diameter and 24 in. stroke.  The two coupled wheels under the barrel of the boiler are 5 ft. 6 in. diameter, and the trailing wheels 4 ft. 6 in.; there are single frames with inside bearings to all the axles.  The boiler pressure is 140 lb., and the tractive force per pound of mean cylinder pressure 106 lb.; the total wheel base is 14 ft. 6 in.  The boiler is 10 ft. 2 in. long and 4 ft. 4 in. diameter, and the heating surface is in the tubes, 858 square feet; fire-box, 90 square feet; total, 948 square feet.  The engine is furnished with wing tanks holding 860 gallons of water, and carries 30 cwt. of coal.  The weight in working order is 38 tons.  These engines have taken a maximum load of twenty-five coaches between London and Brighton, but are mainly employed in working the suburban and branch line traffic; their average consumption of coal is 23.5 lb. per mile, with trains averaging about ten coaches.

Another example is Mr. Webb’s tank engine on the North-Western Railway, which presents a contrast to the foregoing.  It has inside cylinders 17 in. diameter and 20 in. stroke, coupled wheels 4 ft. 6 in. diameter, and a tractive power per lb. of mean cylinder pressure of 107 lb.; the wheel base is 14 ft. 6 in. with a radial box to the leading axle; the heating surface is in the tubes, 887; fire-box, 84; total, 971 square feet; the weight in working order is 35 tons 15 cwt.  The engine is fitted with Webb’s hydraulic brake, and steel, manufactured at Crewe, is largely used in its construction.  The consumption of coal-working fast passenger trains has been 281/2 lb. per mile.  There are many other types, such as the ten wheel bogie tank engines of the London, Tilbury, and Southend and South-Western railways; the saddle tank bogie engines, working the broad gauge trains on the Great Western Railway, west of Newton; and the familiar class working the Metropolitan and North London traffic.  But the same principle is adopted in nearly all—­a flexible wheel base to enable them to traverse sharp curves, small driving wheels coupled for adhesion, and wing or saddle tanks to take the water.  One notable exception is, however, the little six wheel all-coupled engines weighing only 24 tons, which work the South London traffic, burning 241/4 lb. of coal per mile, with an average load of eleven coaches.

Goods engines on all lines do not vary much.  As a rule they are six wheel all-coupled engines, with generally 5 ft. wheels, and cylinders varying between 17 in. and 18 in. diameter and 24 in. to 26 in. stroke; the grate area is about 17 square feet, and the total heating surface from 1,000 to 1,200 sq. ft.; the average weight in full working order varies from 30 to 38 tons.  One noteworthy exception occurs, however, on the Great Eastern Railway, where a type of goods engine with a pony truck in front has been introduced.  The cylinders are outside 19 in. diameter and 26 in. stroke, there are six coupled wheels 4 ft. 10 in. diameter, and the pony truck wheels are 2 ft. 10 in. diameter; the total wheel wheel base is 23 ft. 2 in., but there are no flanges on the driving wheels.  The boiler is 11 ft. 5 in. long and 4 ft. 5 in. diameter, the boiler pressure is 140 lb., and the tractive force per lb. of mean cylinder pressure 162 lb.; the grate area is 18.3 square feet, and the heating surface is in the tubes, 1,334 square feet; fire-box, 122 square feet; total, 1,456 square feet.