It is now about six years since the first American steamship left this city to establish regular communication, on our part, with Europe. Our first steamships were of an inferior character ; they failed to equal in speed the old Cunarders, which had run for eight years. Respecting what had been done by both the English and our own steamships, we said on page 21, Vol. 4, Scientific American, October 7, 1848 ; " no first rate voyage has yet been made across the Atlantic," the shortest at that tine occupying eleven days, made by the Europa. We also said " our American steamships must, can, and shall yet perform the passage across the Atlantic at the rate of fifteen miles per hour ; there is science and genius enough among our engineers and nautical architects to build the very finest steamships." It is now nearly five years since we expressed these opinions ; let us see in the evolution of events if we formed anything like a correct estimate of the future of American steamships. In April 1850, the Atlantic, the first of a new line of American steamships, left this port on her voyage to Liverpool to compete with the celebrated Cunard line, for the mastery ot the seas. Her first voyage was somewhat unfortunate, but from the beauty of her model, a general confidence was placed in her ultimate success. At intervals after that, three noble consorts, the Pacific, Arctic, and Baltic, were added to the line, all built after the same model. For the past two years they have made such voyages as have given them the supremacy over all the English steamships, both in point of speed and every excellent sea quality. This is manfully acknowledged by Capt. McKinnon, of the Royal Navy, in a work recently published, entitled " The Resources and Settlements of America." He came to the United States in one of the Cunard steamships, and went back to England in the Baltic, one of the Collins' Line, respecting which he uses the following language, ' I am only doing justice to these magnificent vessels in stating that they are beyond any competition the firest, the fastest, and best sea boats in the world. I am sorry to be obliged to say this, but as a naval officer I feel bound in candor to admit their great superiority. Their extraordinary easiness in the sea cannot lail to excite the admiration of the sailor. I never beheld anything like it. No sea ever came on board, there was no violent plunging ; the steaming of the Baltic was the absolute poetry of motion." He attributes the superiority of the American steamships to their long gently graduated bows, and the buoyancy of the fore part in being relieved of the weight of the bowsprit. He calls the attention of the English ship builders to the absurdity of a heavy bowsprit on a steamship, and says, " the reason why we allow Brother Jonathan to beat us on our own element is patent to the world, and may be summed up in one sentence, the British model is far inferior to the American.'' Here we have the unbiassed opinion ol a most competent foreign judge, respecting the superiority of American steamships now—they are the finest and fastest in th e world. Although the voyages of American steamships across the Atlantic have been shortened by thirty six hours since we penned the remarks we have quoted at the beginning of this article, still, they have not yet come up to the mark which we then set up before them. We expect to see American steamships making the passage from New York to Liverpool in eight days—the shortest yet made has occupied 9 days and 17 hours. At the average speed of 15 miles per hour this desirable result would be nearly accomplished, and surely, when some of our clipper ships, under canvas, have run over 22 miles per hour, it is not too much to expect that our steamships will make voyages across the Atlantic at an average speed of 15 miles per hour. A new steamship named the Arabia has been added to the Cunard Line. Its perfor-i mances have equalled in speed those of her American rivals. The honest rivalry of two powerful lines like the Collins' and Cunard, will operate healthfully for improvements in steam navigation, not only as it regards speed, but the safety and comlort of passengers also. Our nautical architects and engineers have done well, and deserve the thanks of our country, but they can and must do better (till. The next steamship which is to be built for the Collins' line must go to Liverpool in less than nine days ; this we expect, and we believe we will not be disappointed.