In the year 1809 Trevithick commenced an attempt at tunnelling under the Thames. It was the second time that this difficult undertaking had been tried, Ralph Dodd having been the first of the unsuccessful borers. When a large sum of money had been raised by subscriptions Trevithick commenced boring at Rotherhithe, and in order to save both labour and expense, kept very near to the bottom of the river; but notwithstanding the increased difficulties which he had to encounter on this account, he actually carried the tunnel through a distance of 1011 feet, and within 100 feet of the proposed terminus. At this point an unfortunate dispute arose between him and the surveyor appointed to verify his work, the surveyor asserting that the tunnel had been run a foot or two on one side. This reflection on his skill as an engineer excited Trevithick's Cornish blood, and he is said to have adopted the absurd expedient of making a hole in the roof of the tunnel at low water, and thrusting through a series of jointed rods, which were to be received by a man in a boat, and then observed from the shore. In the execution of this scheme, delays ensued in fitting the rods together, and at length so much water made its way through the gulley formed by the opening in the roof, that retreat became necessary; Trevithick, with an inborn courage, refused to go first, but sent the men before him, and his life nearly fell a sacrifice to his devotion: as he made his escape on the other side, the water rose with him to his neck, owing to the tunnel following the curve of the bed of the river, which necessarily caused the water to congregate towards one part. The work was thus ended almost at the point of its successful completion, being at once a melancholy monument of his folly and his skill. After this unfortunate failure, Trevithick commenced many schemes; among others, his attention was directed towards the introduction of iron tanks and buoys into the Royal Navy. On first representing the importance of this to the Admiralty, the objection was raised, that perhaps, in the case of the tanks, iron would be prejudicial to the water, and consequently to the health of the crews; Trevithick was therefore requested to consult Abernethy upon the subject, which he accordingly did, and received for his answer the following characteristic reply: "That the Admiralty ought to have known better than to have sent you to me with such a question." He likewise, about this period, contributed largely to the improvement and better working of the Cornish engines, and to him the merit is due of introducing into these engines the system of high-pressure steam, and of inventing in the year 1804 the cylindrical wrought iron boiler, (now known as the Cornish boiler,) in which he placed the fire inside instead of outside, as had been the practice before his time.