The following text may have been generated by Optical Character Recognition, with varying degrees of accuracy. Reader beware!

speed and resistance of ship models is the primary purpose of the 6 MARINE REVIEW. a pair of high speed revolving cutters to cut a corresponding line on each side of the model. A number of "level lines'? can be thus transferred from a drawing to a model, and the intermediate wax afterwards removed, and thus the model, which is usually about 12 feet long, is accurately shaped according to the design on the drawing. The shape is always tested carefully in the water, however, by what are called '"'needle trimmers." A number of needle points are set around the sides of the model, each point being a certain fixed distance from the keel, and then the weight of the model, plus the ballast nec- essary to make the points break the surface of the water, should agree with the calculated displacement to the plane of the needle points. In practice the agreement is found to be very close. The needles are then removed, and the and the model, (with a certain amount of ballast on board adjusted to give the required draught and trim,) is placed under the testing carriage. This car- riage can be drawn at any speed (within wide limits) along astraight and level railway, which is about 18 inches above the water, the motive power being a stationary steam engine which winds an endless wire rope to which the car- riage is attached. The model is hooked to a dynamometer on the carriage, and is thus drawn along at the same speed as the carriage, the force necessary to tow the model being registered on a revolving cylinder, by means of a pen attached to 4 multiplying lever of the dynamometer. The time and distance is at the same time being registered by two pens, attached to electric magnetic coils on the same cylinder. The time pen is actuated by a clock which makes electric contact and causes the pen to make a distinctive mark every half sec- ond, The "distance" pen is caused to make a distinctive mark for every 10 feet. The carriage travels, electric contact being made by means of a " trig- ger' projecting from the bottom of the carriage, and striking small blocks of wood placed at 10-foot intervals beside the railway. From the time and dis- tance diagrams the speed of the carriage is ascertained, and from this the cor- responding speed of the ship is deduced by Froude's law of comparison. The registered resistance of the model is also brought up to ship figures by Froude's law, which may be briefly stated as follows: " For similar forms, corres- ponding speeds are as the square root of the length and resistance at corres- ponding speeds are as the cube root of the length." Suppose the ship is twenty-five times the size of the model, then 4 knots for the model would correspond to 4 x ¥ 25 -- 20 knots for the ship ; and sup- pose the resistance of the model is 10 when going at 4 knots, then the resist- ance of the ship would be 10x (25)8 pounds: .'To ascertain the horse power required to propel the ship it is only necessary to find the number of foot- pounds per min., and divide by 33,000 thus: 20 knots = 20% 65080 = 2,026 2,026 x 10 x (25)8 eg O08: 0° ae = 9,600, about. This is in practice subject to two corrections, the first being for density of water. As the model experiments are made in fresh water, the displacement and resistance of the model are too small in the ratio of the density of fresh water to salt water, The second correction is what is known as the "surface friction" correction, and is due to the well known fact that the surface friction per square foot of a short vessel is high compared with the surface friction per square foot of a long vessel, as the friction of the water along the surface of a vessel gradually communicates a more and more forward motion to the water, so that in a long ship the after part of the vessel has very little surface friction, compared with the forward part. A great number "of special experiments were made with thin planes of various lengths, in order to get sufficient data to enable the "surface friction" correction to be made with accuracy. After the net horse power has been obtained in the way thus indicated, an allowance must be made for the friction of the engines and the efficiency of the propeller, and here again Messrs. Denny Bros. are in an exceptional position, for they have secured this data for about 200 ships of various kinds, of single screw, twin screw and paddle wheel steamers. Although the measurement of feet per min , and experiment tank, yet a number of other interesting and valuable experiments are 'made, We will first mention the measurement of the alteration in angle or trim of the model during the experiment. The model is not supported in any way by the dynamometer or any other part of the carriage, so that during the experiment the ends are free to rise and fall, and it is well known that at high speeds the bow of a ship rises and the stern falls. The amount of the rise or fall of the ends is measured by means of a self-recording apparatus at each end of the carriage, consisting of a vertical revolving cylinder, on which a pen, attached to a light rod resting on the model, is made to record the altered vertical height of the model. Experiments are made with model screw propellers with another dynomo- metive carriage which supports the screw shaft or shafts at any desired im- mersion, by means of thin, sharp knife blades depending from a swinging frame, held in one direction by a spring, which is deflected by the thrust of the screw or screws. The deflection is magnified and recorded on a revolving cylinder. The turning force is also automatically registered on the same cyl- inder by a very ingenious arrangement, so that the efficiency of the propellers can be ascertained. The revolutions, time and distance are also registered by pens which are moved by electric contact. The propeller carriage can be connected to the ship model carriage, so as.to make the screw or screws work in any desired position relatively to the model, or the screws can be tried sep- arately by removing the ship model, The resistance of the knife blades, which carry the shaft, and the friction of the driving spindles have to be measured by an independent test, in order to apply a proper correction to the results. Another important, though more recent development at Messrs. Denny's tank, has been the tests of efficiencies of paddle wheels of various designs. In 1887, Messrs. Denny contracted to build two high speed paddle steamers for the Belgian government, under most stringent conditions. The length was not to exceed 300 feet, the draught not to exceed 8 feet, 8 inches, the steamer was to be on level keel, and the speed was not to be less than 203 knots, Absolute rejection of the steamer was to follow failure in any of these conditions. Nearly a dozen different designs to fulfill the conditions were tried in the tank at the Dumbarton works. The difficulty of the problem also caused the build- ers to make experiments with model paddle wheels with such success that they were able to exceed the guaranteed speed by more than a half knot, the actual speed of each steamer being 21.1 knots. Since that time experiments have been made with a great number of model paddle wheels for other vessels, and asa result Messrs. Denny are now famed for building high speed paddle steamers which burn very little coal. The model paddle wheels are driven by a small electro-motor, to which long vertical flexible wires are led, the current being drawn from overhead wires by means of a fixed arm attached to the ex- perimental carriage. The thrust, turning force, and revolutions of the wheels are accurately measured in a somewhat similar manner to the method adopted for screw propellers, and by ringing the changes on size, number of floats, im- mersion, etc., the best design of wheel for a particular ship is ascertained. With a view to finding what is the actual immersion of the floats when running by the side of the model, the wave is very caréfully measured, and it may be mentioned that the waves created by a model in passing through the water, ex- actly correspond to the waves created by the ship at a corresponding speed. The law of comparison is based on this fact, which Messrs. Denny have on several oceasions tested for themselves, by measuring the waves of a ship on the measured mile and comparing them with those observed on the model. The agreement is wonderfully close. It can be readily understood that the cost of conducting--apart from the first cost--such a large and varied number of experiments is quite an important item, as it not only necessitates a staff of experimenters, but workmen are con- stantly employed moulding and finishing models, and making alterations and additions to the apparatus, and there is also a staff of young ladies who have been trained to analyze the results, plot the curves and make drawings and tracings. Altogether there is a staff of seventeen hands constantly employed at experimental tank work, At first this appears to be an expensive luxury for a private ship yard, but on examination it will be found to pay, for some- limes great Sums of money are saved by slightly altering the design of a ship, owing to the reduction in the size and power of the machinery, and if it pays the ship builder to make costly experiments of the nature described, how much more must it pay the ship owner to have an efficient ship, one that will burn very littlec al and give him more cargo cipacity ? Messrs. Denny deserve great credit for their long-sightedness in taking advantage of such an impor- tant invention. Personell of the Light-house Establishment. Special Correspondence to the MARINE REVIEW. Wasuineton, D. C., Aug. 17.--On account of numerous changes of late in the light-house establishment, a revised list of members of the light-house board inspectors and engineers of the different districts has been prepared for distribution under date of Aug 1, 1893. Following are names of members of the board with post-office addresses of each: Hon. John G. Carlisle, secretary of the treasury and ex-officio president of the board, treasury department, Washington, D. C.; Rear-Admiral James A. Greer, U.S. N., chairman, No. 2,010 Hillyer place, Washington, D. C.; Walter 8, Franklin, No. 24 East Mount Vernon place, Baltimore, Md.; Prof. Thomas C. Mendenhall, superin- tendent United States coast and geodetic survey, Washington, D. C.; Col. John M. Wilson, U.S, A., war department, Washington, D. C.; Capt. George Dewey, U.S. N., No. 1730 H street, N. W., Washington, D. C.; Mai. Henry M. Adams, corps of engineers, U. 8. A., war department, Washington, D. C.; Commander Robley D. Evans, U. S. N., naval secretary, No. 324 Indiana avenue, N. W., Washington, D. C.; Capt. Frederick A. Mahan, corps of engin- eers, U.S. A., engineer secretary, No. 1604 K street, N. W., Washington, D. C. The inspectors in the lake districts are: Ninth district, Commander John J. Brice, U.S. N., No. 1,808 Chamber of Commerce building, Chicago, IIl.; tenth district, Commander James G. Greer, U. 8S. N., Post Office building, Buffalo, N. Y.; eleventh district, Commande: William W. Meed, U.S. N., No. 80 Griswold street, Detroit, Mich. Major Milton B. Adams, U. 8. A., No. 18 _ Bagley street, Detroit, Mich., is the engineer in charge of the ninth and eleventh districts, while in the tenth district the engineering work is under the direction of Col. Jared A. Smith, U.S. A., Hickox building, Cleveland, O. The announcement of the appointment of a receiver for the Union Steam- boat Company, which operates the big steamers Owego and Chemung, was 4 surprise in marine circles, as the line, which is the lake adjunct of the Erie railway, was known tobe successful. It was explained later, however, that the move was made to stave off probable attachment on the property by creditors of the Erle, as that company owned the Union Steamboat Company stock.