This was unfortunate, as it removed the incentive to continued research needed to make the water-tube boiler a really satisfactory steam generator. The Scotts, however, continued to work for the successful application of high pressures, and it was this that brought them into contact with the late Mr. Samson Fox, with whom they were closely identified for many years in connection with the development of the corrugated flue and the cylindrical steam boiler. Opinion being adverse to the water-tube boiler, notwithstanding its acceptance by many foreign Navies, there was a strong agitation fostered by engineers to induce the societies for the registry of shipping, and also the Board of Trade, to increase the ratio of the working to the test, pressure in boilers. The British Admiralty allowed the boiler to be worked up to within 90 lb. of the test pressure, whereas in the Merchant Service the working pressure was limited to one-half of the test pressure. In 1888 the Scotts, being convinced that the Admiralty system afforded quite a satisfactory factor of safety, undertook the experiment of submitting a warship boiler, then being built by them to Admiralty specification, to the highest possible pressure, even up to bursting-point. The boiler ultimately leaked to such an extent, after the pressure had been maintained for a long period at 620 lb. per square inch, that it was not considered necessary to proceed further. The stresses at this stage worked out to 48,130 lb. per square inch; and the result proved that there was some justification for a reduction in the minimum scantlings of the shells of marine boilers to, at least, the scale adopted by the Admiralty.[61] These suggestive experiments were carried out in connection with the boilers constructed in 1888-9 for two war vessels built by the Scotts. These vessels were the _Sparrow_ and the _Thrush_. At the same time, the Scotts engined two other vessels of the same type, constructed at the Royal Dockyards. A view is given on Plate XVII. of the _Thrush_, which was commanded by H.R.H. the Prince of Wales on the North American and West Indian stations in 1891. She was a vessel of composite build, of 805 tons displacement, with machinery of 1200 horse-power, to give a speed of 13 knots; but, as is shown by the illustration, she was fitted as a three-masted schooner, and utilised her sails when the wind was favourable. In this respect, she marks the transition stage between the days of the sailing craft and the modern ship, depending entirely on steam for propulsion. Indication is afforded of the progress towards this transformation by Table III. on the opposite page, which shows the improvement in economy in the machinery of warships at various stages in their development. [Illustration: Plate XVIII. ENGINES OF H.M.S. "THRUSH," 1889.]