Elsah, Ill. — What does a man who has written the ''definitive history'' of the preservation movement in the United States do about his own 19th-century Midwestern village and the brick-and-frame 1858 house he calls home?
If that man is Dr. Charles R. Hosmer Jr., of Elsah, he goes avidly about preserving them. And he does so with the kind of enthusiasm that fires other people into revitalizing action as well.
Chuck Hosmer, as his friends call him, is an apostle of preservation who obviously practices what he preaches. Elsah, his village, has a population of 160 - or 900 if you count the students and faculty of nearby Principia College, where he is a professor of history.
In the brief span of a dozen years, Dr. Hosmer and other faculty members and friends have literally dusted off this unique little cluster of unassuming stone , brick, and frame houses on the bluffs overlooking the Mississippi River, rescuing the town from its slow slide into history.
''In 1970,'' the professor recalls, ''Elsah looked pretty awful. Eddie Keller had closed the only grocery store in town. The college had closed the Village Inn, and many of the buildings were neglected and ramshackle in appearance. But within a year, four faculty members had bought houses and commenced to restore them. The college was taking renewed interest in the village at its front door, and the current flowering of tiny Elsah had begun.''
In 197l Dr. Hosmer helped form the Historic Elsah Foundation, draft a historic-zoning ordinance, and create a zoning board. In 1973, while he was serving on the Illinois Historic Sites Advisory Committee, the nomination of Elsah (which he helped formulate and send to Washington, D.C.) earned it a listing on the National Register of Historic Places. A historic district was then created, so that this village - which once throbbed with people off the river boats, farmers, and hardworking townspeople - would be infused with new meaning.
In ''Elsah: A Historic Guidebook,'' which Dr. Hosmer and Prof. Paul O. Williams researched and wrote for visitors who wished to take walking tours of the village, the authors make this statement:
''Occasionally one finds a town in which the prevailing flavor is of the past. Elsah is such a town. Almost all of a piece, it gives a strong hint of the setting of 19th-century life along the river. Mills, warehouses, river shipping, two railroads, and numerous local businesses and throngs of wheat farmers - all have disappeared. But many of the houses and stores remain, and it is they which make coming to Elsah like stepping back in time.''
''I did not truly value Elsah and its place in history until Paul Williams and I began our research for the guidebook,'' Dr. Hosmer recalls. ''We began with Elsah cemetery, then tackled the courthouse. Then Mr. Williams began to study all the old photographs of Elsah, and I began to go through old newspaper and census reports and deeds. Our research awakened in both of us a curiosity and a great appreciation for what was all around us. Such research is an enriching but never-ending process. We are now into the fourth updated edition of our guidebook. New material about our Greek Revival, Franco-American, and Gothic Revival buildings and the families who lived in them keeps coming to light.''
For Dr. Hosmer, 1981 was a stellar year. His definitive history of the US preservation movement, ''Preservation Comes of Age: From Williamsburg to the National Trust, 1926-1949,'' was published, after a decade of research and writing, by the University Press of Virginia. He also put the final preservation touches on the brick McNair-Hosmer house, built about 1858, where he and his wife, Jeri, and their children, Kathryn and Jonathan, have lived since 1972.
''Buying this house put me into the actual business of restoration and renovation. I was taking a house built and lived in by the McNair family for over 50 years, and later bought and partly restored by Minna Johnson, and attempting to make it comfortable and suitable for my growing family.''
The Hosmers spent the first six months simply analyzing the house to determine what was required. Then they added another bedroom and bath for their young daughter, and a garage that would also handle storage. They did their share of ripping out and putting back. What resulted is not pure McNair, but pure McNair-Johnson-Hosmer, a blend of contributions from the three main owners of the house. The ''improvements'' they have put in amounted to 90 percent of the purchase price of their house, and most of the work has been contracted out or done with student help.
''I'm not skilled and I didn't have the time,'' the professor explains. Their total investment is still less than $75,000, which the Hosmers consider a bargain in housing at today's costs.
They have furnished the house with their own medley of possessions - an antique highboy and desk, an Empire bed and rocking chair, a Victorian credenza and four Victorian chairs, an English dining table made about 1820, a Venetian chest from Italy, and an Italian corner cupboard. ''It is a mix that has worked for us,'' Dr. Hosmer reflects.
By heavily insulating the walls and putting 13 to 15 inches of insulation in the attic and window quilts at the windows, the Hosmers assume they must have the warmest house in all of Elsah. They also have a backyard with worlds of archaeological possibilities. The old McNair trash pit in the back has yielded some fascinating bottles of the period.
Dr. Hosmer concludes that his kind of involvement in a village has distinct advantages: ''It gives you a sense of place, and permanence, and of connection with the past, which nothing else could quite do. I feel that I know the McNairs well. They were millwrights and inventors and had a machine shop, and did odd jobs and even took out four patents. They were an industrious family, and I have enjoyed knowing them, through our photographs and research. I feel privileged to live in the house they built. It is good to know a house from its very beginnings. One day, I want to do an album about our house. It will be a record, and it will tell about the changes and the people,, and will include all our photographs, past and present. I think our children would enjoy having it, because they have been involved, too, in our adventure of village and house preservation. It has illuminated a little corner of history for them, too.''