has a series of disks mounted upon a common shaft, and alternating with spiral blades fixed within the casing of the shaft. There are buckets, or cups, upon both the revolving disks and the fixed blades, the fixed buckets being reversed in relation to the moving cups. The steam, admitted first through a set of stationary blades or buckets, impinges at an angle upon the first rotating disk and imparts motion, passing thence through another set of fixed blades to the second disk upon the main shaft, and thus

through the entire series of alternately fixed and rotating buckets. The area of the passages increases progressively to correspond with the expansion of the steam as it is used on the successive disks. The expansion of the steam is accomplished in the turbine itself, instead of in the nozzles, as in the De Laval motor. There is but a single shaft, instead of the two in the De Laval type, and the buckets in a given size of Parsons turbine number about 30,000, as against about 350 in a De Laval motor of the same size."

Whether the same amount of power can be secured

more economically by means of the turbine than by means of the reciprocating engine is apparently not fully established, although greater economy is claimed for the turbine; but the turbine has certain advantages, other than power efficiency, that are valuable. The turbine engine being much more compact than the reciprocating, occupies less space and weighs less. The friction of its moving parts is less, and it is without the vibration which the reciprocating engine has. These advantages of the turbine give it special merits as a marine engine, in which economy of space and weight and reduction of vibration to a minimum are specially important considerations.

The third phase of the technical history of the modern steamship is concerned with the hull. The lines to be followed in constructing a hull to secure seaworthiness and speed were worked out by the builders of sailing vessels during the first half of the nineteenth century, and when the construction of steamships began, about 1840, there were no important changes necessary in the design of the hull.

The first steamships had wooden hulls; and, as was true of the substitution of the screw for the paddle wheel, the change from wooden to iron vessels was brought about slowly. The first ocean-going iron vessel was the Rainbow, a little paddle-wheel ship of 580 tons and 180 horse power, built at Liverpool in 1837-38. The vessel that did most to bring the iron hull into favor was the Great Britain, a ship phenomenally large for its time, of 3,270 tons gross, built in 1839-44. The hull was divided into six water-tight compartments, and was so strongly built that it lay stranded on the Irish coast, without serious damage, for eleven months in 1846 and 1847. The vessel was kept in service on various routes for nearly forty years.

Several causes delayed the general adoption of iron hulls. Until 1860 the people of the United States led Great Britain and other countries in the tonnage of shipping annually constructed, and as wood was abundant and iron was expensive in the United States, our builders adhered to wooden construction much longer than other countries did. The British Government, always conservative, did not favor iron hulls for its navy until after 1850. Until 1852 the British mails could be carried only in wooden ships. Some of the large English steamship companies preferred wooden ships as late as 1860. The Cunard Company built its first iron ship for the Atlantic service in 1856.

The construction of iron ships made steady though not rapid headway in Great Britain during the decade of 1850-60. Thirty per cent of the British tonnage was iron in 1860, and after that date there was but a small wooden tonnage launched. At the present time over ninety-nine per cent of the tonnage of the shipping of the United Kingdom is of iron or steel construction. In the United States, on the contrary, the construction of iron ships made but little headway until after 1870, and since then the change from wood to iron has been so slow that, in 1904, fifty-eight per cent of the tonnage under the American flag was of wooden vessels. It can, however, only be a question of time when wood must give way to metal in shipbuilding, because of the unquestionable superiority of iron and steel ships.

The first change in the material used in the construction of hulls was from wood to iron; the second, was from iron to steel. British builders began to use steel shortly before 1880; and in 1886 the tonnage of steel ships constructed exceeded the figures for iron. Since 1886 the total tonnage and annual construction of iron ships in the United Kingdom has steadily declined; while the figures

for steel vessels have risen, until at the present time over four fifths of the total shipping of that country consists of steel vessels. Steel has now almost displaced iron in the construction of hulls in the shipyards of all countries, and as existing iron ships wear out, their places are taken by steel vessels.

The hull of the modern steamer is divided by bulkheads into water-tight compartments, and is constructed with a double bottom within which water ballast may be carried. The ship carries dynamos, machinery for making ice, for handling cargo, for steering the vessel, and for many other purposes. Almost every year sees some new equipment added to the fitting of the already highly complex and efficient ocean carrier.

The technical progress made in the construction of ocean vessels can be stated most satisfactorily by a brief description of the Minnesota, constructed at New London, Conn., and completed in 1905. This, and its sister ship the Dakota, were built for the Great Northern Steamship Company to carry freight and passengers between Puget Sound ports and transpacific countries. They are the largest ships ever built in America. Their gross tonnage is 20,718, and their net 13,324 tons, and, with the exception of a few vessels on the North Atlantic, were the largest vessels afloat in 1905. These vessels are 630 feet in length over all, 73 feet 6 inches beam, and 56 feet in depth from the keel to the upper deck amidships. Although intended mainly for the transportation of freight, each vessel has accommodations for 250 cabin, 68 second-class, and 1,500 steerage passengers. The hull is constructed with 32 water-tight compartments, and there are nine decks. These vessels have a speed of about fifteen knots an hour.

The Cunard Steamship Company, of Liverpool, is constructing two vessels intended to exceed in size and speed any others that they have thus far built. These vessels,

which will be completed in 1907, are to be nearly 800 feet in length, and are to be propelled by four screws operated with turbine engines having 70,000 horse power. They are to have a speed of 25 knots per hour, and are to be able to maintain an average speed of 24 knots per hour for the transatlantic voyage.

REFERENCES FOR FURTHER READING

MAGINNIS, A. J. "The Atlantic Ferry." Third Edition, 1900. SEATON, A. E. "A Manual of Marine Engineering." Twelfth

Edition, 1895.

FRY, HENRY. "The History of North Atlantic Steam Navigation."

1896.

PREBLE, G. H. "A Chronological History of the Origin and Development of Steam Navigation, 1543-1882." 1883.

MARVIN, W. L. "The American Merchant Marine." 1902. CHAMBERLAIN, E. T. "Annual Reports, Commissioner of Navigation, Washington, D. C."

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SMITH, A. R. 'Shipbuilding.' Twelfth Census of the United States, vol. x, Part IV, pp. 209–39. 1902.

CROWELL, J. F. "The Shipping Industry of the United States, and Its Relation to the Foreign Trade." Monthly Summary of Commerce and Finance, for December, 1900. Bureau of Statistics, Washington, D. C.