Big city museums have great appetites and thrive on change and growth. As a prestigious institution in a rural setting, the Clark Art Institute has the privilege of moving deliberately, as though guided by the rhythms of nature. It is located in Williamstown, Massachusetts, a small college town three hours away by car from New York and Boston. Attendance surges in summer, when the Berkshire Hills draw city dwellers to music festivals; in fall, the institution resumes its role as a scholarly retreat.