Read this excerpt adapted from The Rocket: The Story of the Stephensons, Father and Son by H. C. Knight. It was a critical moment, but [Stephenson] had no fears of the result. Robert often came to Liverpool to consult with his father, and long and interesting discussions took place between father and son concerning the best mode of increasing and perfecting the powers of the mechanism. One thing wanted was greater speed; and this could only be gained by increasing the quantity and the quality of the steam. For this effect a greater heating surface was necessary, and mechanics had long been experimenting to find the best and most economical boiler for high-pressure engines.
Young James, son of that Mr. James who, when the new Liverpool and Manchester route was talked of, was the first to discover and acknowledge George Stephenson's genius, made the model of an improved boiler, which he showed to the Stephensons. He introduces himself to our notice now with a patented model of an improved boiler in his hand, which Stephenson thinks it may be worth his while to make trial of. "Try it," exclaimed the young inventor—"try it, and there will be no limit to your speed. Think of thirty miles an hour!"
The improved boiler was what is called a multi-tubular boiler. An iron boiler is cast, six feet long, and three feet and a third in diameter. It is to be filled half full of water. Through this lower half there run 25 copper tubes, each about three inches in diameter, open at one end to the fire, through which the heat passes to the chimney at the other end. You see this would present a great deal of heating surface to the water, causing it to boil and steam off with great rapidity. The invention was not a sudden growth, as no inventions are. Fire-tubes serving this use started in several fertile minds about the same time, and several persons claimed the honor of the invention; but it was Stephenson's practical mind which put it into good working order and made it available. For he told Robert to try it in his new locomotive.

What is explained in this excerpt that is not explained in "The Steam Engine"?