I recently addressed to Mr. Atherton the following question: "Taking two ships of the same _size, displacement, and power, or coal_, the one a side-wheel, the other screw: What will be their relative _speed and carrying capacity_ in smooth water? What in a sea-way, or in regular transatlantic navigation?" He replied under address, "Woolwich Royal Dock Yard, 14 Sept., 1857: "It is my opinion, based on experiment, that a well-applied screw is quite equal to the paddle-wheel for giving out the power by which it is itself driven, that is, in smooth water. I can not say from observation or experience what is the comparative operation at sea." I addressed the same inquiry to Mr. Robert Murray, of Southampton, who has written an able work, entitled, "The Marine Engine," and who is considered excellent authority, and have from him the following reply, dated Southampton, 19 Sept., 1857: "With regard to the relative efficiency of the paddle-wheel and screw for full-powered mail steamers, I am disposed to prefer the paddle-wheel for _transatlantic_ steaming, in which the vessel has to contend with so much rough weather and heavy sea, and the screw for the Mediterranean and the Pacific routes. "For auxiliary steamers of any kind the screw has manifestly the advantage. "With regard to the actual speed obtained from each mode of propulsion in vessels of the same power and form, and with the propeller in its best trim, I am disposed to prefer the paddle-wheel, either in smooth water, or when steaming head to wind, but in other conditions the screw." What he means by "other conditions," is evidently when the screw is running with a fair wind, which is seldom, so as to use her sails. Bourne also states very clearly in two places that the propeller is by no means so efficient in a sea-way, as a side-wheel steamer, and admits that when a vessel is steaming at eleven or twelve knots per hour, the sails not only do not aid her, but frequently materially retard her motion. (_See Bourne, page 237._)