, and The Blue and the Gray.