The enlarging the fire-place end of boilers or fire-tubes has led to many forms. Trevithick's model of 1796[188] had an oval tube giving a greater spread of fire-*bars; the same is seen in the 1808 steamboat;[189] the Dolcoath boilers of 1811[190] show the oval and also the bottle-neck fire-tube; the Welsh locomotive of 1804[191] had the fire-tube contracted at its bend or return portion; the Tredegar puddling-mill fire-tube of 1801[192] tapered gradually from the fire-bridge to the chimney end; in the London locomotive of 1808[193] the fire-tube took the bottle-neck shape close to the fire-bridge. The accompanying sketch shows the bottle-neck contraction, only on the top and sides of the fire-tube was to give breadth to the fire-bars _d_, and thickness to the fire at bridge _c_, after which the flue portion of the fire-tube was contracted: this boiler was for many years a favourite in Cornwall. The bottle-neck contraction of the 'Echo' boiler was similar to the above, except that the enlargement of the fire-place was downwards instead of upwards, and the fire-tube, instead of going through the end of the boiler, returned to near the enlarged fire-place, when it passed out through the side of the boiler to the chimney, just as in the Tredegar puddling-mill boiler; all those variations were with the object of increasing the fire-grate, and at the same time keeping down the gross size and weight of boiler and its water. [Footnote 188: See vol. i., p. 104.] [Footnote 189: See vol. i., p. 335.] [Footnote 190: See vol. ii., p. 169.] [Footnote 191: See vol. i., p. 181.] [Footnote 192: See vol. i., p. 223.] [Footnote 193: See vol. i., p. 207.] In 1805, Lord Melville failed to keep his appointment with Trevithick, on his proposal to construct a high-pressure steamboat.[194] Rennie, a pupil and friend of Watt, and familiar with Trevithick's high-pressure steam-dredgers on the Thames, was employed by Lord Melville and the Admiralty on the Plymouth Breakwater, where in 1813 Trevithick proposed the use of his high-pressure steam locomotive and boring engine.[195] In 1820 Rennie wrote to Watt, that the Admiralty had at last decided upon having a steamer; at that time fifteen years had passed since Trevithick's offer to propel the Admiralty by steam-puffers, and ten years more were to pass before they could make up their minds to venture on high-pressure steam from his boilers. The Steam Users' Association are equally hesitating, judging from words just spoken by an engineer, the son of an engineer:-- [Footnote 194: See Trevithick's letter, 10th Jan., 1805, vol. i., p. 324.] [Footnote 195: Ibid., vol. ii., p. 24.]