Changed "the oldest public art museum in the United States" to "one of the oldest..." The existing footnote makes it clear that there are several others that are much older. It is possible that it is the oldest purpose-built art museum building still in use in the US but this would require a reference  based on the footnote, the Redwood Library would be a century older, as it still occupies its original building and has an art gallery, but this may not be the original collection.