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Fig. 152.

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-Express Passenger Engine (Combined Compound and Simple).

Great Northern Railway (H. A. Ivatt, Esq., Locomotive Engineer).

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End Elevations and Sections of Great Northern Express Passenger Engine.

within the barrel. The boiler is similar to that in the engine shown in Fig. 141. It is also shown by Fig. 149, Plate IX. Fig. 150 is a very heavy engine with side tanks, shown also by the photo in Fig. 151, Plate IX., also with tube-plate set back in the barrel.

Figs. 152 to 156 illustrate an "Atlantic" type locomotive designed by Mr Ivatt to work continuously, either as a simple or a compound, for hauling the heaviest Scotch expresses, and the fast service for the towns of the West Riding. The boiler is worked to a pressure of 200 lb. It has 141 sq. ft. of heating surface in the fire-box, and 2,359 sq. ft. in the tubes, making a total of 2,500 sq. ft. There are 248 tubes, 24 in. in diameter, with liberal spacing. The details of construction are clearly seen. A special feature is that the large grate area of 31 sq. ft. is obtained without undue lengthening of the fire-box by extending the box laterally over the trailing wheels. The boiler is 16 ft. long between the tube-plates. The barrel is made of steel, in. thick. The fire-box is of copper, in. thick, the fire-box tube-plate is in. thick. The tubes are of iron. The fire-box is 5 ft. 11 in. long, by 6 ft. 9 in. wide.

The compound engines comprise four cylinders -two high pressure of 13 in. diameter and 20 in. stroke situated outside the framing, and two of 16 in. diameter and 26 in. stroke inside it. These can be worked either single or compound continuously by means of a change valve placed over the steam chest of the inside cylinders. When working simple, the change valve admits high-pressure steam to both sets of cylinders, and discharges the exhaust of the outside cylinders into the blast pipe. When the valve stands in the position for compounding, it cuts off the boiler steam from the inside or low-pressure cylinders, and turns the exhaust from the outside or high-pressure cylinders into the inside steam chest; the steam passes on its way round the inside of the smoke-box. The change valve is worked by a small auxiliary steam cylinder in connection with a water dashpot, arranged to lock it in either simple or compound position.

The valves on the outside cylinder are Richardson's balanced, with the back cut out

between the strips, so that the exhaust goes straight through. The steam chest is between the inside cylinders. The valves are balanced by strips working against a rubbing plate placed between them. The outside valve gear is of the Walschäert type (see Link Motions), the inside gear is ordinary link motion. Both are worked from two reversing levers close together on the foot plate. Each reversing shaft can be locked from the foot plate by means of a vacuum lock similar to a vacuum brake placed on the middle of the shaft, and designed by Mr Ivatt.

The total weight of the engine in working order is 69 tons. That of the tender is 40 tons 18 cwt. The driving axles are each loaded to 18 tons. The bogie carries 18 tons. The engine wheel base is 26 ft. 4 in., the driving wheels are 6 ft. 8 in. in diameter. The bogie and trailing wheels are 3 ft. 8 in., and the tender wheels are 4 ft. 2 in. diameter. The tender has a coal capacity of 5 tons, and a water capacity of 3,670 gallons.

Compressed Air Locomotives.-These are used chiefly in mines and situations where smoke and fire are objectionable or dangerous. Their range is comparatively limited, but is sufficient for the purpose required. The power is derived from a store of compressed air carried in a tank or tanks above the framing, in the same situation as that occupied by the boiler of a steam locomotive, and the cylinders and motion work are practically the same as those for steam, compounding being often done. The cylinders are usually ribbed on the bodies, to absorb heat rapidly from the surrounding air. The pressures in the storage tanks range from 500 to 2,000 lb. per square inch, depending on the space available for the tanks. If there is plenty of head and side room, large tanks may be used with air at the lower pressures, but if space is very limited, smaller holders must be used with the air at a very high pressure to carry sufficient volume. Re-charging is done from stations to which tubes are connected up, the operation only taking a minute or so. The air is sometimes reheated on the loco. by tanks of hot water obtained from the charging stations.

Compressed air has been used to a limited

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