A large part of this efficiency was due to the boilers, which were of the Rowan water-tube type, and are illustrated on the opposite page. They had square vertical water-tubes, and through each of these there passed four hot-gas tubes. They evaporated 11 lb. of water per pound of coal, which was 30 per cent. higher than was attained with the best marine boilers of those days. The coal consumption at sea was about 1.86 lb. per indicated horse-power per hour. Unfortunately, there soon developed small holes in the boiler-tubes, owing to erosion of the external surface, probably the consequence of the chemical action set up by the steam for cleaning the tubes mixing with the soot and other deposit.[46] Although for this reason this early water-tube boiler did not succeed, there is no doubt that the performances suggested improvements which have since brought complete success to this system of boiler. At the same time, the efficiency of high steam pressures was completely established and resulted in very considerable progress in the size and power of steamships. [Illustration: A PIONEER IN WATER-TUBE BOILERS.] Another innovation which suggested future developments was the fitting at the base of the funnel in the _Thetis_ of a series of water-tubes for the purpose of utilising the waste heat from the boilers to evaporate water for subsequent condensation to make up the boiler feed. The time was not ripe for such a utilisation of the waste gases--the heat was insufficient to generate the required steam--but now various schemes are applied for absorbing the waste heat in the uptake to heat air for furnace draught and to superheat steam. A number of water-tube boilers were made, and a set was fitted into a corvette built for the French Navy. This vessel, completed in the early 'sixties, was the first ship in the French fleet to be driven by compound engines, and will fall to be described with other vessels in our next Chapter, dealing with the work of a century for the Navy. Perhaps the most significant indication of the success of the Scott compound engine is found in the results of its application to the early Holt steamers. Alfred Holt commenced trading with the West Indies in 1855, while his brother, George Holt, became associated with Lamport in the River Plate trade in 1865. Both lines continue among the most successful in British shipping. The Holt steam line to China was commenced in 1865, and was the only one _viâ_ the Cape of Good Hope which proved at once successful. Built and engined by the Scotts, the early Holt liners, starting from Liverpool, never stopped till they reached Mauritius, a distance of 8500 miles, being under steam the whole way, a feat until then considered impossible.[47] Thence the vessels proceeded to Penang, Singapore, Hong Kong, and Shanghai. Unaided by any Government grants, they performed this long voyage with great regularity. [Illustration: Plate XI. HIGH-PRESSURE MACHINERY IN THE "THETIS."]