Dissolve the yeast in warm water in a large bowl, and let stand 5 minutes. Lightly spoon the flour into dry measuring cups, and level with a knife. Stir in 2 1/2 cups flour and salt. Turn the dough out onto a floured surface. Knead the dough until smooth and elastic (about 10 minutes), and add enough of remaining flour, 1 tablespoon at a time, to prevent the dough from sticking to hands (dough will feel tacky).
Place the dough in a large bowl. Cover and let rise in a warm place (85°), free from drafts, 45 minutes or until doubled in size. (Press two fingers into the dough. If indentation remains, the dough has risen enough.) Punch dough down; cover and let rest for 5 minutes. Divide in half. Working with 1 portion at a time (cover remaining dough to keep it from drying), roll each portion into a 16-inch rope on a floured surface. Place ropes on a large baking sheet sprinkled with cornmeal. Cover and let rise 30 minutes or until doubled in size. Uncover dough. Cut 3 slits in top of each loaf to allow steam to escape.
Preheat oven to 450°.
Throw 1/4 cup water on the floor of oven (avoiding heating element). Place loaves in oven. Quickly close the oven door. Bake at 450° for 20 minutes or until the loaves are golden brown and sound hollow when they are tapped. Remove from pan, and cool on wire racks. Cut each baguette into 12 slices.