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THRE MARINE RECORD. : ceviiee Tn Zi = to whom such figures as curves of stability, or righting levers, ere perhaps stranger than the Greek alphabet, and would ave been about as difficult for him to decipher as to show the kinetic energy of the cow that jumped over the moon, at the moment of impact with the earth, But yet, this man could and did produce time and again very superior models in all respects to his scientific confrere, the reason I opine was this, what we frequently hear spoken of as rule of thamb is really experimental knowledge, acquired quietly, sometimes unconsciously, but none the less real for all that. So this cwpenter had letrned by practice where to fine his models to make them hold their wind, and where to fill on them to make them safe to ran with, Another illustration—Let us take a merely practical old sailor, or marine engineer. I mean of the intellectual and thoughtful class which are to be found in every country. Ask such wen how many tons to the inch his vessel displaces, or how far apart the stays in the flat surface of his boiler must be spaced in order to secure a certain working pressure, the one will answer you correctly, within a ton, the other to within half aninch, that is, when they are quietly looking over the side, or smoking their pipe during their watch on deck, but place the same good men under governm:nt examination fo- a certificate, or worse sill, under examination by a shrewd admiralty lawyer, and see what bulls they will make, why to judge by their acquitment under such circumstances you would say, such men were not competent to navigate an elevator in a building, ora street car, which are certainly not the most dangerously rapid vehicles on this continent. ‘The mistake, even clever men frequently make, is trusting too much to figures without bringing good c mm on sense and eriticul thought to bear on tae result. You may prove the greatest absurdity by figures, so please look upon all extraordinary results shown merely by figures with sus- picion, and then your own good common sense, acquired by : pret experience, will often expose an error and reveal the truth, As an example of the disastrous consequences resulting from the adoption of inyentions of merely weoretical men I will briefly refer to one gigantic failure. The most perfect murine boiler (on psper) I ever saw was the “Purner Boiler.”’ The circulation in this boiler would have been perfect and complete. ‘Ihe stresses to which alone it was subject (according to theory) were perfect, all were tensional. Bat the most disastrous failures I have ever known in convection with patent marine machinery were caused by the adoption of this (in one respect) theoretically perfect cylindrical combination boiler, Again, in this case, there were not wanting rule of thumb engineers, who prophesied this failure, but such warnings were futile against experienced advocates, supplied with highly colored drawings, and still more highly colored cir- culars. It has been said, by the most eminent engineers of the day (and repeated by authors of engineering and shipbuilding works of more or less celebrity), that the principle upon which nearly all marine propeliers work, is the projection ot a mass of water in a direction opposite to that of the required motion of the vessel, Now, whether this be exactly correct or not, does not matter, I think, for the use I propose making of this axiom, and there is no doubt in the world, that the point is a debatable one, indeed, I may say that the assertion bristles with the variety of explanatioas surrounding the course taken by the water after contact with a revolving screw pro- peller, but though this be so, I will assume for the present that ihe water is so accommodating as to operate just as laid down at the commencement of this paragraph, for whether it be exactly so or not, wiil not materially effect the use to which I am pow putting this hypothesis, for I submit that there cannot be any thrust from a screw propeller, auy more than from a side-wheel, unless the water in immediate contact with the blades be projected astern, (when headway motion is the ob- ject of attainment) with a greater velocity than the surround- ing water beyond the periphery of the ‘“‘wheel’’ is being Jeft astern. There must be excess of pressure on the driving side of the wheel, to produce thrust, just as with a screw working in a solid nut, When you screw, the pressure is solely on the driving side of the thread. When unscrewing is to be ef- , feeted, the pressure will all be on the opposite side of the 1 thread, just as obtains with the ordinary marine engine cross— head. You know that all the pressure is on the head-way - guide when the engine is moving ahead, ard all on the back- : way guide when moving astern. Indeed, the action of the ; screw propeller (in water) may be similar to the action of the | piston in the compound cylinder, pressure one side, partial vacuum, the other, the result, work. I make bold to assert, speaking generally, that screw steam- = a ies tT, ers are almost inyariubly overserewed. ‘This anomaly is partly intentional and partly excuseable, but it is wholly indic- ative of want of confidence, It comes about scmewhat in this way. The centre of shaft has to be arraaged as one of the early steps in the design of a ; steamer, and even the most self: confident engineers, and celf- reliant nayal architects, appear to be overcome by the dread of being too low with this centre; hence, it is not uncommon to see screw propellers fitted on steamers, professedly from a few inches to a foot and more too large in diameter. As an example of the gain to be realized by haying screw propellers of proper dimensions and surfaces, | may cite the well known trials and changes effected in the “wheels”? of the “Tris?* By reducing the diameter of her screws from 18 feet 63 inches to 16 feet 33 inches, and incrensing the mean piten from 18 feet 2 inches 1o 19 feet 11} inches, a speed of 164 knots was attained, with 2,400 less indicated horse power, the saving in power being not less than 32 per cent. As a recent example, I may say, that one of the latest British Atlantic ‘Record Breakers’? has had her wheels reduced in diumeter from time to time, till now the diameters are 12 or 18 inches less than they originally were. The ‘Teutonic,’ as is well known, was over-screwed. The diameter of her ‘‘wheels’’ being 19 feet 6 inches, pitch 28 feet 6 inches, and the total disc area about 600 square feet, If a man wanted an excuse or required an apology for pre- ming to touch on screw propulsion, surely such indications the above of puerile and weak kneed designing, would sup- y the excuse and form the apology. ordinary cargo steamers the first thing we have to con- is the wetted skin, and in ordinary Lake steamers, the need to consider (to obtain our horse power) is To ascertain this the following formula will > B equals the extreme breadth, D equals the mean draft, L equals length onkeel. WhereB +2Dx Lx ‘96 = 40 + 532 x 290 x 9 equals 18,792, which is the square feet of wetted surface in one of our ordinary steamers, Now, to be safe for a ten knot average speed, we must allow one-horse power for every fifteen feet of wetted skin, thus: 18,792 +15 equals 1,253, and these steamers can steam fully 10 knots per hour with 1,250 H. P. Or further, if weallow one H. P. for every 3} tons of displacement upto the ordinary load line, this will also secure us asafe 10 knot speed. For the ratio of horse power to displacement in various steamers, I annex the table A. Tere is an ordinary lake steamer of 4,700 tons load displacement, und fully ten-knot speed, now 4,700, divided by 33 equals 1,253, which is the I. H. P, required, [Vo be Continued.| + ee AIDS TO NAVIGATION Congressman VY. A, Taylor while at Cleveland last week. stated that the measures for the benefit of the lakes were in very encouraging shape at Washington and says that most of the appropriations will be obtained without difficulty, Pres- ident M. A, Bradley, of the Lake Carriers’ Association, re- ceived the following telegram trom Secretary Keep, at Wash- ington, stating that ‘Senators McMillan and Sawyer, by hard fighting, induced the Senate Committee on Appropria tion to provide in the sundry civil bill for Bar Point light- ship; floating lights at Limekiln Crossing, Grassy Island, Grosse Isle and Mamajuda ranges; Upper St. Mary’s Range, and authority to use the Eleven Foot Shoal Lighthouse appro- priation for lightships.” With regard to the latter shoal, which is at thej entrance to Green Bay and close to the regular course of all E naba ore carriers, the owners of the latter recently started what seemed to be a successful moveme nt for the establish- ment of a private lightship there, to be paid for by boats in the Escanaba ore trade, but the passage of the sundry civil bill, with this provision, which is likely to occur within a month, will now happily render such a step unnecessary. $e 6 ca LAUNCH OF THE MAHONING. There was launched from the Wyandotte yards of the Dotroit Dry Dock Co, on Saturday afternoon the fiue steel freight steamer Mahoning, built to the order of the Anchor Line, E. T. Evans, manager, Buffalo, N. Y. The Mahoning is known as one of the trio of ‘straight backs” or sheerless steamers, and is 295 feet overall, 275 feet keel, 50 feet beam and 26 feetdeep. She will carry 2600 tons ona 15 feet draft and her average speed is estimated at 13 miles per hour. Her triple expansion engines are 20, 33, and 54 by 42 inch stroke, two Scotch type boilers 13 feet in length by 14 feet dia., tested for 160 pounds steam, and the propeller is 12} feet in dia. by 17 feet lead. She will be fitted with one spar forward and have the best cargo handling equipment devised for speedy work. Extra fine- ness has been given the Lull and the thirteen mile speed is no doubt less than she will be found capable of. The Messrs, Kirbys’, scrutinized by nearly the wh ole pop- ulation of Wyando.te, conducted the launch in their usual prompt and successful fashion and the Mahoning will be ready for business in about three weeks, re ee “A look at the lines will show that such a yacht as the Gloriana will actually shorten her water-line as she heels down on her powerful bilge, and she can only increase her length by increasing at the same time her displacement, which we knOw is theoretically and practically constant, whether sheis at anchor or at her highest speed, At anchor the yacht is larger and more powerful than the others of her class by virtue of the extended area of the loadwater line plane, the advantages of which have always been realized by successful designers—the Herreshoffs in particular, Looking atthe actual value of the long ends, apart from the peculiar form of the boat proper, the first point noticeable is that while they result naturally from the carrying out of the long fore and aft curves to their legitimate endings, a large part of each end might be cut off without loss of power; in fact, so far as speed in smooth water is concerned, and apart from all question ot appearance, the two ends might be cut off pluwnb with the water line, as in the scows to which the Gloriana has 80 often been compared.’’ ED Oe ee A MASTERS LICENSE REVOKED, The local inspectors at Detroit have conciuded their inves- tigation of the late collision between the steamer City of Mackinac and the tug Washburn in Detroit River, by which unfortunate occurrence John Hurley and W. F. Robinson were drowned, ‘The result is that the license of Capt. George Burns, whojsailed the Washburn, is revoked for gross careless- ness. ‘The finding has been approyed by Supervising Inspec- tor Westcott. There is some talk of an appeal being taken, but the opinion generally preyails that nothing of the kind will be done. em A river always appears more shallow than it really is, be- cause the light proceeding from the bottom of the river is re- fracted as it emerges out of the water, A river is ubout One- third deeper than it seems to be. [f, therefore, a river of clear water seems only four and one half feet deep it is actu~ ally six feet deep. any persons get outof their depth in bathing in consequence of this deception, Remember, a riyer is always deeper than it looks to be, Notes. Iv is reported that the only point of difference now remain- ing between the Senate and House Conference Committee on the River and Harbor Bill relates to proposed improvements in the new State of Washington, Capt, BLACKLAW, commander of the British steamship Hay Green, has been presented by King Oscar of Norway and Sweden with a valuable binocular glass encased in silver for heroism displayed in rescuing the crew of Norwegian brig Sophie, while bound to Philadelphia. On the inland waterways, which traverse the World’s Fair grounds from one end to another, there will be plying three kinds of boats for public use. There will be the omnibus, ex— press and cub boas or Iaunches. The omnibue boats will make regular trips around tha waterways, stopping at each building. The express boats will make round trips without stopping, while the cab boats, with carrying cap»city of four persons, may be hailed at any point aud engaged for the trip or by the hour, as is a hansom cab. On the Clyde during last month twenty seven vessels were launched, representing a tonnage of 37,000. The most notable works on hand are a crniscr of 7,700 tons, three vessels of 2,500 tons each, and several other vessels. Juver Netson, of the U. S. Court, at Boston, has ordered a verdict of $17,300 to be entered in favor of the schooner Luey James against the steamship City of Savannah. The Court decided the officers in charge of the steamers were ineompe- tent and inattentive to their duties, and were greatly negligent or ignorant of their duties or both in their calculations of dis— tance on approaching the schooner, THE progress of shipbuilding from earliest times up to the present will be shown by a very extensive exhibit at the World’s Pair by Laird Bros., the big English shipbuilding firm at Birkenhead. The firm’s exhibit of like character at the recent English naysl exhibition attrac.ed 2 great deal of attention, Tue mayor of Philadelphia has received from the State 'De~ partment at Washington, a solid gold goblet which the Em- peror of Russia desires presented to Capt. Sargent, of the steamer Indiana, in rememberance of the part taken by him in carrying food supplies to the Russian Famine sufferers, —_—__—ann ce THE RISE AND FALL OF TIDES, 3The recent singular rise and fall experienced in the level of Lake Superior from Sault Ste. Marie to Duluth, or more es- pecially at each extremity, is phenomenal, In glancing at the phenomena in other parts of the world it is found that in— “Chepstow, in the Bristol Channel; Mont, St. Michael, in the Gulf of St. Malo; Dungeness Spit, near Cape Virgins; and the Basin of Mines, at the head of the Bay of Fundy,— are all places celebrated for a great tidal rise and fall, which in exterme cases amounts at some of them to 70 feet and up wards, “On the other hand, in the open ocean, where the tide wave is untrammelled, the range is but four feet or so, and in in inland seas it is almost insensible. For instance, among the islands of the Pacific Ocean the rise and fall varies from 3 to 6 feet, and in the Mediterranean the average rise and fall does not excced 18 inches, though in places—Sphax for exam- ple—owing to local causes, the rise and fall is full five feet. ‘Lakes and inland seas being comparatively small, the at- traction of the sun and moon is neurly equal at both extremi- ties, therefore their tides are insignificant. Close investiga- tion backs up the theory that the magnitude ef the tidal range depends upon the proportion the size of the lake or sea bears to the diameter of the earth: for instance, the existence of a tide in Lake Michigau has been proyed by a series of ob servations made at Chicago in 1859. The average height of this tide is 1} inches; and the average time of H. W. is 30 minutes after the moon’s transit. The length of Lake Michi- gan is 350 miles, or 1-23 of the earth’s diameter; and its tide is about 1-23 of that which prevails in mid-ocean, Again, the length of the Mediterranean is 2,400 miles, or, roughly, } the diameter of the earth, which gives the average height of its tide as } what it is in the open sea, and this is confirmed hy ooservation, “The tide current, then, is caused by tide waves from the ocean being concentrated and cheeked by local formation, also by the frictional resistance offered by the bottom and sides of a narrow channel. In passing through contracted spaces, these wayes, as already stated, ure heaped up und urged on by the continued pressure of the water be- hind, whose motion is less retarded than that of their own; and thus, in seeking to find its level, an actual current is ere~ ated, “The tide current must not, however, be mixed up with the general ocean currents, which are progressiye movements of the water, due partly to prevailing winds, and partly to dif- ferences of temperature and density, which, by disturbing the equilibrium, cause a constant circulation to be going on in the — waters of the globe; and this, be it remembered, takes p! in a vertical as well as a horizontal direction,” Po ee Tue John Pridgeon was the first yessel to pass 0 tire length of the new straight channel, at ‘Lol stakes are yet up, and vessels canndt use for the present.