"THEY ARE NICE people and they do nice work." That was one environmentalist's recent comment to us about the Smithsonian Institution's scientists. Because that sentiment is widespread, and one we tend to share, some readers have wondered why this newspaper featured a story Feb. 28 on how the agency got the land for its Chesapeake Bay Center for Environmental Studies south of Annapolis.

The center was born when a wealthy Marylander willed a 366-acre Rhode River estate to the Smithsonian in 1962. Because nearby landowners wanted to expand bayside research, the Smithsonian was able to get over $1.5 million in foundation grants and buy lands and interests totaling 2600 acres of wetlands, woods and farms. The most innovative agreement was reached with attorney Ernest N. Cory Jr. For $125,000, including the assumption of mortgages on Mr. Cory's new home and an old boat, the agency got future rights to a key 17-acre tract. The Corys retained the right to live there for 20 years or more, and took a tax loss of foregoing development of the property.

Whatever impression the article may have left you with, there's nothing wrong about this. Everyone gained - the local gentry, the Smithsonian and the public that benefits from the research. What makes the tale worth telling is the way it illustrates two aspects of the Smithsonian's general approach. One is a rare blend of sagacity and serendipity. The second is the flexibility that comes from using non-federal funds to launch an enterprise.

And there's the rub: having gotten this acreage without reference to Congress, the Smithsonian has sought more and more federal operating support. Appropriations for the Center began with just $25,000 in fiscal 1966. The annual figure is $600,000 now. That is just part of the picture. Between July 1975 and October 1976, the Center spent $757,000 in federal money counts, $211,000 in research grants from other federal agencies and $28,000 in outside support, for a total of $1,135,000.

Thus the little Center is somewhat larger than Congress may think. And that is true of the Smithsonian as a whole. The institution's far-flung exhibits and research consume over $100 million in appropriations per year, augmented by around $11 million from its own trust funds and heaven-knows-what from other grants. Until recently nobody outside the agency had reviewed its total operations or the relationship between appropriations and other funds. Now Congress is beginning to exercise more thorough oversight. A major General Accounting Office study is underway. The purpose of those inquiries is to find out what use this vast, prestigious institution is making of the tax-payers' money. That's fair enough, it seems to us. The Smithsonian should not be regarded as ecxempt from scrutiny, simply because its work is so impressive and eclectic - and its manner is so . . . genteel.*