wheels; the vertical height of P by the scale on the right hand multiplied by 240 gives the total pressure in pounds on the four blocks. F is the line showing the retarding effect of the four blocks upon the one pair of wheels before the wheels began to slide upon the rails; and ƒ shows the effect while the wheels were sliding upon the rails. The vertical height of F or f, according to scale B, multiplied by 60, gives the retardation in pounds. It will be seen that the stop was made in half the time with the wheels braked but not skidded of that required when the wheels were skidded. The accompanying Diagram 3 shows in another way the comparative retarding effect of the brakes when acting on the revolving wheels and when applied with sufficient force to skid the wheels. This experiment was made by keeping the van at a uniform speed on a rising gradient of 1 in 264-the line T shows the strain on the draw-bar during the experiment. The line s shows the speed of revolution of the braked wheels, when the revolution was checked and the friction diminished as shown by the line f; the strain, T, on the draw-bar diminished in a corresponding ratio. From this it is evident that the retardation which arises when the wheel is sliding on the rail is far less than the retardation produced by the effect of the brake blocks when applied to the wheels so as to allow the wheels to continue revolving. In order to understand this it is necessary to consider the general action of railway brakes. When a train is 3r 150. 1.10. 130 120 110 1005 80 90 70. 80 60 70. 50 60 40 50 30 40. 20 f 35 N: 4. Seconds 04 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 27 28 29 30 moving at a given velocity the adhesion of the wheels on the rails causes them to revolve; every point on the surface of the tyre moves round at the same rate as that at which the train itself is moving forward; but every such point in relation to the forward movement of the train comes successively to rest at the moment when it comes in contact with the rail. Now when the brake is applied with a slight pressure only, the wheel continues to move round at the same rate as that at which the train is moving, but it moves with more difficulty, and this increased difficulty in moving is shown either by an increase in the tractive force required to keep up the forward motion, or, in cases where the accelerating force is not kept up, by the tendency of the moving mass to come to rest in a shorter time than would otherwise be the case. But if the pressure with which the brake is applied be increased, a point is reached when the friction between the brake block and the wheel first approaches, then equals, and finally exceeds, the adhesion of the wheel on the rail. When this happens, the wheel first begins to revolve more slowly, and then ceases to revolve and slides along the rail, or, as it is usually termed, is skidded. The retardation is then no longer due to the friction between the brake block and the tyre of the wheel; but the vehicle is transformed for the time from a vehicle on wheels into a sledge, and the retardation is due to the excess of resistance which is produced by making the vehicle slide along the rails over that produced by making the vehicle move forward on wheels revolving freely. The reason why the retardation caused by the brake blocks applied to revolving wheels exceeds that caused by the skidded wheels became obvious from the fact next discovered, viz., that the coefficient of friction between the brake blocks and the wheels varied inversely according to the speed of the train, a higher proportionate percentage of brake-block pressure being required to obtain a given amount of friction at high speeds, and a lower pressure at lower speeds. This is illustrated by the Diagrams 4, 5. In these diagrams P represents pres N° 5. same at the same speed, some simple rule might be deduced which would give the pressure required at each speed for obtaining a certain amount of retardation; but when the speed of the van was kept nearly uniform by the effort of the engine, the friction of the blocks decreased; and this occurred notwithstanding a continued increase of the brake-block pressure: showing that, through some cause not yet fully determined, the holding-power of brake-blocks at all speeds is considerably less after some seconds of application than when first applied. This peculiarity is illustrated by Diagram 6, and is also 120 110 400 90 80 70 60 50 40. 30 20 40 5 40 55 30 20 Seconds 0 1 2 3 4 5 6 7 8 9 15% Velocity. Coefficient of friction. Miles per Feet per Extremes Mean. *179 *083 '127 59 *194 088 '140 51 44 20 361 29 22 143 28 73 II 20 Under 5 *197 *087 *142 166 Diagram 7 shows the curves of this decrease obtained from a few of the experiments. It would seem as if the coefficient of friction due to each speed becomes nearly uniform after a certain number of seconds have elapsed. The experiments were, however, necessarily limited to something between twenty and thirty seconds each, so that this point has not been fully determined. The decrease in the coefficient of friction, arising from time sometimes overcomes the increase in the coefficient of friction arising from a decrease in speed; especially when, either from the stop being on a descending gradient or from a small proportion of the train only being fitted with brake power, the train takes considerable time in coming to rest. Therefore, a higher brake pressure is required in such cases than when the stop is made in a short time. The accompanying diagram (8) shows a uniform force of friction with a practically uniform speed, as ob tained by means of an increasing brake-block pressure. If the position of the brake-blocks were always the The line P, shows the pressure, F the friction, and s the portion of the lower body, until it reached its summit at a'; from this moment it would begin to descend the next incline, from a to b; provided the force, P, acting in the direction of the arrow, S, would leave it time to do so, the incline from b to c would have to be mounted next, causing a certain amount of resistance during the time the body traversed the distance dc. But if we increase the speed in the direction of the dart s, so that the body will require less time to traverse a'd than to fall through db, in such case a' would not arrive at c, but at some other point, b', and then only the portion of the 5 7 9 11 13 15 17 13 21 23 25 27 29 31 33 35 210 200 480 180 470 170 460 160 450 460 440 440 430 -130 120 -120 410 100 -100 -090 090 The fact that the coefficient of friction diminishes with speed sufficiently explains why a skidded wheel affords less resistance than one which still rotates, because the resistance occasioned by the rotating wheel is only limited by the adhesion of the wheel on the rail, and this, as already shown, is the same as static friction, since the point of the wheel is stationary as regards the forward movement of the train at the moment it touches the rail; whilst when the wheel is skidded and slides, the friction is that due to the speed at which the wheel moves on the rail, and is therefore less than the other. DOUGLAS GALTON (To be continued.) GEOGRAPHICAL NOTES ON Monday the French Geographical Society held an extraordinary meeting, in the large hall of the Sorbonne, for the reception of Major Serpa Pinto, the African explorer. We understand that Major Serpa Pinto and Lieut. Lucien N. B. Wyse have both promised to attend the coming meeting of the British Association at Sheffield, and will give accounts of their recent explorations in Africa and the American Isthmus. Some other very interesting papers, we hear, are in preparation for the geographical section, over which Mr. Clements R. Markham will preside. THE principal novelty in this month's Petermann's Mittheilungen is an elaborate paper "On the Geographical Distribution of some Plague Epidemics," by Dr. Carl Martin. Dr. Emin Bey has a short paper "On the River Obstructions of the Bahr el Jebel," and we regret to say, Dr. Gerhard Rohlfs writes from Bengazi on June 10, that he has resigned the leadership of the expedition of the German African Society, which was organised for the purpose of reaching the Congo by starting from Tripoli. Dr. Rohlfs gives as his chief reason for resigning, the length of time the expedition is likely to last, and the value of even a single year at his age. He has, however, done his best to remove all difficulties from the way of the expedition in setting out, and these have not been few. He proposes Dr. Stecker to succeed him, and hopes the This simple illustration is taken from an article in the Chicago Railway Gazette, by M. Krajewski. Society will approve. Though Dr. Rohlfs' resignation is to be regretted, he cannot be blamed, and we trust the expedition will be able to carry out its original programme. THE French Government will present to their Parliament a bill for taking preliminary steps in order to establish a Soudan railway from Algiers to Senegal, vid Timbuktu. An official commission has been appointed to report on that subject. THE Engineering and Mining Journal states :-At a late meeting of the New York Academy of Sciences, Prof. Arnold Guyot, of Princeton College, presented before the Geological Section a paper upon "The Topography of the Catskills," containing the results of several years of study of these mountains, and which he is about to publish. From the contents of this interesting paper, it will appear, that this region of country-in the midst of the oldest settlements, and long celebrated as a summer resort-has remained comparatively an unknown wilderness, even to this day; for Prof. Guyot, within the past few summers, has actually discovered and named an extensive group of mountains rising into peaks in some cases over 4,000 feet high (the "Southern Catskills," or "Shandakeens"), which are not laid down on any map, or described in any gazetteer. These works of reference refer to this region as "a hilly country," merely, and the fact that it contains mountains higher than the true Catskills, is quite new to science, and it has been reserved to Prof. Guyot to make an interesting geographical discovery in the very heart of the State of New York. For the paper in extenso our readers are referred to the Proceedings of the Academy, and for a fuller abstract, to the American Naturalist for July, to which we are indebted for the brief notice here given. CAPT. JAMES B. EADS, who is constructing the jetties to deepen the channel at the mouth of the Mississippi River, has written a letter to the New York Tribune, in which he proposes to substitute for the contemplated ship canal across the Isthmus of Darien a railway by which the largest vessels may be conveyed across in twenty-four hours. This project he claims to be entirely practicable, and says it would cost considerably less than the canal, and might be completed in three or four years. The ship could be raised by a lock and the usual hydraulic methods, and he suggests two methods that are practicable, and with precautions to prevent straining. He recommends turn-tables instead of curves in the railway where changes of direction are necessary. The car, or cradle, to carry the ship should be built in sections, each about 100 feet long, and each section supported by about 200 wheels, some of them driving wheels moved by engines. The weight of the largest merchant steamers and their cargoes would not exceed 10,000 tons. Such a vessel Capt. Eads would place on five of these sections, supported by 1,000 wheels bearing on eight or ten rails, so that each wheel would support about 12 tons. thinks his plan entirely practicable, and urges it very strongly. Indeed the scheme adopted at the recent conference for an interoceanic canal meets with no favour in America, and Mr. Troutwine thinks it will never be finished, the difficulties are so great. Perhaps national, as much as engineering reasons, influence American opinion on the subject. He DR. H. A. A. NICHOLLS, who, we believe, is SurveyorGeneral of Dominica, has addressed to the Colonies and India some notes of considerable interest on that little. known island and its boiling lake. In many parts, he tells us, fine undulating uplands extend from the heads of the valleys far into the interior, and one runs across the broadest part of the island at an elevation of 800 feet above the sea, containing many thousands of acres of fine, well-watered land, with a virgin forest of lofty tin 1 trees. The chief mountain peak reaches a heig 4,747 feet. Only a small portion of the island is cultivated, but of the rest, which is covered with the primeval forest, large tracts are suited for the growing of coffee, cocoa, spices, limes, and other tropical products. The Boiling Lake, which is at an elevation of 2,425 feet above the sea, has been visited on three occasions by Dr. Nicholls, who, on his second visit, ascertained that the temperature at the edge was 180° F., gradually increasing towards the centre. The lake was first seen in recent times by an exploring party organised and led by Mr. Watts, a colonial magistrate, and Dr. Nicholls. They thought they were its discoverers, but it has been found that the volcano is mentioned in a very rare medical work published in 1797. Prof. Georg GERLAND concludes in the current num ber of Globus a long and elaborate examination into the future of the American Indians. The conclusion he comes to is that facts do not warrant the inference that the Indians are dying out, nor that they have been deleteriously affected by contact with civilisation. IN the course of this month a highly interesting geographical work will be published by Karl Graeser, of Vienna, by order of the Austrian Minister for Education. Its author is Prof. Friedrich Umlauf, and its title "Wanderungen durch die oesterreichisch-ungarische Monarchie; landschaftliche Characterbilder in ihrer geographischen und geschichtlichen Bedeutung." Geography will take place at Brussels from September 27 THE second International Congress for Commercial to October I next. It will be divided into five sections; the first will consider commercial routes and exploring expeditions, the second natural and artificial products, the third and fourth questions relating to emigration, colonisavoted to the discussion of general questions. tion, and instruction, while the fifth section will be de HERR CARL BOCH, who has now finished his natural history exploration of the western highlands of Sumatra, is about to explore, on behalf of the Dutch Government, the north-eastern part of Borneo-the district of Koetai. There is a powerful and friendly Sultan at Koetai, who has been requested by the Dutch Government to give all possible assistance to Herr Boch. ONE of the newest of French geographical societies, that of Montpellier, which has assumed the title of Société Languedocienne de Géographie, now publishes a Bulletin about every two months, which for size is imposing enough, for the last number runs to 180 pages. It contains, among other matter, observations on the creation of an inland sea in the Eastern Sahara, and papers on Natal, the Transvaal, and Zululand, and on the River Ogowé, as well as a summary of M. Soleillet's account of his recent attempt to reach Timbuktu, við Ségou. canals connecting the Caspian Sea with the Black Sea is A NEW project for the construction of a system of now being considered by the Russian Government, and is discussed in a recent number of the Journal of the Russian Imperial Office for Public Works of Communication. The author of the new project is the engineer, M. A. Daniloff. He proposes to construct (1) a canal of some 300 versts in length, from the River Terek to the water-shed of the River Manytsch, which connects the Don with the Caspian Sea, but the bed of which is generally dry; (2) a canal of about 320 versts, from the mouth of the River Kalans (a tributary to the Manytsch), eastward to the Wolga, near Astrachan; (3) a canal from the same spot, westward to the Don (about 350 versts); (4) a branch from the eastward canal to the Serebriakowskaya Station on the Caspian Sea; (5) a branch from the westerly canal to the Black Sea. Other Russian news states that the Government has commanded the Khan of Khiva to furnish 5,000 workmen for the works connected with ecting the Oxus River into the Caspian Sea. |