“In an iron steam-ship, built head to the south, the attraction due to machinery is added to that of the hull, whereas in one built head to the north, the attractive forces of hull and machinery are, in the northern hemisphere, antagonistic, and a position of small, or no ‘semicircular’ deviation for the compass may generally be obtained. To iron steam-vessels engaged on the home or foreign trades in the northern hemisphere, this direction of build is therefore to be preferred.” And, again, at p. 355, he remarks: “As every piece of iron not composing a part of, and hammered in the fabrication of the hull,—such as the rudder, funnel, boilers, and machinery, tanks, cooking galleys, fastenings of deck houses, &c.,—are all of a magnetic character differing from the hull of a ship, their proximity should be avoided, and, so far as possible, the compass should be placed so that they may act as correctors of the general magnetism of the hull. “A compass placed out of the middle line of the deck is affected by the nearest topside, and its deviations must necessarily be much increased if that topside has the dominant polarity, as in ships built east or west.” [119] “The principal reason of an iron vessel being so much healthier is on account of her coolness and her freedom from all manner of smell; in an iron vessel there is no disagreeable smell of bilgewater, which there is in a wooden vessel in a tropical climate; it is, in fact, the difference between carrying water in a cask, and in a tank.” (Evidence of Mr. McGregor Laird, p. 58, “Steam Navigation to India.”) [120] Mr. Robert Stephenson thought it possible, that if you had a dock filled with sulphate of copper, you might treat an iron vessel as you do a small teapot, and electrotype it with a thin coating of copper. (Evidence, 1851, 26th June, before Committee of the House of Commons.) [121] The Liverpool underwriters, in their book of registry for iron vessels (established 1862), in the edition of that work for 1863 and 1864, offer the following remarks: “Experience has shown that iron ships are much more durable than was at first supposed. By the use of cement inside, and by careful attention to outside coating, a well constructed iron ship can be reckoned upon to last, _in first-class condition_, for a period of at least twenty years. Wear and tear of equipment, and of the wood used in their construction, must in all cases be excepted.” [122] Mr. McGregor Laird states in his evidence (Question 553, p. 59) before the Select Committee on “Steam Navigation to India,” 1834, “A strong iron vessel will not weigh one-half of that of a wooden one, and therefore will draw considerably less water;” further (Q. 554), “Her capacity for stowage will be much greater, her sides, including strong iron frames, not exceeding 4 inches in thickness, while those of a wooden vessel will be 12 inches thick.”