May 16, 1985

CHOOSING a comfortable armchair is a project that requires thought. Real comfort has as much to do with your own sitting habits as it does with the chair's shape. When considering the possibilities - and trying out a chair is mandatory before buying it - think about how you sit: Do you position yourself upright or in a sprawl? Do you cross your legs or stretch them out? Do you curl up, feet tucked under? What will you be using the chair for principally - watching television, reading, for social gatherings? If you are as likely to sit on the arms as on the seat, look for a chair with arms of substance. As in all upholstered furniture, inner construction has a lot to do with comfort. When shopping ask the sales clerk for specific information and turn the chair upside down to inspect it.

Columbia University graduated 7,173 students yesterday in ceremonies interrupted briefly by the walkout of about 200 students and faculty members opposed to the university's investments in companies operating in South Africa. The protesters, many with signs reading ''Divest Now'' pasted on their mortarboards, held what they called an alternative commencement on the steps of Hamilton Hall, 30 yards from the main ceremony on the school's central plaza and the scene of a three-week demonstration that ended last month. Among the protesters were 15 seniors whose diplomas had been withheld because they had participated in the earlier demonstration. The walkout began just as Columbia's president, Michael I. Sovern, began his commencement address.

The Allied Corporation, one of the most rapidly expanding automotive and aerospace companies in the nation, announced the biggest industrial merger in history yesterday, agreeing to buy the Signal Companies, another major aerospace producer, for nearly $5 billion in cash and stock. The deal, unanimously approved by directors of both organizations, will make the new company one of the biggest aerospace contractors in the world. It will operate under the name Allied Signal, with headquarters in Morris Township, N.J., Allied's present home. Edward L. Hennessy Jr., chairman and chief executive of Allied, who will head the new company, said of the deal, ''We're going to be in every airplane that flies.''

''We should not forget the time it took to reach agreement on the Austrian treaty.

The chairman of the State Senate Investigations Committee said today that he would hold public hearings on the operation of the New York City Police Department because of recent charges of brutality. At the same time the Cuomo administration is considering empaneling a Moreland Act Commission - an investigative body with sweeping subpoena powers - to examine accusations of brutality by police departments not only in New York City, but also around the state, according to administration officials. Also today, the United States Attorney for the Eastern District of New York, Raymond J. Dearie, announced that Federal prosecutors and the Federal Bureau of Investigation were investigating reports of torture and beatings at a Queens precinct station house. He said a Federal grand jury would look into the allegations.