Remove coarse chard stems, reserving them for another use. In a saucepan cook chard in water clinging to its leaves, covered, over moderately high heat, stirring once or twice, until wilted and tender, 3 to 5 minutes. Drain chard well in a colander and pat dry between paper towels. In a food processor chop chard fine and season with salt and pepper.
Preheat oven to 350 degrees F. Butter a loaf pan, 9 by 5 by 3-inches, and line bottom with buttered waxed paper.
Peel potatoes and quarter. In a kettle cover potatoes by 1 inch with salted cold water and simmer until tender, about 20 minutes. Drain potatoes and while still warm force through a ricer or medium disk of a food mill into a bowl. With an electric mixer beat in butter, eggs, and salt and pepper to taste until smooth.
Transfer half of potato mixture to another bowl and beat in chard until smooth. Spread chard mixture in prepared pan and spread plain potato mixture evenly over it. Terrine may be prepared up to this point 3 days ahead and chilled, covered.
Put loaf pan in a baking pan and pour enough hot water into baking pan to reach halfway up sides of loaf pan. Bake terrine in middle of oven 40 minutes, or until a thin knife inserted in center comes out clean, and let stand in loaf pan on a rack 5 minutes. Run knife around edge of terrine and invert onto a plate.
Serve terrine cut into slices.