Down at the lower end of the road the approach winds down the side of the rocky heights. Here it is supported for nearly half a mile on a great stone wall, which gradually grows smaller and smaller as the approach nears the level of the river. At one point another great mass of rock got in the way of the workmen, and they blasted their way right through its centre. The carriages will disappear in this cut as though they had been swallowed up by the rocks, and come out again on the other side as they wind their way down toward the straight part of the road along the river-bank. Over forty thousand cubic yards of rock were cut out of this place alone, and the workmen used all this and much more to fill in the cribs when they sunk them in the river below. The big wall that supports the approach was another difficult part of the work. In one place this is over forty feet high, and more than half as thick at the bottom. Just think of a solid stone wall as high as a house and more than half as thick at its base! It narrows down to two or three feet in thickness at its top, like a pyramid of masonry, and above this will be a railing to prevent people from falling off, for there is to be a sidewalk along the outer edge of the driveway here. It took many, many months to build that wall alone. There will be two sidewalks in most parts of the new boulevard, but people will be allowed to cross from one to the other only at certain points, and then under the roadway. It would be dangerous to cross where fast horses are constantly passing, so there will be two or three tunnels, or transverse culverts, as the engineers call them, at different parts of the driveway. These tunnels will pass under the road-bed, connecting both sidewalks with stone steps at either side. Sewer culverts, too, have been built at a number of points along the driveway, for the amount of rain that drains off the slopes at the side of the boulevard after a storm would almost undermine it if there were not proper outlets for the water. [Illustration: THE COFFER-DAM AT THE BEGINNING OF THE LEVEL STRETCH.] Another engineering difficulty was found when the workmen reached the lower end of the approach, for the rocky bluffs end suddenly there before the approach has reached the level of the crib-work. Here they had to dig down forty feet in the mud to get a hard bottom for the rest of the support. A wooden wall was built around the spot to keep the water out, and inside of this "coffer-dam," as the engineers call it, the masons laid the foundations for the last end of the stone wall. It was almost impossible to keep the wooden sides from leaking too, and they had to keep pumps at work almost all the time to prevent the inside from filling with water.