being abandoned. At the same time the temporary appropriation of the Western Gallery, D, to the Portrait Gallery and the examination-rooms, will give them an advantageous increase of accommodation. Hence, by this first instalment of the new works, a considerable improvement on the present state of things will be effected; but the space will still be much below what has been estimated as necessary by the Committees who have investigated the matter. 38. The next portion to be undertaken may be the building with façades at the eastern end, marked м' on the drawing, and coloured red. This is estimated to cost 54,1837, and it will furnish 33,750 square feet of additional floor-space. When this is built there will be, in all, 113,750 square feet available, i.e. enough not only to accommodate the present collections, with some increase, but also to receive the Portrait Gallery, and to provide examinationrooms, if required. At this time, therefore, there will no longer be any need to hire from the Commissioners of 1851 the Western Gallery, D, and thus an expenditure of 2000l. per annum will be saved. 39. The accommodation can afterwards be extended from time to time, as and when means may be voted for the purpose, by the erection of the other portions shown on Drawing No. II., as follows :— 40. The entire floor-space gained by the new buildings, when completed according to Drawing No. II., will be 157,100 square feet. To this must be added the space in the existing southern galleries, which will be assumed still to remain available. They contain, at present (as we have already stated), 51,500 square feet; but, in the process of building the new erections, a portion of the old ones will have become absorbed therein, and the space will be reduced to 41,818 square feet. The total available space will therefore amount to 198,918 square feet. The total estimated cost of the new work shown on Drawing No. II. is 222,8037. 41. In submitting this Report to the Treasury, we desire to state to their Lordships that one of the principal considerations guiding us has been to prepare a plan which admitted of being executed in parts, but which, when completed, should suffice for as long a period as we think it necessary to foresee. We have taken as our starting-point the demand of 160,000 square feet of area, and we have shown how it may be provided without more than a strictly temporary use of the Western Gallery, which does not belong to the Government. 42. We have been invited to express an opinion as to whether there would be space, in the completed plans, to provide for the collections now housed in the Museum in Jermyn Street, and the instruction now given there. We believe that there would be space for the purpose. We have the honour to be, Sir, TRANSMISSION OF POWER BY COMPRESSED AIR A MOST interesting experiment is about to be tried in Birmingham. A Company, whose engineer is Mr. J. Sturgeon, has obtained Parliamentary powers to supply power from a central station by compressed air through pipes laid in the streets. The application to Parliament was supported by the Birmingham Corporation, and the powers extend over an area of between four and five square miles. It is at first intended to restrict operations to about one square mile and a half. This area will include twenty-three miles of main pipes. The central works are designed for the production of 15,000 horsepower, of which the engines laid down at first will supply 6000 horse-power. The authorised capital expenditure for the whole is 276,800/., of which 150,000l. will be spent at once for the initial 6000 horse-power. In this journal we have nothing to do with the financial aspects of the project, but we mention these figures to show that within a short time the system may be expected to be in operation on such a scale as will very fairly test its mechanical efficiency. At a recent meeting of the directors it was determined to start clearing the ground and commencing the foundations for the central station at once, so that by next summer we may see considerable advance made towards the realisation of the project. This is the first time that an experiment of this kind has been tried in Britain. Power is distributed from a central station at Hull by the hydraulic system, but transmission by air has hitherto only been tried in small installations at mines, quarries, in sinking piers, as at the Forth Bridge, and in tunnel-boring. In mines and tunnels it has very evident advantages, in that it keeps up a continual supply of fresh, cold air where ventilation is very much needed; and therefore its undoubted success at the St. Gothard works does not demonstrate its certainty of success for the distribution of power on a large scale to the workshops of a town where the atmosphere is bearably pure Moreover, the pipe systems of these small installations have not been sufficiently long and complicated to test in any severe sense the liability to loss by friction, leakage, and variation of temperature. The results of the present experiment will therefore be of the utmost scientific value to engineers, and will be watched with corresponding interest. No fairer field for such an experiment could be found than in Birmingham, which is marked out from all other towns by the enormous number of its small workshops requiring minute amounts of driving-power, and the total turn-over of each of which is too small to enable the owner to afford skilled tendance to his boiler and engine. In these small shops the power is required only intermittently throughout the day. At times the engine may actually stand altogether for an hour or two, while it is only rarely that it is called to exert more than a comparatively small fraction of its full power. Meanwhile the large loss due to furnace and boiler inefficiency-that is, to waste of heat by radiation and by hot gases passing up the chimney-goes on steadily at a pretty uniform rate. Under such circumstances, the advantages of generating the power at a great central station are so evident as not to require demonstration. The question of chief technical interest is really as to whether the best means of distribution is by air, by water, by electricity, or by cheap gas to be used in gas-engines. That question can only be finally settled by expensive experiment. In passing, the writer may indicate his own opinion that there lies in the future a magnificent field for enterprise on the part of the gas companies of large towns in supplying cheap gas for heating and the production of mechanical power, and it is most decidedly their interest to improve the efficiency and lower the prime cost of gas-engines. The site of the central works is a triangular plot of ground adjoining Garrison Lane, at the intersection of the London and North-Western and the Midland Railways. The Birmingham and Warwick Canal forms one boundary of this plot. The fuel to be used under the boilers is gas obtained from Wilson's eight-hundredweight producers. Eighteen of Lane's water-tube boilers will supply six engines to produce the 6000 horse-power aimed at at first Air is admitted to the furnaces through gridiron sliding shutters, by means of which the supply is hand-regulated. It mixes with the gas in a mixing-chamber immediately below the front end of the furnace. The roof of this mixing-chamber is an arch of perforated bricks, and these bricks becoming highly heated the mixed air and gas is raised to a high temperature before being ignited. No special means have so far been considered necessary to prevent risk of lighting back into the mixing-chamber. The production of gas in the producers is controlled by the steam jet blown in at the foot of each. The steam for these jets is supplied from a special donkey boiler. The whole of the steam jets are throttled down by the action of a governor that runs, so to speak, in equilibrium with the air-pressure in the mains. The engine drives a small air-pump, which forces air into one end of a small cylinder, to the other end of which the air from the mains is admitted. If the pressure rises in the mains above standard, the piston of this cylinder is moved, and this movement is communicated by suitable gearing to the throttle-valve regulating the steam jet to the producers. The production of gas, and therefore the production of heat by its combustion under the boilers, are thus automatically regulated in accordance with the requirements, so that the airpressure in the mains is prevented from varying outside certain narrow limits. In connection with this part of the scheme we may point out that it seems to be a mistake not to throttle the entrance-areas for the air to the furnaces automatically and simultaneously with the regulation of the gas supply. The chief advantage in using gas instead of solid fuel lies certainly in the power of obtaining perfect combustion by thorough admixture and careful proportioning of air to fuel. This advantage is sacrificed if the air supply is not diminished and increased simultaneously with, and in the same proportion as, that of gas. We suspect also that it will be found desirable not to rely solely on the throttling of the steam blast as at present intended; the more direct and rapid action of a throttlevalve between the producer and the boiler-furnace will be highly advantageous, if not necessary. By means of simple mechanical relays, actuated either by the steam or by the compressed air, there can be no difficulty in controlling these three sets of throttle-valves by the action of a single governor. The steam-pressure is to be 160 pounds per square inch. Each set of three boilers supplies an engine of 1000 horse-power. The engine is of the triple-expansion type; the high-, intermediate-, and low-pressure cylinders having the diameters 20, 30, and 49 inches, and a common stroke of 48 inches. The areas of the three pistons are thus in the ratios 1, 24, and 6. The cranks are at 120° to each other. The high-pressure and intermediate cylinders are steam-jacketed at the sides. The low-pressure cylinder is not jacketed, but a novel arrangement of steam-jacketing its piston is adopted. The piston is hollow, and steam is led into its interior by a tube which is parallel to the piston rod, and moves to and fro through a stuffing gland in the cylinder cover, projecting into a larger tube screwed on the gland and supplied with steam direct from the boiler. The argument in favour of this arrangement is that side-jacketing of the low-pressure cylinder involves a large absolute waste of heat that goes towards heating the exhaust-steam as it leaves the cylinder on its way to the condenser; this loss of heat by the jacket steam being noxious, not only because it is pure waste, but also because it raises the back pressure against the piston. The fresh steam in the hollow piston sweeps over the inside surface of the cylinder just in front of the incoming working steam, and thus heats the metal and prevents undue condensation of the working steam, while it is comparatively inactive in heating the back-pressure steam. In criticism of this argument, it may be remarked that towards the end of the stroke (during the last quarter of the stroke) this piston-jacket surface giving heat to the exhaust-steam is greater than a side-jacket would offer. For three-quarters of the stroke, however, it is less. Each cylinder is connected with the fly-wheel shaft by a cross-beam. Over each end of each beam stands a single-acting, air-compressing cylinder of 26 inches diameter and 48-inch stroke. Each engine thus drives six of these air-pumps; and, since the speed is ninety double strokes per minute, the volumetric capacity of the compressors of each engine is close on 8000 cubic feet per minute. 14'7 = The pressure in the mains is to be 45 pounds per square inch above the atmosphere, and the delivery-valves are expected to lift a little before three-quarters of the compressor piston-stroke is finished. Thus the volume of air compressed to the above pressure delivered per minute by each engine is taken as about 2000 cubic feet. The ratio of pressures is 59'7 406. Thus, if the compression curve were isothermal, the valves would lift, as above assumed, at 75 of the stroke. If it were adiabatic, this pressure ratio would correspond to a ratio of final to initial volume of 367, and the valves would lift at 63 of the stroke. If the curve lay exactly mid-way between these two, or were according to the law pv, the ratio of final to initial volume would be 31, and the valves would lift at 69 of the stroke. In the latter case the volume delivered would be 31 × 8000 = 2480 cubic feet per minute. Calculating simply from the product of this volume by 45 pounds per square inch pressure (i.e. from the work the air could do in an air-engine without clearance, without expansion, and without more than atmospheric back pressure), this would give about 487 horsepower delivered in the consumer's engines for each engine developing 1000 horse power at the central station. Two indicator-cards taken from two air-compressing cylinders at Frood Colliery, near Wrexham, give very different results, possibly because one compressor was near the steam-engine cylinder, and was heated by it, while the other was not. The compression-curve from the one cylinder corresponds with the relation pv-1237, while that from the other corresponds to px -166. The latter curve is thus much steeper even than the adiabatic, and would indicate that the air was actually heated by conduction or radiation during its compression. Such heating could hardly have taken place to such an extent as to account for the above very high index, and the more probable explanation is that the air was steam-laden as it was taken in, and that the extra rise of pressure is really due to that of the steam in the mixture consequent on the rise of the temperature. It is desirable to keep down the compression-curve as nearly as possible to the isothermal line, because by doing so the area of the compressor indicator-card, and therefore the work to be done by the engines, is kept down to its minimum; whereas no advantage can be derived from the increase of temperature obtained by adiabatic compression, because this is rapidly lost by cooling in the pipes long before the air is utilised in the air-engine it drives. It is worth noticing that, because of the air being discharged from the compressors through valves which automatically lift when a certain designed pressure is reached, this loss of power due to cooling in the pipes is effected rather by a contraction of volume than by a diminution of pressure. The decreasing-pressure gradient along the pipes is very small, and is due solely to frictional and viscous resistance to the flow, and to variation of velocity consequent on variation of section. In order to approximate to isothermal compression, Mr. Sturgeon has adopted very special cooling arrangements for his compressors. Firstly, the air used is all taken through the roof of the engine-house, and thus heating by contact with the boilers and engines below is avoided. It is filtered of deleterious dirt in entering through the roof. Secondly, the compressor cylinders are surrounded by ample water jackets, through which a continual fresh-water circulation is kept up. Thirdly, the delivery-valve-it is a single large disk of slightly greater diameter than the cylinder-is made hollow, and through it a cold-water circulation is kept up, the water being spread out in a thin radial stream across the valve face over which the air flows as it leaves the cylinder. This cooling-water is supplied to the hollow valve through a tube sliding in a stuffing-box in the cylinder cover. A further development of this system would be a supply of cooling water to the face of the piston after the manner that the steam is supplied to the piston-jacket of the lowpressure engine cylinder; but this refinement has not been deemed necessary in the design as at present adopted. The compressor piston-face travels a little beyond the position assumed by the flat face of this delivery-valve when the latter is closed. During the momentary pause at the end of the stroke, the valve therefore falls into actual contact with the piston-face, and the two descend together until the valve is landed on its seat. Thus the clearance space is reduced absolutely to zero. The suction-valves are somewhat similarly arranged so as to reduce the clearance at the other end of the stroke to a very small amount. The cooling-water is circulated by gravity from a tank giving a head of 20 feet. The water is pumped into this tank from the canal, and the power spent in pumping this water is a partial set-off against the economy resulting from the approximation to isothermal compression; but the power thus gained greatly outweighs the work spent in this pumping. As at present designed, the air-pipes are of wroughtiron plate, riveted, but a new design for plate-steel tubes is being considered. The pipes are to be laid in concrete tunnels, which free them from all pressure of superincumbent soil or paving, and will always be very accessible for examination and repair. They are of 24 inches diameter near the central station, and diminish to 7 inches in the smallest branches. The joints are given a small degree of flexibility. In one design they are formed by two angleirons riveted to the outside ends of the two pipes, a hard rubber ring of circular section being placed between the flanges thus formed, and the flanges being drawn together by bolts. In another design a sort of double-socket coupling-piece covers the ends of both pipes for a few inches; the end of each pipe has formed on it two slightly projecting rings, and between these is poured, in the molten state, through a hole in the socket-coupling, a soft metal that expands during solidification. We rather doubt whether this last design will give sufficient tensive strength to the joint. Tensive strength is required simply because there are necessarily bends in the pipe here and there. The air is supplied to the consumer through a registering meter. This meter is similar in construction to Beale's gas exhauster. It consists of two cylinders, one inside the other. Both are 4 inches long; the outer one has a diameter of 14 and the inner a diameter of 9 inches. The outer one is fixed, and is furnished with an inlet and an outlet opening. The inner cylinder revolves freely on a fixed axis, distant (14-93) = 2 inches away from the centre of the outer case, so that the two cylinders always touch along a fixed line. Two sliding shutters project from a slot through the centre of the revolving Connection For Counting Gear cylinder. By means of a pin and a pair of sliding blocks running in circular grooves cut on the inner surface of, and concentric with, the fixed cylinder, these shutters are drawn out and in from the revolving cylinder so as always to keep in contact with the fixed one. During one revolution these shutters sweep through the meter a volume of air about 17 cubic feet. This rotation is reduced three times by worm-gearing in being transmitted to the counter-box, so that a single dial with two concentric circular scales, which are read by two fingers like the hour- and minute-hands of a common clock, is sufficient to register up to a million cubic feet. Fig. I shows this meter. It is driven by a small difference of air-pressure between the inlet and outlet. The Company intend to charge at the rate of 5d. per 1000 cubic feet at standard pressure of 45 pounds per square inch. If the air were used in an engine without expansion, without clearance space, and without back pressure above the atmosphere, this would correspond to hour per indicated horse-power of a cost per 60 X 33000 X 5 Under the conditions of actual practice the writer calcu- pure rolling action at one or other side of its tread. The indicated horse-power will cost per hour from 2d. down to as low as Id., excluding cost of engine attendance and depreciation, and interest on first cost of engine. FIG. 3. the roller, the point of contact for zero pressure coinciding with the centre of the disk. It also seems a pity, when a Bourdon tube that measures the pressure exists in any case in the meter, that its measurement of the pressure should not be made visible by the simple addition of a The standard pressure at which the air is sold at the above price being 45 pounds per square inch, a reduction of price per cubic foot has to be made if the pressure of the supply be less than this pressure. This is effected by introducing a variable velocity-gear between the volume-pointer and graduated dial. meter and the dial-counter. This arrangement is shown in Fig. 2. The rotation is transmitted to a small roller on a spindle capable of sliding in its bearings parallelly to its own axis. It drives a disk on the counter-arbour by rolling contact. The end of the roller-spindle is linked to the end of the tube of a Bourdon pressure-gauge. As the pressure rises, the roller is thus pushed nearer the centre of the disk, and gives this disk, therefore, an increasing fraction of a revolution per revolution of the roller. The roller really lies between two disks, but the one is "idle" and serves simply to support the roller in pressing against the driven disk. This integrator is wholly wrong in principle, and it is badly designed in detail. The roller has a rubber tyre round it, and therefore touches the disk at different radii, and thus must rapidly wear away, owing to the want of The registrations of all the meters in the whole district are telegraphed to the central station and added up on one large central counter, so that the engineers in charge may have means of continually comparing the actual consumption with the duty of the engines, known from ordinary engine continuous counters, and of detecting any serious leakage that might occur in consequence of breakage of a main or branch pipe. The telegraphing apparatus is shown in Fig. 3. The counting disk is divided into ten equal divisions, each representing 1000 cubic feet, by small metal projections. As these come successively underneath a contact-maker, they allow the passage of a current, which moves the finger of the central counter through a corresponding division. One main wire, with branches to the separate meters, is sufficient for the whole district, the earth return being used. As the counter-disk moves slowly, special means must be taken to break the contact instantaneously after it is made; otherwise all but one of the indications of several meters, whose times of contact with the tooth on the disk overlapped, would fail to be registered at the central station, and should the stoppage of any one engine in the district happen to occur while this tooth of its meter was in contact the whole registering apparatus would cease to act for an indefinite time. The contact-breaker is shown in Fig. 3, at the left-hand side. The momentary current caused on making contact magnetises an electro-magnet, which, by attracting its armature, draws the contact-maker (which is mounted on a piece of watch-spring) past the tooth into such a position that it catches behind a small plate of insulating material at the back of the tooth, which prevents it springing again into contact with the latter when the armature of the magnet is released. Fig. 4 explains the calculation of the thermodynamic efficiency of this mode of transmission of power. It is drawn for unit volume of atmospheric air drawn into the air-pumps. The pressures are reckoned in atmospheres. ABCDE is the indicator-diagram showing the work done by the compressor-pump. The compression-curve CD is taken according to the law pc v1.2 because it seems probable that this index may be reached with the efficient water-cooling system adopted. The suction-line A B is 10 taken atmosphere below atmospheric pressure. The point F is taken on the same isothermal as C; thus D F is the loss of volume consequent on the air cooling in the pipes down to atmospheric temperature. The diagram E F G H is the indicator-diagram for an engine driven by the air without loss of initial pressure below the compressor pressure, without clearance, without expansion, and with a back pressure atmosphere above atmospheric pressure. The same back pressure is used for all the other engine diagrams. The diagrams EFIKH, EFLMH, and EF N H are diagrams for engines with similar conditions, and with ratios of expansion 1, 2, and 2; that is, with cuts off 3, 4, and 2, the last being that that brings the final pressure down to 1 atmosphere. The expansion-curve FILN is taken as adiabatic. atmosphere be lost in frictional and viscous resistance to flow through the pipes, by obstructions at bends, passage through meter, &c., or by sudden change of section of pipe, then the admission line is lowered to P Q. The effect of clearance is to cut off a part of the diagram by a vertical line at the left-hand end. This vertical line is not drawn in the diagram, because its position varies with the grade of expansion employed. In calculating the following results the clearance has in each case been taken as the volume of the cylinder. The area of the compressordiagram is 16, and the efficiency is in each case obtained If by dividing the engine-diagram area by 16, and multi-engine. This can hardly be accomplished even if the engine plying this quotient by. This is the ratio between be situated close to the central works. It need hardly be the compressor-diagram and that of the central station pointed out that the expansion will not usually be carried engine which drives it, the mechanical inefficiency of this so far as to bring the working pressure to near equality central plant being taken as. The results are most with the back pressure; in fact, to do so is decidedly very clearly shown in tabular form. bad practice, and does not lead to economy in the brakepower, especially when depreciation and interest on first cost of the engine is taken into account. With good management, from 30 to 50 per cent. efficiency may be expected. Table of Efficiencies of Transmission of Power by Air compressed to 45 pounds per square inch Ratio of expansion No loss of initial pressure = I Efficiency I 2 No clearance ... 2 72 *60 In a paper read by Mr. Sturgeon before the British Association last summer, he gives a table of calculated efficiencies ranging from 32 to 84. These calculations include allowances of 2 per cent. for valve-resistance and 69 leakage past compressor-piston; 13 per cent, for leakage, friction, and wire-drawing in the pipes; and 8 per cent. for clearance and back pressure in the consumer's engine. Except the last, these allowances are much more liberal than those that have been made in calculating the above table. On the same basis as ours have been made, Mr. Sturgeon's calculations would have given considerably higher figures than the above 32 to 84. But the higher figures in Mr. Sturgeon's table are obtained by supposing that the consumer heats the air by a gas-stove, before passing it into his engine, up to temperatures from 212° F. to 320° F. How the resulting figures can be in any sense 57 The last two sections of this table comprise the limits of practicable results. The highest efficiency shown is 60 per cent. This could only be obtained by avoiding absolutely all loss of pressure between compressors and air |