Chapter 5

If [theta] be the angle between the path of the projectile and the face of the plate, then v in the above formulæ becomes v sin [theta].

When we come to back the plates, their power to resist penetration becomes greater, and our formula changes. The Gavre formula, given above, is used to determine the velocity necessary for a projectile to pass entirely through an iron plate and its wood backing.

Compound and steel armor are said to give about 29 per cent. more resisting power than wrought iron, but in one experiment at the proving ground, at Annapolis, a compound plate gave over 50 per cent. more resisting power than wrought iron.

The Italian government, after most expensive and elaborate comparative tests, has decided in favor of the Creusot or Schneider all-steel plates, and has established a plant for their manufacture at Terni, near Rome.

The French use both steel and compound plates; the Russians, compound; the Germans, compound; the Swedes and Danes use both. Spain has adopted and accepted the Creusot plate for its new formidable armored vessel, the Pelayo; and China too has recently become a purchaser of Creusot plates.

Certain general rules may be laid down for attacking armor. If the armor is iron, it is useless to attack with projectiles having less than 1,000 feet striking velocity for each caliber in thickness of plate. It is unadvisable to fire steel or chilled iron filled shells at thick armor, unless a normal hit can be made. When perforation is to be attempted, steel-forged armor-piercing shells, unfilled, should be used. They may be filled if the guns are of great power as compared to the armor. Steel and compound armor are not likely to be pierced by a single blow, but continued hammering may break up the plate, and that with comparatively low-powered guns.

Wrought iron must be perforated, and hard armor, compound or steel, must be broken up. Against wrought iron plates the projectile may be made of chilled cast iron, but hard armor exacts for its penetration or destruction the use of steel, forged and tempered. Against unarmored ships, and against unarmored portions of ironclads, the value of rapid-firing guns, especially those of large caliber, can hardly be overestimated.

The relative value of steel and compound armor is much debated, and at present the rivalry is great, but the weight of evidence and opinion seems to favor the all-steel plate. The hard face of a compound plate is supposed to break up the projectile, that is, make the projectile expend its energy on itself rather than upon the plate, and the backing of wrought iron is, by its greater ductility, to prevent the destruction of the plate. It seems probable that these two systems will approach each other as the development goes on. An alloy of nickel and steel is now attracting attention and bids fair to give very good results.

The problem to be solved, as far as naval armor is concerned, is to get the greatest amount of protection with the least possible weight and volume, and this reduction of weight and volume must be accomplished, in the main, by reducing the thickness of the plates by increasing the resisting power of the material. In the compound plate great surface hardness is readily and safely attained, but it has not yet been definitely determined what the proper proportionate thickness of iron and steel is.

A considerable thickness of steel is necessary to aid, by its stiffness, in preventing the very ductile iron from giving back to such an extent as to distort the steel face and thus tear or separate the parts of the plate. The ductile iron gives a very low resisting power, its duty being to hold the steel face up to its work. If now we substitute a soft steel plate in the place of the ductile iron, we will get greater resisting power, but our compound plate then becomes virtually an all-steel one, only differing in process of manufacture. The greatest faults of the compound plate are the imperfect welding of the parts and the lack of solidity of the iron. When fired at, the surface has a tendency to chip.

In the all-steel plate we have the greatest resisting power throughout, but there are manufacturing difficulties, and surface hardness equal to that of the compound plate has not been obtained. The manufacturing difficulties are being gradually overcome, and artillerists are in high hopes that the requisite surface hardness will soon be obtained.

The following may be stated as well proved:

1. That steel armor promises to replace both iron and compound.

2. That projectiles designed for the piercing of hard armor must be made of steel.

3. That the larger the plate, the better it is able to absorb the energy of impact without injury to itself.

4. That the backing must be as rigid as possible.

* * * * *

[FROM ENGINEERING.]

THE COMPRESSED AIR SYSTEM OF PARIS.

The demand for compressed air as a motive power is constantly increasing in Paris; the company, according to its official reports, is financially prosperous, and it seems difficult to understand how it should continue as an actively going concern, unless it at all events paid its way. The central station of St. Fargeau, originally started on modest lines, for maintaining a uniform time by pneumatic pressure throughout Paris, has grown rapidly to very large proportions, though it has never been able to supply the demand made on it for power; and at the present time a second and still larger station is being constructed in another part of Paris. We confess that we do not understand why such large sums of money should continue to be spent if the enterprise is not commercially a sound one, nor how men of such eminence in the scientific world as Professor Riedler should, without hesitation, risk their reputation on the correctness of the system, if it were the idle dream of an enthusiast, as many persons--chiefly those interested in electric transmission--have declared it to be.

In describing the developments that have taken place during the last two years, we shall confine ourselves entirely to the details of a report recently made on the subject by Professor Riedler. As soon as it became evident that a very largely increased installation was necessary, it was determined that the new central station should be as free as possible from the defects of the first one. These defects, which were the natural results of the somewhat hasty development of an experimental system, were of several kinds. In the first place, so large a growth had not been contemplated, and the extensions were made more or less piecemeal, instead of being on a regular plan; the location of the central station itself was very unfavorable, both as regards the facilities for obtaining coal and other supplies; the cost of water was excessive, and the amount available, inadequate.

This evil was partly remedied by elaborate arrangements for cooling the injection water so that it could be repeatedly used, a device costly and ineffective, and resulting in extravagant working, to say nothing of the high charges made by the Paris company for supplying water. To these drawbacks had to be added others of an even more serious character. The engines first laid down were not economical, and the compressors employed gave but a very inferior result; with each extension of the plant, the efficiency of both engines and compressors was increased, the most satisfactory, we believe, having been those supplied by the Societe Cockerill, and one of which was exhibited at the Paris exhibition in 1889. Still it was clearly recognized that much better results were possible, results which Professor Riedler claims have been attained and which will be embodied in the new installation now in progress.

This central station is located on the left bank of the Seine, close to the fortifications, opposite Vincennes and not far from the terminal stations of the Orleans and the Paris, Lyons, and Mediterranean Railways; the plan, Fig. 1, shows the position. The works are separated from the river by the quay, over which a bridge will be constructed for the transfer of coal from the landing stages belonging to the company, into the works; as will be readily seen from the plan, it would be quite easy to run junction lines to the two adjacent railways, but with all the advantages given by water carriage, it was considered unnecessary to incur the expense. The river also affords a constant and unlimited water supply, so that none of the difficulties existing at St. Fargeau Station in imperfect condensation and cooling will be met with.

The new installation, called the Central Station of the Quai de la Gare, is laid out on a very large scale, the total generating energy provided for being no less than 24,000 horse power; of this it is intended that 8,000 horse power will be in operation this year, and an extension of 10,000 horsepower in 1892; the power now in course of completion comprises four engines of 2,000 horse power each. Four batteries of boilers will provide steam for these engines. Figs. 2, 3, and 4 show the first section of the installation now in progress; the four groups of engines (three-cylinder condensing) are shown at 1, 2, 3, and 4; the four groups of boilers ranged behind them at F, F; the feed water heaters belonging to each group at V V.

The end of the building abuts against the Seine, and the position of the water conduits for inlet and discharge are indicated at C and A respectively. The installation, when completed, will include very extensive arrangements for transporting and storing coal, and the interior of the boiler houses will be furnished with an overhead system of rails and carriers for handling the coal automatically, as far as possible. All the principal mains and steam pipes are made in duplicate, not only for greater security, but in order that each set of engines and boilers may be connected interchangeably without delay. The Seine supplies an ample quantity of water, but not in a condition either for feeding the boilers, for condensation, or for the air compressors.

Special provisions have therefore to be made to filter the water efficiently before it is used. For this purpose the water is led to a group of four filters (see L, Fig. 4); from them it passes into the tanks, JJ, and is pumped into the heaters. The filters can be rapidly and automatically cleaned by reversing the flow of water through them. Figs. 5 and 6 show the general form of the type of engine adopted, as well as the engine house, some of the mains, etc. They are vertical triple-expansion engines, and are being constructed by MM. Schneider et Cie, of Creusot, with a guarantee of coal consumption not to exceed 1.54 lb. per horse power per hour, with a penalty of 2,000 francs for every 100 grammes in excess of this limit. It is evident that with this restricted fuel consumption, a large margin for economy will exist at the new works, as compared with the St. Fargeau station, where the best engines cannot show anything like this result, while some of the earlier ones are distinctly extravagant, and the whole installation is handicapped with imperfect means of condensation.

Moreover, according to Professor Riedler, the consumption of steam by the new Schneider engines will be only 5.3 kilos. per horse power and per hour as compared with some of the large engines requiring 9 kilos., and the Cockerill engines--using 8 kilos. per hour, not to speak of the older motors that are very extravagant in the use of steam. The St. Fargeau station is worked under a further disadvantage. The constantly increasing demand from subscribers taxes the resources of the station to their fullest extent, so that practically there is no reserve power.

In the new installation the work will be equally constant, but care will be taken always to have a sufficient reserve. Electric lighting will form a considerable part of the duty to be done from this station, and in all cases it is intended to work with accumulators, so that the resistance to be overcome by the engines, so far as this part of the duty is concerned, will be well known and uniform. The engineers of the Compressed Air Co., of Paris, have during the last five years acquired an experience which could only be attained at a high price and at the expense of a certain amount of failure; this period, it is claimed, is now passed, and in the new installation it is possible to put into practice all the valuable lessons learned at St. Fargeau, to say nothing of the more favorable natural conditions under which the extension is being started and the improvements in the compression of the air made by Mr. Popp and Professor Riedler, and to which we shall refer later.

Chiefly in consequence of the high value of the ground, vertical engines were adopted at the new station; the proximity to the river made the foundations somewhat costly, and the risk of occasional floods rendered it desirable to set the level of the engine bedplates 20 inches above the floor of the building; the foundations of the engines are continuous, but are quite independent of the building. There are three compressing cylinders in each set of engines, one being above each steam cylinder. Two of these are employed to compress the air to about 30 lb. per square inch, after which it passes into a receiver and is cooled; it is then admitted into the third or final compressing cylinder and raised to the working pressure at which it flows into the mains. In the illustrations, h, m, and b are the high, intermediate, and low pressure cylinders of one set of engines; as will be seen, each cylinder is on a separate frame connected by girders; directly above the cylinders are the two low and the one high pressure air cylinders, b¹, m¹, and h¹ respectively. The former deliver the air compressed to the first stage into the receiver, T¹ (see Fig. 5), whence it passes into the third compression cylinder, and thence by a main into the cylinders, R R, which are in direct communication with the delivery mains; these mains terminate in the subway, T. The water for condensation is brought into the engine house by the channel, C, and the condenser pumps, a, draw direct from this supply; the discharge main back to the river is shown at A. The relative positions of the engine and boiler houses are indicated in Figs. 2 to 5, where F shows the end of one group of boilers; the air supply for the compressors is led from the central raised portion, S, of the roof.

Professor Riedler's first experiments in improving the efficiency of air compressors were made with one of the Cockerill compressors in use at the St. Fargeau Station, and considerable difficulty attended this work, because the machinery was necessarily kept almost in constant operation. These compressors were designed by MM. Dubois and Francois, of Seraing. Two of their leading features were the delivery of the compressed air at as low a temperature as possible, and with a relatively high piston speed of about 400 ft. a minute. The former object is attained by the injection of a very fine water spray at each end of the air cylinder, and its rapid removal with each stroke; the free as well as the compressed air flows through the same passages, one at each end of the cylinder; the inlet valves being placed at the side of these passages, and the outlet or compressed air valves at the top, the compressed air, entering a chamber above the cylinder, common to both valves, and passing thence to the reservoir. The compressed air valves, which are seven in. in diameter, are brought back sharply to their seats at each stroke, by a small piston operated by compressed air flowing through a by-pass from the chamber. The illustrations published by us on page 686 of our forty-seventh volume show the construction of these compressors. The engravings on page 683 of the same volume illustrate the compressors used in a somewhat older part of the installation; they were made by M. Blanchod, of Vevey, and a passing reference may be made to them. The air is admitted through valves in the cylinder, and is forced out through spring-loaded valves; water is admitted into the cylinder to cool the air.

Fig. 7 indicates the modification made by Professor Riedler in one of the Cockerill compressors: a receiver, A, was placed under the two compressing cylinders, B and C. The first stage is completed in the large cylinder, B, the air being compressed to about 30 lb. per square inch; from this it is discharged into the receiver, A, through the pipe, B¹, where it meets with a spray injection that cools it to the temperature of the water. The final stage is then effected in the smaller cylinder, C, which, drawing the air from the receiver through the pipe, C¹, compresses it to about 90 lb. and delivers it through the pipe, d, to the mains. We hope shortly to publish drawings of this compressor in its final form; in its elementary stage Professor Riedler claims to have obtained some very remarkable results. He says that the waste spaces in his modification were much smaller than in the Cockerill compressor, while the efficiency of the apparatus was largely increased. The actual engine duty per horse power and per hour was raised, as a maximum, to 384 cubic feet of air at atmospheric pressure, and compressed to 90 lb. per square inch, a marked increase on the duty of the compressors in use at the St. Fargeau station. The Cockerill compressors experimented on at the same time showed a maximum duty of 306 cubic feet of air. A considerable advantage is claimed in drawing clean and cool air from the outside of the building, and beyond the main feature of carrying out the compression in two stages, Mr. Riedler appears to have shown great skill in introducing several minor alterations and improvements in the plant.

Figs. 8, 9 and 10 are diagrams showing the comparative efficiency of the three types of compressors at St. Fargeau--Fig. 10 being a diagram of the Riedler compressor--and indicate the gain derived from the intermediate cooling. The loss is shown to be only 12 per cent., as compared with a loss of 43 per cent. in a large part of the plant, and of 105 per cent. in the earlier compressors of the St. Gothard type. The table given herewith contains a summary of trials made by Professor Gutermuth, and are intended to show the comparative results of an extended trial with three kinds of compressors at St. Fargeau.

PERFORMANCES OF COMPRESSORS AT THE ST. FARGEAU CENTRAL STATION.

+-------+--------+------+-------+--------+--------+---------+ | R p | | E | | | | | | e o e | Horse- | f |Amount |Quantity| Cubic | | Compressors. | v f r | Power | f |of Air | of Air |Feet of |Final Air| | o |Absorbed| i |Passing| Passing|Air per |Pressure.| | l E m | by | c |through| through| Horse- | | | u n i |Compres-| i | Inlet | Valves | Power | | | t g n | sors. | e | Valves| per | and per| | | i i u | | n | each | Hour. | Hour. | | | o n t | | c |Revolu-| | | | | n e e | | y | tion. | | | | | s . | | . | | | | | --------------+-------+--------+------+-------+--------+--------+---------+ | | | | cubic | cubic | |lb. per | 1. | | | | feet | feet | |sq. in. | _Sturgeon_ | | | | | | | | _Compressor_ | 37 | 302 | .87 | 41.67 | 91,507| 261.3 | 90 | Diameter of | 37 | 258 | .87 | 38.13 | 84,650| 276.1 | 90 | cylinder, | | | | | | | | 23.62 in. | | | | | | | | and 21.66 in.;| | | | | | | | stroke, | | | | | | | | 48.63 in. | | | | | | | | | | | | | | | | 2. | | | | | | | | _Cockerill_ | 40 | 337 | .83 | 46.61 | 111,864| 281.83 | 90 | _Compressor._ | 45 | 353 | .83 | 46.61 | 125,844| 302.66 | 90 | Diameter of | 40 | 342 | .88 | 49.43 | 118,632| 296.65 | 90 | cylinder, | 46 | 377 | .85 | 48.02 | 132,534| 298.77 | 90 | 25.98 in.; | 38.67 | 324 | .89 | 50.14 | 116,434| 306.19 | 90 | stroke, | 38.5 | 337 | .89 | 50.14 | 115,818| 294.18 | 90 | 47.24 in. | 38.6 | 329 | .91 | 50.84 | 117,740| 305.13 | 90 | | | | | | | | | | | | | | | | | 3. | | | | | | | | _Riedler_ | 52 | 615 | .985 | 77.34 | 241,300| 353.50 | 90 | _Compressor._ | 60 | 709 | .985 | 76.98 | 277,128| 353.50 | 90 | Diameter of | 38 | 422 | .985 | 77.34 | 176,330| 376.12 | 90 | low-pressure | 39 | 424 | .985 | 77.34 | 181,030| 384.60 | 90 | cylinder, | | | | | | | | 42.91 in.; | | | | | | | | diameter of | | | | | | | | high-pressure | | | | | | | | cylinder, | | | | | | | | 26.38 in.; | | | | | | | | stroke, | | | | | | | | 47.24 in. | | | | | | | | --------------+-------+--------+------+-------+--------+--------+---------+

The results thus obtained were so satisfactory that the designs were prepared for the great compressors to be operated at the new central station on the Quai de la Gare by the 2,000 horse power engines.

The transmission of the compressed air through the mains is unavoidably attended with a certain percentage of loss, which, of course, increases with the length of the transmission, the presence of leakage at the joints, etc. Professor Riedler has devoted considerable time to the investigation of this source of waste, and we shall presently refer to the results he has recorded; in the first place, however, we propose to consider what he has to say on the subject of utilizing the air at the points of delivery, and the means employed for obtaining a relatively high efficiency of the motor.