We are indebted to Mr. Charles S. Storrow, an American engineer of reputation, for a great deal of very interesting information upon the subject of the ventilation of tunnels, and the state of the atmosphere in them during and subsequent to the passage of trains. Mr. Storrow was sent to Europe in 1862, by the commissioners of the State of Massachusetts, which had been appointed by legislative action, in relation to the construction of the Great Hoosac tunnel already referred to. The tunnel first visited by Mr. Storrow, in England, was the Box Tunnel, of which in his report he furnishes a section. Its length is 3,227 yards; its gradient throughout is 1 in 100. It has five shafts, each being twenty-five feet in internal diameter. But this diameter is, as just stated, now considered to be unnecessary. The deepest shaft is 300 feet.[139] Mr. Storrow experienced no inconvenience as regards ventilation when going through the tunnel in passenger trains. On one occasion he proceeded through the tunnel in a hand car—“a passenger train passed us” says Mr. Storrow, “on the other track to that which we were on. It filled the tunnel with smoke and produced perfect darkness, so that the other ends of the tunnel (which had been seen most distinctly previous to the passage of the train) could not any longer be seen, and on removing the lights we were carrying, the person who sat at my side talking with me and touching me, was absolutely invisible. With all this, however, there was nothing troublesome to respiration. As we proceeded and successively passed the large shafts, a ray of light appeared directly under them, which, a moment afterwards was lost in intense darkness, and this continued until we reached the other end of the tunnel, after an interval of fifteen or twenty minutes from the passage of the express train.” Mr. Storrow was informed that “the ventilation of the tunnel depends a good deal on the weather. Whenever a strong clear wind blows, the smoke disappears very readily, but in fogs it is quite troublesome to the workmen. I could not find, however, that it seriously interfered with their work. Indeed there never was a time when it could not be done, if required. No artificial means had ever been used to ventilate the tunnel, but the passage of quick trains going through it in the ordinary course of the operations of the road, is a powerful agent for this purpose. With the steep grade of 1 in 100, the steam, on the descent of the tunnel, is nearly or quite shut off, and the train passes quickly through. The first engine which passed us, moving down the grade, produced no sensible effect upon the air, but in ascending the tunnel there is a very great expenditure of steam; the trains moving slowly, and heavy trains are assisted by a second engine. All this of course vitiates the air. The quick passage of a train always produces a current in the direction of its motion, and if made by a train running downward, and therefore using but little