Calder Watch: Hart Senate Office Building employes may get to walk unobstructed across their building's atrium -- not to mention enjoy seeing Calder's "Mountains and Clouds" -- as early as this week.

The mobile part of the sculpture -- the "clouds," which have been grounded on the atrium floor since last fall -- will be hoisted 90 feet up to the ceiling sometime between tomorrow and Thursday, said Pat Ellenwood, co-owner of Crystallizations Systems Inc., the mobile's fabricators.

Ellenwood blamed delays from vendors and the amount of special machining that has to done for the slow progress on the installation, which was to have been completed by Christmas. Meanwhile, the mobile pieces have been lying on the floor in the atrium, and the stabile -- the "mountains" -- have been in storage in Waterbury, Conn.

Finally installed, the sculpture promises to dazzle. "We're very excited, because Calder's genius was spatial relationships, and the use of space and the movement of the mobile will be very impressive," said Ellenwood, who added that it will be the world's largest indoor sculpture. Its surface area, she said, is five times that of the Calder sculpture in the East Building of the National Gallery of Art. The stabile is 52 feet high, and one piece of the mobile is 43 by 32 feet. A computer will direct the mobile's movements.

Ellenwood's husband and business partner Nelson Young engineered the mobile's fabrication. It has been a time-consuming venture for both of them. "We've been living this project for almost two years," she said. Scholarships for Violinists

Washington area high school violinists can start practicing for a new award. Classical radio station WGMS and Chevrolet are cosponsoring the WGMS/Chevrolet Young Artists Scholarship Awards, worth $1,500 each, to be given to 10 violinists annually. Any public or private high school student in grades 10-12 in the metropolitan area may enter.

Audition tapes must be submitted to WGMS by March 21. Awards will be announced and presented on June 1. William Steck, National Symphony Orchestra concertmaster, and Jody Gatwood, music professor and Montgomery Chamber Orchestra concertmaster, are among this year's judges. Picasso at Work

It is a singular peek at a genius at work. "The Mystery of Picasso," a 1955 French film of Pablo Picasso working on six paintings and a sculpture, is scheduled to open in Washington April 4 at the Circle West End. The film, directed by Henri-Georges Clouzot, is being released by Samuel Goldwyn Jr., who obtained the rights after protracted negotiations with the Picasso family.

In the film, Picasso paints on a transparent screen, using specially prepared paints. The six works created on camera were destroyed after filming, in accordance with Picasso's wishes. "Centaur," the sculpture, is owned by the Los Angeles County Museum of Art. The film itself is of visual interest: photography is by Claude Renoir, Auguste Renoir's grandson. The National's Free Programs

Are the free arts programs at the National Theatre in trouble? "Let's put it this way: Our programs are always in a vulnerable position," says Kathleen Barry, producer of the National's Monday night and Saturday morning series.

The fate of Monday Night at the National's 1986 season is no longer uncertain: First American Bank just donated $25,000 to finance it.

However, Saturday Morning at the National -- the free children's series that features live performances of music, storytelling, dance and theater -- has lost "over half" its funding, said Barry. The Saturday series had been cosponsored by both the Marriott Corp. and the J. Willard Marriott Hotel. Barry said the corporation is continuing its support, but the hotel has withdrawn. At this point, there is enough money for the Saturday Morning season to continue through May. Barry says she still hopes to find the funds to continue beyond that. End Notes

The National Chamber Orchestra, once known as the New World Players Chamber Orchestra, presents an intriguing inaugural concert at 7:30 p.m. tomorrow in the Kennedy Center's Terrace Theater. "Music of Black Composers" will spotlight the classical work of Washingtonian George Walker, as well as Carman Moore, Ulysses Kay, Hale Smith, Howard Swanson and David Baker. Two of the works -- Moore's "Youth in a Merciful House" and Kay's "Suite from the film 'The Quiet One,' " a 1948 documentary about a troubled youth, are being performed as American premieres. Swanson's "Vista No. 2," written in 1969, is having its world premiere. Music director and conductor Stephen Robert Kleiman unearthed Swanson's forgotten manuscript and raised funds to prepare the music for orchestral performance . . . The Washington Opera has elected Grace Bender, Michael K. Deaver, Alan I. Kay, Marc E. Leland, E.F. Livaudais Jr. and Roger W. Mehle to its board of trustees.