Meantime, greater boiler efficiency was being obtained. The "Britannia" worked with a pressure of 12 lbs. This was gradually increased to 30 lbs. in boilers constructed in 1868, and this was practically the range of pressure during the period of single-expansion engines. The salt water used in these boilers caused them to become quickly salted up, which not only diminished their efficiency but shortened their lives, and it was not until the compound engine was invented by John Elder that cylindrical boilers, working at a pressure starting at 60 lbs. and increasing to 190 lbs., were introduced. These proved a great success. By the use of fresh feed water and replenishing it from the condensers, salting was prevented and the life of a boiler greatly increased. No further great improvement in the boiler has taken place. The water-tube boiler is still in an experimental stage, and attention is now being directed to oil fuel, which will reduce the engine-room staff, ensure greater cleanliness and quicker despatch. The result of these improvements in marine engines and boilers has been to reduce the consumption of coal from 4 lbs. per indicated horse-power to 1.4 lbs., which cannot be considered otherwise than a great achievement. The future high cost of coal is sure to stimulate invention, and we may at no distant date expect developments in internal combustion engines adapting them to high powers which may open up a new and great era for mechanically-propelled vessels, and again entirely change the world's outlook. We have also always before us the probability of further discoveries in electricity; the recent developments in wireless telegraphy teach us that we are only on the threshold of discoveries which will bring this mighty but mysterious power more and more into the service of man. CHAPTER IV THE MAKERS OF OUR SHIPPING TRADE These sketches of the growth and development of our shipping trade would be incomplete without some reference to those who built up its great prosperity--men who are entitled not merely to our consideration but to our admiration; men whose memories should be treasured by Liverpool people, because they afford to generations yet to come examples of industry and perseverance in the face of difficulties which should not be without beneficial effect if kept in remembrance. Things move so rapidly, and our memory is so limited that we are apt to view the things of to-day as of our own creation, and lose sight of the strenuous spade work done by our forefathers.