Corrugated pistons, or irregular cams, _C D_, are adopted, forming chambers within the cases. In the engine the steam enters at _A_, at the bottom of the case, and presses the cams apart. The only packing used is in the ends of the long metal cogs, which are ground to fit the case and are kept out by the momentum of the cams, a.s.sisted by a slight spring back of the packing-pieces. The friction on the pump (Fig. 124) is said to be less than in the engine. This is the reason given in support of the claim that the rotary engine forces water to a given distance with from one-fourth to one-third the steam-pressure necessary to drive all reciprocating engines. The smaller amount of power necessary to do the work, the less strain and consequent wear and tear upon the whole machine, are said to make it more durable and reliable. The pump being chambered, its liability to injury by the use of dirty or gritty water is lessened, and it is stated that it will last for years, pumping gritty water that would soon cut out a piston-pump. The pump used with this engine is, as shown in the above ill.u.s.tration, somewhat similar to the rotary engine driving it. Each of the revolving pistons has three long teeth bearing against the cylinder, and packed, to prevent leakage, like the engine-cams. They are carried on steel shafts coupled to the engine-shafts. The water enters at _E_ and is discharged at _F_, and the pa.s.sages are purposely made large in order that sand, chips, and dirt, which may enter with the water, may pa.s.s through.

The rotary engine is gradually coming into use for various special purposes, where small power is called for, and where economy of fuel is not important; but it has never yet competed, and may perhaps never in the future compete, with the reciprocating-piston engine where large engines are required, or where even moderate economy of fuel is essential. This form of engine has a.s.sumed so little importance, in fact, in the application of the steam-engine, that comparatively little is known of its history. Watt invented a rotary engine, and Yule many years afterward (1836) constructed such engines at Glasgow.

Lamb patented another in 1842, Behrens still another in 1847. Napier, Hall, Ma.s.sey, Holly, La France, and others, have built engines of this cla.s.s in later times. Nearly all consist either of cams rotating in gear, as in those above sketched, or of a piston set radially in a cylinder of small diameter, which turns on its axis within a much larger cylinder set eccentrically, the piston, as the former turns, sliding in and out of the smaller cylinder as its outer edge slides in contact with the inner surface of the larger. In some forms of rotary engine, a piston revolves on a central shaft, and a sliding abutment in the external cylinder serves to separate the steam from the exhaust side and to confine the steam expanding while doing work. Nearly all of these combinations are also used as pumps.

Fire-engines, made by the best-known American builders of engines, with reciprocating engines and pumps, such as are in general use in the United States, have become standard in general plan and arrangement of details. These are probably the best ill.u.s.trations of extreme lightness, combined with strength of parts and working power, which have ever been produced in any branch of mechanical engineering. By using a small boiler crowded with heating-surface, very carefully proportioned and arranged, and with small water-s.p.a.ces; by adopting steel for running-gear and working parts wherever possible; by working at high piston-speed and with high steam-pressure; by selecting fuel with extreme care--by all these expedients, the steam fire-engine has been brought, in this country, to a state of efficiency far superior to anything seen elsewhere.

Steam is raised with wonderful promptness, even from cold water, and water is thrown from the nozzle at the end of long lines of hose to great distances. But this combination of lightness with power is only attained at the expense of a certain regularity of action which can only be secured by greater water and steam capacity in the boiler. The small quant.i.ty of water contained within the boiler makes it necessary to give constant attention to the feed, and the tendency, almost invariably observed, to serious foaming and priming not only compels unintermitted care while running, but even introduces an element of danger which is not to be despised, even though the machine be in charge of the most experienced and skillful attendants. Even the greatest care, directed by the utmost skill, would not avail to prevent frequent explosions, were it not for the fact that it rarely, if ever, happens that accidents to such boilers occur from low water, unless the boiler is actually completely emptied of water. In driving them at fires, they frequently foam so violently that it is utterly impossible to obtain any clew to the amount of water present, and the attendant usually keeps his feed-pump on and allows the foaming to go on. As long as water is pa.s.sing into the boiler it is very unlikely that any portion will become overheated and that accident will occur.

Such management appears very reckless, and yet accident from such a cause is exceedingly rare.

The changes which have been made in LOCOMOTIVE-CONSTRUCTION during the past few years have also been in the direction of the refinement of the earlier designs, and have been accompanied by corresponding changes in all branches of railroad-work. The adjustment of parts to each other and proportioning them to their work, the modification of the minor details to suit changes of general dimensions, the improvement of workmanship, and the use of better material, have signalized this latest period. Special forms of engine have been devised for special kinds of work. Small, light tank-engines (Fig.

125), carrying their own fuel and water without "tenders," are used for moving cars about terminal stations and for making up trains; powerful, heavy, slow-moving engines, of large boiler-capacity and with small wheels, are used on steep gradients and for hauling long trains laden with coal and heavy merchandise; and hardly less powerful but quite differently proportioned "express"-engines are used for pa.s.senger and mail service.

[Ill.u.s.tration: FIG. 125.--Tank-Engine, New York Elevated Railroad.]

[Ill.u.s.tration: FIG. 126.--Forney's Tank-Locomotive.]

A peculiar form of engine (Fig. 126) has been designed by Forney, in which the whole weight of engine, tender, coal, and water, is carried by one frame and on one set of wheels, the permanent weight falling on the driving-wheels and the variable load on the truck. These engines have also a comparatively short wheel-base and high pulling-power. The lightest tank-engines of the first cla.s.s mentioned weigh 8 or 10 tons; but engines much lighter than these, even, are built for mines, where they are sent into the galleries to bring out the coal-laden wagons.

The heaviest engines of this cla.s.s attain weights of 20 or 30 tons.

The heaviest engine yet constructed in the United States is said to be one in use on the Philadelphia & Reading Railroad, having a weight of about 100,000 pounds, which is carried on 12 driving-wheels.

[Ill.u.s.tration: FIG. 127.--British Express Engine.]

[Ill.u.s.tration: FIG. 128.--The Baldwin Locomotive. Section.]

[Ill.u.s.tration: FIG. 129.--The American Type of Express-Engine, 1878.]

A locomotive has two steam-cylinders, either side by side within the frame, and immediately beneath the forward end of the boiler, or on each side and exterior to the frame. The engines are non-condensing, and of the simplest possible construction. The whole machine is carried upon strong but flexible steel springs. The steam-pressure is usually more than 100 pounds. The pulling-power is generally about one-fifth the weight under most favorable conditions, and becomes as low as one-tenth on wet rails. The fuel employed is wood in new countries, c.o.ke in bituminous coal districts, and anthracite coal in the eastern part of the United States. The general arrangement and the proportions of locomotives differ somewhat in different localities.

In Fig. 127, a British express-engine, _O_ is the boiler, _N_ the fire-box, _X_ the grate, _G_ the smoke-box, and _P_ the chimney. _S_ is a spring and _R_ a lever safety-valve, _T_ is the whistle, _L_ the throttle or regulator valve, _E_ the steam-cylinder, and _W_ the driving-wheel. The force-pump, _B C_, is driven from the cross-head, _D_. The frame is the base of the whole system, and all other parts are firmly secured to it. The boiler is made fast at one end, and provision is made for its expansion when heated. Adhesion is secured by throwing a proper proportion of the weight upon the driving-wheel, _W_. This is from about 6,000 pounds on standard freight-engines, having several pairs of drivers, to 10,000 pounds on pa.s.senger-engines, per axle. The peculiarities of the American type (Fig. 128) are the truck, _I J_, or bogie, supporting the forward part of the engine, the system of equalizers, or beams which distribute the weight of the machine equally over the several axles, and minor differences of detail. The cab or house, _r_, protecting the engine-driver and fireman, is an American device, which is gradually coming into use abroad also. The American locomotive is distinguished by its flexibility and ease of action upon even roughly-laid roads. In the sketch, which shows a standard American engine in section, _A B_ is the boiler, _C_ one of the steam-cylinders, _D_ the piston, _E_ the cross-head, connected to the crank-shaft, _F_, by the connecting-rod, _G H_ the driving-wheels, _I J_ the truck-wheels, carrying the truck, _K L_; _N N_ is the fire-box, _O O_ the tubes, of which but four are shown. The steam-pipe, _R S_, leads the steam to the valve-chest, _T_, in which is seen the valve, moved by the valve-gear, _U V_, and the link, _W_. The link is raised or depressed by a lever, _X_, moved from the cab. The safety-valve is seen at the top of the dome, at _Y_, and the spring-balance by which the load is adjusted is shown at _Z_. At _a_ is the cone-shaped exhaust-pipe, by which a good draught is secured. The attachments _b_, _c_, _d_, _e_, _f_, _g_--whistle, steam-gauge, sand-box, bell, head-light, and "cow-catcher"--are nearly all peculiar, either in construction or location, to the American locomotive. The cost of pa.s.senger-locomotives of ordinary size is about $12,000; heavier engines sometimes cost $20,000. The locomotive is usually furnished with a tender, which carries its fuel and water.

The standard pa.s.senger-engine on the Pennsylvania Railroad has four driving-wheels, 5-1/2 feet diameter; steam-cylinders, 17 inches diameter and 2 feet stroke; grate-surface 15-1/2 square feet, and heating-surface 1,058 square feet. It weighs 63,100 pounds, of which 39,000 pounds are on the drivers and 24,100 on the truck. The freight-engine has six driving-wheels, 54-5/8 inches in diameter. The steam-cylinders are 18 inches in diameter, stroke 22 inches, grate-surface 14.8 square feet, heating-surface 1,096 feet. It weighs 68,500 pounds, of which 48,000 are on the drivers and 20,500 on the truck. The former takes a train of five cars up an average grade of 90 feet to the mile. The latter is attached to a train of 11 cars. On a grade of 50 feet to the mile, the former takes 7 and the latter 17 cars. Tank-engines for very heavy work, such as on grades of 320 feet to the mile, which are found on some of the mountain lines of road, are made with five pairs of driving-wheels, and with no truck. The steam-cylinders are 20-1/8 inches in diameter, 2 feet stroke; grate-area, 15-3/4 feet; heating-surface, 1,380 feet; weight with tank full, and full supply of wood, 112,000 pounds; average weight, 108,000 pounds. Such an engine has hauled 110 tons up this grade at the speed of 5 miles an hour, the steam-pressure being 145 pounds. The adhesion was about 23 per cent. of the weight.

In checking a train in motion, the inertia of the engine itself absorbs a seriously large portion of the work of the brakes. This is sometimes reduced by reversing the engine and allowing the steam-pressure to act in aid of the brakes. To avoid injury by abrasion of the surfaces of piston, cylinder, and the valves and valve-seats, M. Le Chatelier introduces a jet of steam into the exhaust-pa.s.sages when reversing, and thus prevents the ingress of dust-laden air and the drying of the rubbing surfaces. This method of checking a train is rarely resorted to, however, except in case of danger. The introduction of the "continuous" or "air" brake, which can be thrown into action in an instant on every car of the train by the engine-driver, is so efficient that it is now almost universally adopted. It is one of the most important safeguards which American ingenuity has yet devised. In drawing a train weighing 150 tons at the rate of 60 miles an hour, about 800 effective horse-power is required.

A speed of 80 miles an hour has been often attained, and 100 miles has probably been reached.

The American locomotive-engine has a maximum life which may be stated at about 30 years. The annual cost of repairs is from 10 to 15 per cent. of its first cost. On moderately level roads, the engine requires a pint of oil to each 25 miles, and a ton of coal to each 40 or 50 miles run. One of the best-managed railroads in the United States reports expenses as follows for one month:

Number "train-miles" run per ton of coal burned 53.95 " " " " quart of oil used 34.44 Pa.s.senger-cars hauled 1 mile per ton of coal 275.7 Other " " " " " 634.8 Cost repairs per mile run $2 43 " fuel " " 3 64 " oil and waste per mile run 62 " wages of engine-men per mile run 6 22 All other expenses per mile 1 91 Total cost per "train-mile" run 14 82

Although the above sketch and description represent the construction and performance of the standard locomotive of the present time, there are indications that the compound arrangement of engines will ultimately be adopted. This will involve a considerable change of proportions, greatly increasing the volume and weight of steam-cylinders, but enabling the designer to more than proportionally decrease the weight of boiler and the quant.i.ty of fuel carried. There is no serious objection to their use, however, and no insuperable difficulty in the construction of the "double-cylinder" type of engine for the locomotive. A few such engines have already been put in service. In these engines the high-pressure cylinder is placed on one side and the larger low-pressure cylinder on the other side of the locomotive, thus having but two cylinders, as in the older plan. The valve-gear is the Stephenson link, as in the ordinary engine. At starting, the steam is allowed to act on both pistons; but after a few revolutions the course of the steam is changed, and the exhaust from the smaller cylinder, instead of pa.s.sing into the chimney, is sent to the larger cylinder, which is at the same time cut off from the main steam-pipe. When the engine is ascending a steep gradient the steam may, if necessary, be taken from the boiler into both cylinders, as when starting. Compound engines of this kind have been used on the French line of railroad from Bayonne to Biarritz. They were designed by Mallet and built at Le Creuzot. The steam-cylinders are of 9-1/2 and 15-3/4 inches diameter, and of 17-3/4 inches stroke of piston. The four driving-wheels are 4 feet in diameter, and the total weight of engine is 20 tons. The boiler has 484-1/2 square feet of heating-surface, and is built to carry 10 atmospheres pressure. When hauling trains of 50 tons at 25 miles an hour, these engines require about 15 pounds of good coal per mile.

The total length of the railways in operation in the United States on the 1st day of January, 1877, was 76,640 miles,[93] being an average of one mile of railway for every 600 inhabitants. The railways are as follows:

[93] January, 1884, over 120,000 miles.

Miles.

Alabama 1,722 Alaska 0 Arizona 0 Arkansas 787 California 1,854 Colorado 950 Connecticut 925 Dakota 290 Delaware 285 Florida 484 Georgia 2,308 Idaho 0 Illinois 6,980 Indiana 4,072 Indian Territory 281 Iowa 3,937 Kansas 3,226 Kentucky 1,464 Louisiana 539 Maine 987 Maryland 1,092 Ma.s.sachusetts 1,825 Michigan 3,437 Minnesota 2,024 Mississippi 1,028 Missouri 3,016 Montana 0 Nebraska 1,181 Nevada 714 New Hampshire 942 New Jersey 1,594 New Mexico 0 New York 5,520 North Carolina 1,371 Ohio 4,680 Oregon 251 Pennsylvania 5,896 Rhode Island 182 South Carolina 1,352 Tennessee 1,638 Texas 2,072 Utah 486 Vermont 810 Virginia 1,648 Washington 110 West Virginia 576 Wisconsin 2,575 Wyoming 459 ------ Total 76,640

In 1873 came the great financial crisis, with its terrible results of interrupted production, poverty, and starvation, and an almost total cessation of the work of building new railroads. The largest number of miles ever built in any one year were constructed in 1872. The greatest mileage is in Illinois, reaching 6,589; the smallest in Rhode Island, 136, and in Washington Territory, 110. The State of Ma.s.sachusetts has one mile of railroad to 4.86 miles of territory, this ratio being the greatest in the country. The longest road in operation is the Chicago & Northwestern, extending 1,500 miles; the shortest, the Little Saw-Mill Run Road in Pennsylvania, which is but three miles in length. The total capital of railways in the country is $6,000,000,000, or an average of $100,000 per mile. The earnings for the year 1872 amounted to $454,969,000, or $7,500 per mile. The largest net earnings recorded as made on any road were gained by the New York Central & Hudson River, $8,260,827; the smallest on several roads which not only earned nothing, but incurred a loss.

The catastrophe of 1873-'74 revealed the fact that the latter condition of railroad finances was vastly more common than had been suspected; and it is still doubtful whether the existing immense network of railroads which covers the United States can be made, as a whole, to pay even a moderate return on the money invested in their construction. At the period of maximum rate of extension of railroads in the United States--1873--the reported lengths of the railroads of Europe and America were as follows:[94]

[94] _Railroad Gazette._

RAILROADS IN EUROPE AND AMERICA IN 1873.

----------------------------+------------+-------------+------------ COUNTRIES. | Railroads, | Population. | Area, | Miles. | | Sq. Miles.

----------------------------+------------+-------------+------------ United States | 71,565 | 40,232,000 | 2,492,316 Germany | 12,207 | 40,111,265 | 212,091 Austria | 5,865 | 35,943,592 | 227,234 France | 10,333 | 36,469,875 | 201,900 Russia in Europe | 7,044 | 71,207,794 | 1,992,574 Great Britain, 1872 | 15,814 | 31,817,108 | 120,769 Belgium | 1,301 | 4,839,094 | 11,412 Netherlands | 886 | 3,858,055 | 13,464 Switzerland | 820 | 2,669,095 | 15,233 Italy | 3,667 | 26,273,776 | 107,961 Denmark | 420 | 1,784,741 | 14,453 Spain | 3,401 | 16,301,850 | 182,758 Portugal | 453 | 3,987,867 | 36,510 Sweden and Norway | 1,049 | 5,860,122 | 188,771 Greece | 100 | 1,332,508 | 19,941 ----------------------------+------------+-------------+------------

The railroads in Great Britain comprise over 15,000 miles of track now being worked in the United Kingdom, on which have been expended $2,800,000,000. This sum is equal to five times the amount of the annual value of all the real property in Great Britain, and two-thirds of the national debt. After deducting all the working expenses, the gross net annual revenue of all the roads exceeds by $110,000,000 the total revenue from all sources of Belgium, Holland, Portugal, Denmark, Sweden and Norway. An army of 100,000 officers and servants is in the employ of the companies, and the value of the rolling-stock exceeds $150,000,000.

SECTION III.--MARINE ENGINES.

The changes which have now become completed in the marine steam-engine have been effected at a later date than those which produced the modern locomotive. On the American rivers the modification of the beam-engine since the time of Robert L. Stevens has been very slight.

The same general arrangement is retained, and the details are little, if at all, altered. The pressure of steam is sometimes as high as 60 pounds per square inch.

[Ill.u.s.tration: FIG. 130.--Beam-Engine.]

The valves are of the disk or poppet variety, rising and falling vertically. They are four in number, two steam and two exhaust valves being placed at each end of the steam-cylinder. The beam-engine is a peculiarly American type, seldom if ever seen abroad. Fig. 130 is an outline sketch of this engine as built for a steamer plying on the Hudson River. This cla.s.s of engine is usually adopted in vessels of great length, light draught, and high speed. But one steam-cylinder is commonly used. The cross-head is coupled to one end of the beam by means of a pair of links, and the motion of the opposite end of the beam is transmitted to the crank by a connecting-rod of moderate length. The beam has a cast-iron centre surrounded by a wrought-iron strap of lozenge shape, in which are forged the bosses for the end-centres, or for the pins to which the connecting-rod and the links are attached. The main centre of the beam is supported by a "gallows-frame" of timbers so arranged as to receive all stresses longitudinally. The crank and shaft are of wrought-iron. The valve-gear is usually of the form already mentioned as the Stevens valve-gear, the invention of Robert L. and Francis B. Stevens. The condenser is placed immediately beneath the steam-cylinder. The air-pump is placed close beside it, and worked by a rod attached to the beam. Steam-vessels on the Hudson River have been driven by such engines at the rate of 20 miles an hour. This form of engine is remarkable for its smoothness of operation, its economy and durability, its compactness, and the lat.i.tude which it permits in the change of shape of the long, flexible vessels in which it is generally used, without injury by "getting out of line."

[Ill.u.s.tration: FIG. 131.--Oscillating Engine and Feathering Paddle-Wheel.]

For paddle-engines of large vessels, the favorite type, which has been the side-lever engine, is now rarely built. For smaller vessels, the oscillating engine with feathering paddle-wheels is still largely employed in Europe. This style of engine is shown in Fig. 131. It is very compact, light, and moderately economical, and excels in simplicity. The usual arrangement is such that the feathering-wheel has the same action upon the water as a radial wheel of double diameter. This reduction of the diameter of the wheel, while retaining maximum effectiveness, permits a high speed of engine, and therefore less weight, volume, and cost. The smaller wheel-boxes, by offering less resistance to the wind, r.e.t.a.r.d the progress of the vessel less than those of radial wheels. Inclined engines are sometimes used for driving paddle-wheels. In these the steam-cylinder lies in an inclined position, and its connecting-rod directly connects the crank with the cross-head. The condenser and air-pump usually lie beneath the cross-head guides, and are worked by a bell-crank driven by links on each side the connecting-rod, attached to the cross-head. Such engines are used to some extent in Europe, and they have been adopted in the United States navy for side-wheel gunboats. They are also used on the ferry-boats plying between New York and Brooklyn.

[Ill.u.s.tration: FIG. 132.--The Two Rhode Islands, 1836-1876.]

Among the finest ill.u.s.trations of recent practice in the construction of side-wheel steamers are those built for the several routes between New York and the cities of New England which traverse Long Island Sound. Our ill.u.s.tration exhibits the form of these vessels, and also shows well the modifications in structure and size which have been made during this generation. The later vessel is 325 feet long, 45 feet beam, 80 feet wide over the "guards," and 16 feet deep, drawing 10 feet of water. The "frames" upon which the planking of the hull is fastened are of white-oak, and the lighter and "top" timbers of cedar and locust. The engine has a steam-cylinder 90 inches in diameter and 12 feet stroke of piston.[95] On each side the great saloons which extend from end to end of the upper deck are state-rooms, containing each two berths and elegantly furnished. The engine of this vessel is capable of developing about 2,500 horse-power. The great wheels, of which the paddle-boxes are seen rising nearly to the height of the hurricane-deck, are 37-1/2 feet in diameter and 12 in breadth. The hull of this vessel, including all wood-work, weighs over 1,200 tons.

The weight of the machinery is about 625 tons. The steamer makes 16 knots an hour when the engine is at its best speed--about 17 revolutions per minute--and its average speed is about 14 knots on its route of 160 miles. The coal required to supply the furnaces of such a vessel and with such machinery would be about 3 tons per hour.

or a little over 2-1/2 pounds per horse-power. The construction of such a vessel occupies, usually, about a year, and costs a quarter of a million dollars.

[95] The steam-cylinders of the engines of steamers Bristol and Providence are 110 inches in diameter and of 12 feet stroke.

[Ill.u.s.tration: FIG. 133.--A Mississippi Steamboat.]

The non-condensing direct-acting engine is used princ.i.p.ally on the Western rivers, driven by steam of from 100 to 150 pounds pressure, and exhausts its steam into the atmosphere. It is the simplest possible form of direct-acting engine. The valves are usually of the "poppet" variety, and are operated by cams which act at the ends of long levers having their fulcra on the opposite side of the valve, the stem of which latter is attached at an intermediate point. The engine is horizontal, and the connecting-rod directly attached to cross-head and crank-pin without intermediate mechanism. The paddle-wheel is used, sometimes as a stern-wheel, as in the plan of Jonathan Hulls of one and a half century ago, sometimes as a side-wheel, as is most usual elsewhere. One of the most noted of these steamers, plying on the Mississippi, is shown in the preceding sketch.

One of the largest of these steamers was the Grand Republic,[96] a vessel 340 feet long, 56 feet beam, and 10-1/4 feet depth. The draught of water of this great craft was 3-1/2 feet forward and 4-1/2 aft. The two sets of compound engines, 28 and 56 inches diameter and of 10 feet stroke, drive wheels 38-1/2 feet in diameter and 18 feet wide. The boilers were steel. A steamer built still later on the Ohio has the following dimensions: Length, 225 feet; breadth, 35-1/2 feet; depth, 5 feet; cylinders, 17-3/8 inches in diameter, 6 feet stroke; three boilers. The hull and cabin were built at Jeffersonville, Ind. She has 40 large state-rooms. The cost of the steamer was $40,000.

[96] Burned in 1877.

These vessels have now opened to commerce the whole extent of the great Mississippi basin, transporting a large share of the products of a section of country measuring a million and a half square miles--an area equal to many times that of New York State, and twelve times that of the island of Great Britain--an area exceeding that of the whole of Europe, exclusive of Russia and Turkey, and capable, if as thoroughly cultivated as the Netherlands, of supporting a population of between three and four hundred millions of people.

The steam-engine and propelling apparatus of the modern ocean-steamer have now become almost exclusively the compound or double-cylinder engine, driving the screw. The form and the location of the machinery in the vessel vary with the size and character of the ship which it drives. Very small boats are fitted with machinery of quite a different kind from that built for large steamers, and war-vessels have usually been supplied with engines of a design radically different from that adopted for merchant-steamers.

[Ill.u.s.tration: FIG. 134.--Steam-Launch, New York Steam-Power Company.]

The introduction of _Steam-Launches_ and small pleasure-boats driven by steam-power is of comparatively recent date, but their use is rapidly increasing. Those first built were heavy, slow, and complicated; but, profiting by experience, light and graceful boats are now built, of remarkable swiftness, and having such improved and simplified machinery that they require little fuel and can be easily managed. Such boats have strong, carefully-modeled hulls, light and strong boilers, capable of making a large amount of dry steam with little fuel, and a light, quick-running engine, working without shake or jar, and using steam economically.

[Ill.u.s.tration: FIG. 135.--Launch-Engine.]