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112 THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. [April 1, 1868. s ignal-box contained sixty-seven levers, of which thirty-seven worked s ignals, and thirty worked points. The signals locked the points and each other, so that no contradictory signals could be given ; nor could the permission for ingress to or egress from any platform be given, until the points were arranged in accordance with the signal for that par ticular platform. An idea of the duty performed by this apparatus, which was erected by Messrs. Saxbv and Farmer, might be formed from the fact, that seven hundred and seventy-five trains had passed under the signal bridge in a single working day. One morning, lately, thirty-five trains were signalled and passed in or out of the station in thirty-five minutes. Mr. Walker’s electric telegraph apparatus, which worked a miuiature semaphore distance-signal in each box, was used for signalling the trains on the block system. The cost of the works of the City Terminus Extension was £505,336, and of the whole Charing Cross Railway, including the ex tension, £1,160,118 or including land, somewhat more than three million sterling. In this sum, it was to be remembered, were included about 41 miles of railway for a double line, two large bridges over the river Thames, a considerable number of expensive street bridges, and two of the most extensive metropolitan termini. The importance of the traffic, which was not at present fully developed, might be gathered from the fact that, during the year ending the 1st of January 1868, being the first year since the City Terminus Extension Railway was opened, about eight million passengers used the Cannon Street Station, of which number about three million and a-half were local passengers ’ between Cannon Street and Charing Cross At the present time about twenty-six thousand passengers used the Cannon Street Station daily, and the South Eastern Railway now conveyed about fifteen million passengers annually. April, 1868.—The Paper read was “ On the Experimental De termination of the Strains on the Suspension Ties of a Bow-String Girder,” by Mr. W. Airy, Assoc. Inst. C.E. Although in this communication the case of a Bow-String Girder, as ordinarily constructed, only was considered, yet the Author believed that the principle, on which the strains had been ascertained, was equally applicable to all mechanical structures with complex bracing, as, for instance, station roofs, where the labour and uncertainty of a theoretical calculation rendered an experimental investigation ex ceedingly desirable. The model on which the experiments were made was composed of a bow of steel, and had a span of 6 feet with a rise of 1 foot; the string being constructed of two slips of oak, and the suspension ties being of steel wire, guage No. 6 (96 feet to the oz.) The process by which the tensions were ascertained was the follow ing : the ties, on being sounded, gave a good resonant musical note, and advantage was taken of this to compare the note of any string with that of a free string suspended in a frame, and cut off by a sliding bridge to the length of the string under comparison. The free string supported a small scale-pan, and this scale-pan was loaded with weights till the note of the free string and that of the string under comparison exactly- coincided. This was determined by ear with the greatest accuracy, the effect of i oz. in 80 oz. being clearly perceptible. The tension of the string on the girder was thus measured by the weight in the scale-pan of the free string; and this was done for every string in every case. The determination of the thrusts 'was arrived at by a differential process, thus :—A uniformity distributed weight was applied on the girder, and the tension of every string was taken ; then a travelling weight was introduced in addition, and hung at any one point, and the tension of every string was again taken: the difference of the tensions in the two cases of each string being regarded as the thrust, or tension, of that string produced by the travelling load. The reduced results of the experiments, comprising the effect of every possible arrangement of loads that could come on a girder, were given in diagrams. It was verified by experiment : 1°, that the ten sion, or thrust, of every string was proportional to the weight causing it; and 2°, that when several weights were applied at the same time, the effect on every string was that due to the sum of the effects which would be produced by each of the weights separately. The rules by which the strength of ties should be regulated, as de duced from the experiments, were, for an evenly distributed stationary load, that all the bars were in tenison, that the end uprights were most strained and the middle ones least, and that, with respect to the diagonals those were most strained which radiated outwards from the points where they met the string—the strain on each of which might be taken at one-half the load due to a bay—those that radiated in wards from the points where they met the string being strained to the extent of one-fourth the load due to a bay. In the case of a single moveable load, the uprights were liable to a tension of from 10-34 to l-6th of the weight, as the load advanced from either end towards the middle, and the greatest tension to which the diagonals were liable was one-fourth, and the greatest thrust two-thirteenths of the weight. LIQUID FUEL. By Benjamin H. Paul. The economy of fuel is a subject of so much importance in a variety of aspects, and it aflbrds so much scope for improve ment, that any suggestion made with that object is always deserving of full consideration; and, even if such suggestions should be impracticable or erroneous, it is at least worth while to demonstrate clearly the circumstances which may be con sidered as justifying an adverse opinion. That such a course is appropriate in regard to a project which is expected to involve a reconstruction of our navy and a radical revolution in steam navigation, will, I apprehend, be readily admitted. The proposal to substitute for the coal now used as fuel in steam vessels some kind of liquid combustible, is an off-shoot of the excitement which has prevailed during the last few years in regard to the discovery of vast quantities of petroleum in America; and it was that material which was in the first instance recommended as the substitute for coal. A commis sion appointed in America some years ago, to investigate the subject reported that petroleum was beyond doubt more than twice as effective as anthracite coal in the production of steam, and that steam could, by the use of this material, be produced in less than half the usual time. It was an inference by no means unnatural that if this were the case, and if coal could be superseded by this material as the fuel of steam vessels, a very great portion of the space required in merchant steamers for the stowage of coal would be rendered available for more profitable cargo; that steam packets might become independent of coal depots at various points of their passage; and that vessels of war would be enabled to keep the sea for a very much longer time than they now do with coal. Any prospect of such advantages as these being attainable might reasonably have been expected to justify a more thorough and searching investigation of this subject than it has yet received in this country. Besides petroleum, several other analogous materials have been proposed as substitutes for coal; for instance, the oil obtained by distilling particular kinds of coal, or the shale which occurs in coal formations, and more recently the oil known as “ dead oil,” which is one of the products obtained in rectifying the coal tar of gas works. All these materials resemble each other closely in being composed chiefly of carbon and hydrogen, which are, in various proportions, the combus tible and heat-producing constituents of all kinds of fuel. For the application of these materials and of liquid fuel generally, various methods have been proposed, but before speaking of them it is desirable to consider what is the evaporative power of these materials respectively, since that is a very important point to determine in regard to the question as to the relative merits of these different kinds of fuel. The heat generated by combustion has been made the subject of the most careful investigation; and since the time of Lavoisier, Laplace, and Rumford, the more precise measurement of the amounts of heat capable of being produced by the combustion of carbon and hydrogen, has been repeated by several physicists with results which agree so closely, that they may safely be regarded as well established. The names of Dulong, Despretz, Andrews, Favre, and Silbermann are, moreover, an unquestion able guarantee that these results, and the methods by which they were obtained, are perfectly trustworthy. According to these results, the maximum heat-producing capabilities of carbon and hydrogen are on the ratio of 1 to 4'5. The actual quantities of heat generated by the combustion of a pound of carbon or of hydrogen are as follows :— Relative Calorific Power. Pound. Heat units. Carbon .... 1 ... . 14,500 .... 1'00 Hydrogen ... 1 ... . 62,032 .... 4-28 The heat unit here referred to is the quantity of heat which raises the temperature of one pound of water one degree Fahr, (from 40 deg. to 41 deg.). Therefore, the numbers given in the table represent the quantities of water capable of being heated one degree Fahr, by the conversion of one pound of carbon into carbonic acid gas, or of one pound of hydrogen into water. As there arc, in the Fahrenheit thermometric scale, 180 degrees between the freezing point and boiling point of water, those numbers divided by 180 give the corresponding quantity of