drives the engine in the opposite direc- studs or segments of Fig. 82. B is a tion. In the first instance resistance is cylinder fixed to the base J and open at introduced in the main circuit. In the the top. Closely fitting in cylinder B is second instance resistance is removed the cylinder A, open at the bottom. from the circuit. Supported by a tube passing through Imperfections due to the oxidization the base, and closely fitted in cylinder A, of contact points in the electric engine, is the piston C. Insulated upon one and the occasional failure of the engine side of cylinder B is a stationary contact to respond to changes in its circuit, led plate E, to which one pole of the generto the abandonment of what was other- ator is attached. Insulated upon the wise a successful regulator, and the sub- opposite side of the cylinder are the 18 stitution therefor of hydraulic-cylinder and piston mechanism, constituting the regulator shown in Fig. 84, which is also from a photograph. The internal arrangements of the regulator are shown in Fig. 85, in which J is a hollow metal base containing the various resistances connected with the 18 pieces F, corresponding to the 10 grooved central plates F. Running in the grooves of E and F are two contact arms, G and H, whose pressure upon E and F is regulated by spring I. The current, therefore, passes by way of plate E to arms H and G and a contact plate F outwardly, as shown in the regulator connections, Fig. 82. The ends of the cores of an electro magnet, K, provided with an armature lever, L, pass through pounds, it obviously falls with the same the piston head, and serve to open and force. With a piston area of 80 square close the valves NM. When the magnet inches and 20 pounds pressure of water, K is not energized, a coiled spring a circuit is operated, with singularly forces the armature away, and valve N is rapid changes in the direction of moveopened to the flow of a stream of water, ment of the cylinder, with an upward and valve M is closed against its escape force in the cylinder of 1,600 pounds. into the lower chamber and outwardly There is apparently no limit to the through the exit tube O. Therefore the powerful effects thus producible by cylinder A rises. When the magnet is minute changes in the strength of curenergized, the valve N is closed and the rent supplied to a system of lamps. By ingress of water stopped, while valve M means of these regulators, the changes is opened and the water contained in the in the circuit occasioned by the Sawyer upper chamber is allowed to escape. switches for graduating the light are The cylinder A then falls by its own instantly balanced; but the fact remains weight as rapidly as the escaping water that as much power is expended in drivpermits. By a suitable arrangement the ing the generator when there are a few waste water is used to cool the various as when there are many lamps in circuit, artificial resistances. The local circuit and in a general distributing system, of the magnet K is established by the where economy is the prime consideraback contact of the relay lever. The tion, such regulators, however perfect in movements of the cylinder A are smooth their operation, can have no practical and the changes rapid. The contacts of application." G and H with F and E are of a firm and substantial character. With a pressure of 10 pounds of water and a piston area of 10 square inches, the cylinder A is raised above its weight with a force of 50 pounds; and as its weight is 50 Quite as valuable as the above is the author's presentation of the subjects of "General Distribution" and "The Commercial Aspects." But for these topics. we must refer the reader to the book itself. STREET CARRIAGEWAY PAVEMENTS. By GEORGE FREDERICK DEACON, M. Inst. C. E. tabulate its results in a manner which, if As a branch of civil engineering street paving has never occupied a very high place. It is true that, when once the conditions of its maximum economy and efficiency are ascertained, exceptional skill is not required to put them into practice; but those conditions have The present paper will be confined to rarely received at the hands of engineers street carriageways, and will exclude the the consideration in detail which they consideration of such roads as form lines demand; and when the very high degree of communication between centers of is duly appreciated, in which the welfare population, or between scattered rural of the community is influenced by the dwellings and market towns, railway method of paving adopted, it must be termini, or harbors. The former diviconceded that the divergent opinions sion of the subject of road making offers and practice at present extant should at abundant matter for consideration in a once be formulated and fairly compared. single paper, and the latter division has The proceedings of the institution have been ably dealt with by Mr. W. H. contained no discussion on the subject Wheeler, M. Inst. C. E. The details and for many years. The author therefore cost of construction of certain pavements proposes to state his practice, and to will be given, and their relative ad vantages will then be generally con- without much loss of time in selecting sidered. each set as it is placed. Two or more longitudinal joints in nearly the same CONSTRUCTION. CARRIAGEWAYS OF line, or a single rather wide longitudinal HAMMER-DRESSED STONE, OR SET joint, almost invariably initiates I. PAVEMENTS. groove. a Figure of Sets.-Sets when properly The square form which, for stability, secured in position and formed of the would otherwise have been the best, has hardest stones are so durable, even under therefore been altered in the author's the heaviest traffic, that it is not worth practice to an oblong of the width above while to make them more than from 6 to stated, and of a length varying from 5 to 7 inches deep. Two depths are em- 7 inches, which, when the set is fixed in ployed in Liverpool: the first for moder- position in the manner about to be ate traffic, averaging 6 inches; and the described, is not found to be too great in second for the heaviest traffic, averaging relation to the depth to secure stability. 7 inches. In relation to its other These conclusions may be summarized dimensions, a set should be of such as follows: The FOR FOOTHOLD. Depth. Width. Sets for moder- Length. in. in. 4 to 14 5 to 7 4 to 14 57 ESSARY FOR FOOTHOLD. Sets for light Depth. Width. Length. in. in. 3 to 14 5 to 7 depth that when the wheel of a heavy HARD STONE, IN WHICH JOINTS ARE NECESSARY cart passes over one edge of its upper surface it should not tend to tip. resultant direction of pressure of the load and adjoining sets should, in short, always tend to depress the whole set vertically. Where, as frequently happens, this precaution is neglected, the maintenance of a uniform surface is im- SOFTER STONES, IN WHICH JOINTS ARE UNNEC possible. This consideration would lead per se, for sets of the above-mentioned depth, to a surface of 4 or 5 inches square. Sets of this figure remain comparatively stable, even on bad foundations and with defective joints. There are other influences, however, which lead to a modification of this form. It is desirable that the width of each set measured along the street should be small. If it is large, a horse drawing a heavy load attempting to find a joint slips back, and requires an exceptionally wide joint to pull him up. The wide joint induces increased wear of the upper corners of the sets, and increased noise. traffic or in- Materials of Sets.-Among the igneous, plutonic, and metamorphic rocks are to be found a great variety of stones suitable for paving, of all degrees of hardness, brittleness, and toughness; but the durability under traffic, the tendency to wear round rather than flat and to become slippery, are dependent upon no conditions which can readily be foreseen. The size and distribution of the crystals, In practice, the author has found that, the nature and hardness of the constitufor all but the softer stones, upon which, ent minerals, operate in such a manner even if there were no joints, horses that two opposite combinations often could travel with facility, it is desirable produce nearly the same result; while that the widths of the sets should be rocks of nearly equal hardness frequentsuch that four taken at random, and ly behave very differently under traffic. placed side by side, will not measure The specific gravity, moreover, appears more than 14 inches across. As in this to afford no assistance in the determinameasurement the irregular projections of tion. the sets are taken into account, the average surface width is less than 3 inches; and if with this width the surfaces are square, the number of longitudinal joints is so great that it is impossible for the paviors to break joint properly, and at the same time to keep the joints close, The only sedimentary rocks which it is desirable to use for street pavements are some portions of the millstone grits and other more laminated grits of the coal measures, and the use of these should be confined to steep gradients and to streets of little traffic. Specimens of rocks used |