against iron vessels, gave remarkable impulse to their increase. But strong prejudices, unreasonable doubts, and real difficulties had still to be overcome before the suitableness of iron ships for ocean navigation could be established. Another of the chief and more tenable objections to the extended use of iron vessels was the perturbation of the compass. Moreover, one or two unfortunate accidents had been attributed to this cause, though this more, probably, served as a plausible excuse for bad seamanship or negligence. In the course, however, of a few years iron packets began to be used along our coasts; and the art of building them advanced gradually towards perfection. Iron vessels soon afterwards, therefore, acquired a merited confidence. [Sidenote: The _Rainbow_, 1837.] Their superiority had become apparent to the more intelligent persons of the period who directed their attention to engineering and maritime pursuits. In 1833 and 1834, Mr. Fairbairn launched two passenger steamers of iron to ply on the Humber between Selby and Hull. Mr. Manby also built one of considerable dimensions for general purposes; and in 1837 Messrs. Laird built two iron vessels of about 350 tons and 60 horse-power each, ordered by the East India Company for the navigation of the River Indus. In the same year Messrs. Laird constructed for the General Steam Navigation Company an iron vessel, the _Rainbow_,[117] to ply between London and the outports. In that year Muhammed Ali placed upon the Nile an iron steamer built by the same firm, while they also launched from their yard the iron vessels in which Colonel Chesney explored the course of the Euphrates, and which, having been shipped in pieces, were put together by Birkenhead artisans on the banks of that river. Though the value of iron was now fully established for shipbuilding purposes, many years elapsed ere that material came into general use for the construction of over sea _sailing_ vessels, the principal objections being the greater liability of the compass to err,[118] and the difficulty of preventing animalculæ and sea-weeds from adhering to the bottom. But these difficulties were in time overcome, and iron vessels propelled by sails are now nearly as common as steamers built of that material. Experience by degrees successfully met almost every objection; and science was again triumphant over prejudice and ignorance. Iron had been made not merely to float, but to ride buoyantly over the crest of the wave, amidst the raging elements. [Sidenote: Messrs. Tod and MacGregor.]