_Mississippi Jetties._--The broad bar and shallow waters at the mouth of the Mississippi involved such an obstruction to commerce that in 1872 it received the attention of Congress, resulting in the building, by Capt. Eads, of the celebrated jetties. They were begun in 1875 and finished in 1879, and cost $5,250,000. The channel obtained was 30 feet deep and 200 feet wide. Its construction involved the building across the bar and out into the Gulf of Mexico two long reaches of parallel embankments, called jetties. This was effected by sinking mattresses of willow branches bound together and weighted with stone. These were laid in four layers, and when submerged, and resting upon the bottom, were covered with a layer of loose stone, and this in turn was surmounted with a capping of concrete blocks, as seen in cross section in Fig. 234. These jetties so concentrated the flow of waters into a narrow channel as to cause its increased velocity to wash out the mud and silt and deepen the channel. The immensity of the work may be measured by the quantity of material used in its construction, which included 6,000,000 cubic yards of willow mattresses, 1,000,000 cubic yards of stone, 13,000,000 feet (board measure) of lumber, and 8,000,000 cubic yards of concrete. The mattresses were laid 35 to 50 feet wide at the bottom, which width was considerably increased by the superimposed layer of stone, and the jetties extended 2¼ miles into the sea. Their influence upon commerce is indicated by the fact that before their construction the annual grain export from New Orleans was less than half a million bushels, and in 1880, the year following their completion, it was increased to 14,000,000 bushels. [Illustration: FIG. 235.--INTERIOR CONSTRUCTION MODERN STEEL BUILDING.] _High Buildings._--A distinct feature of modern architecture is the enormously tall steel frame building known as the “sky scraper.” The increasing value of city lots first brought about the vertical extension of buildings to a greater number of stories, and the necessity for making them fireproof, coupled with the desire to avoid loss of interior space, due to thick walls at the base, made a demand for a different style of architecture. To meet this a skeleton frame of steel is bolted together in unitary structure, the floors being all carried on the steel frame, and the outer masonry walls being relatively thin, and carrying only their own weight. In Fig. 235 is shown an example of the interior structure of such a building. The vertical columns are erected upon a very firm foundation, and to them are bolted, on the floor levels, horizontal I-beams and girders, stayed by tie rods, which I-beams receive between them hollow fireproof tile to form the floor. The outer masonry walls are built around the skeleton frame, as seen in Fig. 236, and the details of connections for the floor members appear in Fig. 237. [Illustration: FIG. 236.--ENCLOSURE OF STEEL FRAME BY MASONRY.]