Mr. Laird was followed by other builders of iron vessels at Liverpool; the high estimation in which they were held having led to a constantly increasing demand for them. About this time Messrs. Tod and MacGregor, of Glasgow, began to take a leading position in this occupation; the _Princess Royal_, long engaged on the line between Glasgow and Liverpool, and launched from their yard, having been one of the finest and fastest iron packets of her time. [Sidenote: The _Great Britain_, 1839-1843.] From that period iron shipbuilding on the Clyde increased with great rapidity, but the most magnificent specimen of an iron ship of any description produced at that time was the _Great Britain_, to which reference will be made hereafter, constructed by Mr. Patterson at Bristol, for the Great Western Steam Packet Company. [Sidenote: Advantages of iron ships.] [Sidenote: Action of salt water on iron inconsiderable.] For the information of the general reader, I may here state that the advantages of iron vessels consist principally in their durability, strength and safety, increased capacity for stowage, greater economy, and salubrity.[119] With regard to the perturbations of the compass, Professor Airy, previous to the time when Mr. Evans made his report, had published a very concise series of instructions for correcting the compass on board of iron ships; and the progress of science now bids fair to obviate any difficulty whatever ensuing from this cause. Prior to experience it was apprehended that the saline property of the sea-water would tend to corrode the iron, and, further, that this metal would be rapidly destroyed by oxidation. But experience has shown that the effect of salt water on iron _alone_ is so small as hardly to bear a comparison with its effect upon iron in connection with wood. This remarkable difference has been observed in iron vessels in which timber had been used for the keel; the bolts driven through the keel to form its proper connections having been so rapidly acted on as almost to destroy them before the external iron plates of the hull had been perceptibly diminished in thickness: it is further of importance that the vessel should be kept in use rather than be laid up in ordinary. Vessels built in the earliest stage of this art, subsequently to that of building mere canal-boats, bore many years’ service with little need of repair, and remained in a perfectly good condition for a longer period than that to which the durability of wooden vessels ordinarily, and under similar circumstances, extends. But there is a great difference in iron plates, some are inferior and soon oxidize, while others, as will be presently shown, last for many years. As the inner surface of the plates may be almost wholly protected from oxidation, it is only from the external wear that danger may be apprehended. But, though the outer surface of the metal can be protected in a great measure from corrosion, yet iron vessels are subject to the