Although the United States Government had seemed a little ungracious in its delay, it yet rendered, this year and the next, most important service. Already it had prepared the way, by the deep-sea soundings, which it was the first to take across the Atlantic. It now rendered additional and substantial aid in lending to this enterprise the two finest ships in the American navy--the Niagara and the Susquehanna. The former was built some dozen years before by George Steers--a name celebrated among our marine architects as the constructor of the famous yacht America, that "racehorse of the sea," which had crossed the Atlantic, and carried off the prize in the British Channel from the yachts of England--and was designed to be a model of naval architecture. She was the largest steam-frigate in the world, exceeding in tonnage the heaviest line-of-battle ship in the English navy, and yet so finely modelled that, propelled only by a screw, she could make ten or twelve miles an hour. Notwithstanding her bulk, she was intended to carry but twelve guns--being one of the first ships in our navy to substitute a few heavy Dahlgrens for half a dozen times as many fifty-six-pounders. This was the beginning of that revolution in naval warfare, which was carried to such extent in the Monitors and other ironclads introduced in our civil war. Each gun weighed fourteen tons--requiring a crew of twenty-five men to wield it--and threw a shell of one hundred and thirty pounds a distance of three miles. One or two broadsides from such a deck would sink an old-fashioned seventy-four, or even a ninety or hundred-gun ship. But as the Niagara was now to go on an errand of peace, this formidable armament was not taken on board. She was built with what is known as a flush deck, clear from stem to stern, and being without her guns, was left free for the more peaceful burden that she was to bear. When the orders were received from Washington, she was lying at the Brooklyn Navy-Yard, but began immediately to prepare for her expedition. Bulkheads were knocked down, above and below, to make room for the huge monster of the deep that was to be coiled within her sides. These preparations occupied four or five weeks. On the twenty-second of April, she made a trial trip down the bay, and two days after sailed for England, in command of Captain William L. Hudson, one of the oldest and best officers in our navy, who, to his past services to his country, was now to add another in the expeditions of this and the following year. He had with him as Chief Engineer Mr. William E. Everett, whose mechanical genius proved so important in constructing the paying-out machinery.