from the weights of the moving parts and their velocities as measured. It will be seen from the diagram that a great deal of power is required to give velocity, that the engine has to be reversed to stop it when it has made sixteen revolutions, and that a great deal of back pressure is required to bring the machinery to a stop. It will also be seen that a great deal of power was lost in driving the steam out of the cylinder. This might be obviated, partly by altering the adjustment of the valves and partly by adopting some system of expansion, and very likely this may have been remedied since these diagrams were made.

Fig. 524 shows a similar diagram for the Monkwearmouth engine, already described in pp. 392 and 404, and Figs. 499 and 511, but the line representing the power required to lift the weight of coal and ropes, instead of being a straight line, is very much bent. This is due to the action of the balance-chains which facilitate the starting and stopping of the engine. (If there were a third balance-weight equal to one of the other two, the dotted line represents the power that would then be required to lift the weight of coal and rope.)

It will be seen that at the start the power required to lift the load is equal to about 500,000 foot-pounds per revolution, while at the end it is about 260,000 foot-pounds per revolution. Following out the action of the balance-weights, it will be seen that the load-line quickly falls during the first five revolutions, owing to the winding up of the rope with the loaded cage and the descent of the rope with the empty cage. By the end, however, of the fifth revolution, the balance-weights have begun to rest at the bottom of the staple-pits, and by the beginning of the eighth revolution both bunches of chain cable are resting on the ground; thus their weight no longer assists the engine. As the load-line has risen up to as much as it was at the start, again it quickly descends, owing to the winding up and unwinding of the ropes, till at the beginning of the twelfth revolution the engine begins to pick up the flat chain, which causes a slight increase of the load. Again, however, the load-line falls by the winding and unwinding of the ropes until the end of the sixteenth revolution, when the bunches of cable are partly lifted, and as they are raised from the bottom the load against the engine increases till the beginning of the nineteenth revolution, when both balance-weights are suspended, and again the load-line begins to fall. It must be noticed that, in a great measure owing to the balance-weight, less counter-pressure is required in this case to stop the engine than was necessary with the Denaby engine.

Fig. 525 shows a similar diagram for the Douglas Bank Colliery. In this case a spiral drum is used, giving a more even load-line

than in the case of the other two. This engine was a coupled horizontal engine, two cylinders, each 30 inches diameter and 5-feet stroke; maximum pressure in cylinders, 49 lbs. ; maximum

FIG 524.-The Wilson winding-engine, Monkwearmouth Colliery. Explanatory diagram. Each dotted rectangle contains 100,000

foot-pounds.

FIG. 525.-Winding-engine, Douglas Bank Colliery. Explanatory diagram. Each dotted rectangle contains 100,000 foot-pounds.

velocity of piston, 462 feet a minute; mean velocity, 260 feet a minute; the minimum diameter of the drum, 18 feet, and the maximum diameter, 25 feet 4 inches; weight of drum about 82,000 lbs.; total weight of moving parts, including engine, drum, ropes, pulleys, chains, and coals, 133,900 lbs.; steel-wire ropes, 11 lbs. per fathom; cage, 2240 lbs. of steel, containing 4 tubs, each weighing 336 lbs. and holding 6 cwt. of coal. Depth of pit, 1530 feet; time of winding, 52 seconds.

It will be observed that the larger diameter is only about 29 per cent. greater than the smaller diameter. In the case, however, of the engine at Harris's Navigation (Figs. 502 and 503), the maximum diameter of the drum is 32 feet, and the minimum diameter is 18 feet, or nearly 44 per cent. less than the maximum diameter. The consequence of this increased difference is a more effective balancing, the power required to lift the load of coal and ropes being nearly even throughout the journey.

This is proved by the following calculations, which show the work to be done by the engine in lifting the dead load during the first revolution, the middle revolution, and the twenty-fourth revolution.

The work done in lifting the dead load is calculated in foottons as follows:

The above weights are made up as follows :—

It is evident, from the above calculations, any alteration in the section of the rope, or weight of cage, tubs, and coal lifted, will materially affect the dead load on the engine.

Engine Power. From these diagrams it may be seen that the total power required by an engine is from two and a half to three times that which would be necessary merely to balance the dead weight, and that at least as much power is expended at the start in giving velocity as in lifting the load. This is not only because of the high speed attained by the cages in the shaft, which in some cases reaches a mile a minute, but because of the short time allowed for attaining such a high speed. Engines are often going at a great velocity within ten seconds of starting, and within thirty seconds have reached nearly their full speed, though this speed continues to increase until the steam is shut off.

Lubrication. It is important that all the parts of a windingengine should be well lubricated to reduce the friction. Thirty years ago tallow was almost universally used for lubricating the valves and piston. It has, however, been found that tallow has an injurious effect when used inside a steam-cylinder, corroding the metal. Mineral oil or grease is now generally used, some heavy oils being specially made suitable for the interior of valvechests and cylinders. Mineral oil is also chiefly used for the slides and bearings. Metallic packing has also come into use for the stuffing-boxes, and some engineers prefer it to all kinds of soft packing.