The compressed air method was suggested by Mr. Haskin, the promoter and the first builder of the Hudson River tunnel. In 1874, when he began to sink the shaft for the construction of his tunnel, several subaqueous tunnels had already been driven by means of shields. Mr. Haskin had ideas of his own, and thought he could dispense with the shield and could trust to compressed air, since he was firmly convinced that compressed air alone could expel the water and temporarily support the roof of the excavation prior to the building of the lining masonry. In other words, he expected to substitute a core of compressed air for the core of earth removed. In the patent granted him for this method of tunneling, he expresses himself as follows: “The distinguishing feature of my system is that, instead of using temporary facings of timber or other rigid material, I rely upon the air pressure to resist the caving in of the wall and infiltration of water until the masonry wall is completed. The pressure is, of course, to be regulated by the exigencies of the occasion. The effect of such a pressure has been found to drive water in from the surface of the excavation, so that the sand becomes dry.” The compressed air method was soon found to be inefficient, even in the construction of the Hudson tunnel where the roof of the excavation was supported by timbering in the manner indicated in the pilot system. Thus large subaqueous railway tunnels cannot be driven exclusively by the compressed air method; still it has been successfully employed in the construction of small tunnels driven for aqueduct purposes. But the use of compressed air marked a great progress in the art of submarine tunneling. THE MILWAUKEE WATER-WORKS TUNNEL. The following description of the Milwaukee Water-Works Tunnel is an example of subaqueous tunnels driven through good soil in the usual manner employed in land tunnels; but afterward when treacherous material was encountered, the work was continued by means of compressed air.