The truly great artist of today seem able to negotiate freely from one medium to another leaving behind works of art that transcend narrow, artificial boundaries imposed by art critics, historians, and collectors. As a painter we're most familar with his childlike swirls and squigles of paint transforming huge, tubes of airbourne, passenger-laden metal into moving artworks soaring across the skies at speeds his earlier "moving" works could not approach. Alexander Calder is one of the few artists who can be said to have "invented" a kind of art work. Marcel Duchamp coined the name "mobile" for the free-floating bits of painted sheet metal, wood, and wire that made up Calder's first sculpture. The year was 1931.

Born 100 years ago, Calder's father and grandfather were both sculptors, his mother contributed his background as a painter. His earliest sculptural innovation came when he first dabbled in wire sculpture at the age of nine. His educational background included a degree in mechanical engineering. He studied art only later. But it was a course in applied kinetics that inspired his signiture works in which time and movement added two new basic elements to the medium of sculpture. Some of his moving sculptures were hanging, free-floating pieces, in others he experiemented with motorized, "programmed" movement. Albert Einstien is said to have gazed upon one such piece for almost an hour.

After living in Europe for many years, Calder returned to the U.S. in 1933 and rented a farmhouse near Roxbury, Mass. There, working out of an old ice house for a studio, he explored the relationship of art and movement, creating a lifetime ouevre of over 16,000 pieces. He died in 1976. Two years later one of his largest mobiles was installed in the the new East Building of the National Gallery of Art in Washington D.C. There, until July 12, of this year, it will be joined by 266 other pieces of his work in a retrospective that will move on to the San Francisco Museum of Modern Art from September 4 to December 1 1998. The post office will soon release a series of stamps featuring his mobiles. For the first time, his work will not only be "moving", but moving MAIL.