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The character of the plan before us consists in the mode of effecting the excavation, by removing no more earth than is to be replaced by the body of the Tunnel, retaining thereby the surrounding ground in its natural state of density and solidity.

In order so to effect an excavation 35 feet in breadth by 20 feet in height, the author of this plan proposes to have the body of the Tunnel preceded by a strong framing of corresponding di- mensions, as represented in the accompanying drawings. The object of this framing is to support the ground, not only in front of the Tunnel, but to protect at the same time the work of exca- vation on all sides. The body of the Tunnel, which is to be con- structed in brick, is intended to be fitted close to the ground; and in proportion as the framing is moved forward, so the brick-work is made to keep pace with it. But as this framing could not be forced forward, all in one body, on account of the friction of its external sides against the surrounding earth, it is composed. of twelve perpendicular frames, which admit of being moved singly and independently of each other, in proportion as the ground is worked away in front. These several frames are provided with such mechanism as may be necessary to move them forward as well as to secure them against the brick-work, when they are stationary. It is to be observed, that six alternate frames are stationary, while the six intermediate ones are left free for the purpose of being moved forward, when required; these, in their turn, are made stationary for relieving the six alternate ones, and so on, Thus the progressive movement of the framing can be effected.

In order that a sufficient number of hands may be employed together with effect and security, each frame is divided into three small distinct apartments, which may properly be denominated cells. By this disposition, the twelve frames form thirty-six cells, in which the men are to operate for excavating the ground. It is to be observed that the ground in front of each cell is kept from falling in, by means of a powerful shield consisting of small boards, strongly secured by the frames. It is from within these cells that each workman is to cut out the ground, just in the way that he would do if he were to cut out a recess into a wall for the purpose of fitting a closet into it; but, with this precaution, that he is to remove each board, one after the other, and replace each in succession as he cuts the portion of the ground corre- sponding with it. When he has thus gained from 3 to 6 inches over the whole surface.(an operation which it is expected may be made in all the cells nearly in the same time,) the frames are

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