square or circular facets equally distributed over the surface of the tool, as shown in Fig. 8. This is done to expedite the polishing. When the polish is brought up to the best (the best polish is no doubt the finest scratching we are able to do) the glass is allowed to come to a normal temperature, and is then studied by the admirable methods devised by M. Foucault for curved, and by Steinheil and Dr. C. S. Hastings for flat surfaces. Very seldom are the surfaces found free from defect. In order to clearly understand the method which I use for the correction of errors in producing a regular curve, let us take the former case of Fig. 1, where the Foucault test shows a decided over correction or hyperboloid of revolution on the concave surface. The zone a is to be depressed and at the same time new errors, especially zonal errors, are to be avoided. The iron tool, which is of the same diameter as the surface to be worked, is laid off into six points diametrically opposite with the dividers set to the radius of the tool; as in Fig. 3. The tool is now warmed and the pitch is spread over the leaflike spaces, which are given the proper curve by being pressed down on the (previously wetted) concave surface. The pitch and tool are now cooled quickly by an abundant flow of water. In the shaping of this leaflet lies the whole secret of success. The zone, a, Fig. 1, needing the greatest amount of abrasion, the leaflet is made widest at that point, but in order that no zonal errors may be introduced, as in Fig. 2, it is gently tapered in each direction, the amount of taper being somewhat governed by the amount of lateral stroke given to the polisher, as well as the amount of departure of the zone from the normal curve. After the proper shape is given to the correcting or figuring tool, the pitch is again slightly warmed, pressed on the wetted surface, laid aside for an hour or so, and the work of correcting or figuring is then begun. When the polisher has worked long enough to transfer its own peculiarities to the surface under treatment, the glass is allowed to come to a normal temperature and again tested. If any change in the shape of the leaflets is needed, an inspection of the surface will indicate the character of the change required. Cooper Key many years ago graduated the square facets of his polisher. Elliptical, ring and other forms of polishers have been tried from time to time with varying success, and I have myself tried many forms, but with none have I had such uniform success as with the form which I have just described. It has all the ad vantages of a local polisher without its defects, and as these leaflets can be so readily shaped, and so easily manipulated, we have a ready means of giving any desired form to the optical surface we are manipulating. Figs. 4, 5 and 6 show the various forms of these polishers which are designed to correct different forms of errors. Fig. 7 shows a polishing or figuring tool which will give fine results, when time is not an element in the work. Such a polisher would break down almost any form of irregular surface, and give a regular curve, the kind of curve - oblate spheroid, spherical, elliptical, paraboloid or hyperboloid, depending on the length of lateral motion given to the polisher; indeed almost any idiosyncrasy which a curve may present can be successfully treated with a slight modification of this form of polisher. Flat surfaces may also be treated by modifications of the same general form of tool, and overworking the edge zone, so difficult to avoid in hand polishing, can be readily and easily overcome. It is beyond the limits of this paper to discuss the various difficulties which the practical optician has to deal with besides those noted; but I would mention one thing that seems to be an insurmountable barrier to the production of an ideal optical surface, in the lack of homogeneousness in material. It is a fact well known to every one who has to deal with minute measurements that no two pieces of glass, speculum metal or other optical material made by artificial means are ever absolutely homogeneous when they come into the hands of the optician; hence every piece of material must have its special study, and in many cases idiosyncrasies present themselves which say "Thus far shalt thou come, but no farther." If, in this brief paper, I have said anything that will add to the interest of this study, intimately associated with the names of Newton, Herschel, Ross, Lassell, Foucault, Nasmyth, Dr. Draper and many eminent opticians of to-day, I shall feel more than repaid for my work. THE GIANT'S CAUSEWAY AND PORTRUSH ELECTRIC TRAMWAY (IRELAND). By WILLIAM A. TRAILL, C. E., Portrush, Co. Antrim, Ireland. [ABSTRACT.1] THIS road has a finished length of seven miles and a gauge of three feet. It follows the turnpike which skirts the cliffs over1 Abstract made from the full paper by J. Burkitt Webb, Secretary of the Section. looking the sea, and is built upon a sort of sidewalk raised a few inches above the road and separated from it by a curbstone. Forty-two lb. Bessemer steel flanged rails are used for the most of the line and answer also for the return conductor, the main conductor being a 19-lb. T rail placed at the side of the road about 18 in. above the ground. The alignment is an unfavorable one, having curves with a minimum radius of 42 feet and grades of 1 in 50 and even 1 in 25, and some of the worst curves are on heavy grades. The road rises in places to 185 feet above the sea. At present the cars stop at Bushmills, two miles short of the Causeway and six miles from the town of Portrush. A single track is used, with passing places about a mile apart, and these are always on inclines, so that one car is run by gravity while passing the other. The current is taken from the three-inch-broad top of the T rail by springs, which rub along on it, and as there are numerous openings, generally 10 ft. wide, in this rail, to allow the turnpike travel to pass, each car has two springs far enough apart to reach across such openings. The cars are run by D, or D, Siemens motors, running respectively, 850 and 1200 revolutions per minute, and these speeds are reduced by cog wheels and chain gearing connected with the car axles; an average speed of about twelve miles an hour is attained, and a second car is generally drawn behind the electric car. The electricity is generated on the Bush river, three-quarters of a mile from Bushmills, and is carried the whole length of the line with but a slight loss. Two American turbines (Alcott's) of 50 H. P. each are used, one of them, with its gates half to three-quarters open, being sufficient for ordinary traffic. The dynamo used is a SD Siemens" compound wound” weighing 35 cwt., and furnishing a current up to 100 ampères with a maximum potential of 250 volts. This maximum potential is fixed by law so that no dangerous shock can be received by contact with the rails. The road was opened in the fall of 1883, and has been in successful operation since that time; thus the Lichterfelde and the Charlottenburg railways, respectively one and a quarter and one and three-quarter miles long, constructed by Siemens Bros. in Berlin, antedate this railway by a year or two. In the first of these the two rails are used for the direct and return current, with much leakage of current; in the second, two conductors are carried along the road on telegraph poles from which connection is made to the car by a flexible running conductor. The greatest difficulty experienced so far has been in the attempt to regulate the flow of water to the wheel by an automatic governor, the trouble being that the stoppage of a car throws off instantly 80 to 85 per cent of the work from the turbine, so that an immediate corresponding decrease of the water supply must be made; on the contrary, in starting the car the water must be turned on again gradually. When it is added that Sir Wm. Thompson now occupies a position in the board of managers in place of the late Sir William Siemens, it will be seen that the electrical arrangements of this road are and have been in the best of hands. The following details may be of interest. Two loaded cars upon the worst gradient require a current of 60 ampères, the maximum potential being 250 volts. The resistance of conductors from the generating station to Portrush and back is less than 2 ohms, while the resistance of the insulation ("insulite") of the conductors varies from 500 to 1000 ohms, according to the weather. The total leakage along the line may be as much as 2.5 ampères, or ₫ H. P. In eight months after the opening, 13000 "electric-car miles” were run, the daily run in summer being 100 car miles with 600 to 800 passengers. For nearly a year previous to Nov., 1883, steam cars were used on the line so that a reliable comparison between the cost of running by electricity and by steam can be made. The cost of running by steam, including the wages of attendants, coke, oil, etc., was ten cents per mile run, while with electricity this was reduced to less than four, and with such additional traffic as could easily be accommodated with the existing plant it would not be over two and one-half cents. Experience also warrants the prediction that "on a more favorably circumstanced tramway, with a service of single cars, running at short intervals, the working expenses might be reduced tod (one cent) per mile run, and that even though the electricity should have to be generated by steam power." It would seem from an inspection of the detailed expenses of running, that, owing to the higher wages paid here, the comparison, while leaving a large margin in favor of electricity, would not be so overwhelmingly against steam except in localities where abundant water power is available. The cost of the line has been $150,000, as follows: "parlia mentary law, engineering and preliminary, $15,000; construction of six miles of tramway, one mile of branch and sidings, buildings, carriage and engine sheds, $80,000; water-power generating station turbines, electric conductors and cables, $22,500; electric plant, generating dynamos and motors, $10,000; rolling stock; two tramway engines, nine tramway cars, fourteen wagons and mineral trucks, $22,500." The road is understood now to be not only in successful and continuous working, but in a paying condition, which recommends it to the careful study of those interested in such matters in the United States. ELECTRIC TRAMWAYS. By M. HOLROYD SMITH, Fern Hill, Halifax, England.1 [ABSTRACT.] To demonstrate the practicability of Mr. Smith's system of electric tramways an experimental line has been laid in a field near the works of Messrs. Smith, Baker & Co., Manchester, England. The track is 110 yards long and 4' 8" gauge; it is essentially level and the car is driven by the traction of the ordinary wheels, but it contains some sharp curves. On the latter account one wheel on each axle is loose and the power is distributed between the two wheels on the axle by differential bevel gears, so that both wheels drive equally well on curves and straight line. The movable bevel gear in this arrangement is driven by a steel chain from a Siemens motor and the current is brought up from an underground conduit, which forms the main feature of the line. This conduit is constructed in much the same manner as a cable line, with a copper conductor in place of the cable. This copper conductor is double and consists of two half tubes,—as if a tube had been cut in two lengthwise and vertically and the two halves separated somewhat from each other. In this tube, then, a shoe slides, being connected both electrically and by leather straps with the car, the leather straps breaking should the shoe be accidentally obstructed, as by pebbles jammed in the groove. The rubbing part of the shoe, or shuttle, consists of spirally-grooved rollers, whose axis is that of the tube, and whose slow revolution, caused by friction, keeps the tube cleaned out perfectly. The rails answer 1 Mr. Smith not being present the paper was read by Mr. Preece. Abstract by the Secretary. |