The engines of these vessels are in duplicate. Two screw shafts are placed on each side of the vessel, driven respectively by a high- and a low-pressure turbine; to each of the low-pressure turbine shafts a small reversing turbine is permanently coupled for going astern, the estimated speed astern being fifteen knots and a half, and ahead thirty-five knots; two propellers are placed on each shaft.[G] [G] On her second trial trip the Viper attained a mean speed of 34.8 knots, her fastest trial being over 35 knots, or about 41 statute miles per hour, with an indicated horse power of 11,000. This vessel is of about 350 tons displacement. The latter of these two vessels has commenced her preliminary trials, and has already reached a speed of thirty-two knots. The manipulation of the engines is a comparatively simple matter, as to reverse it is only necessary to close one valve and open another, and, owing to there being no dead centers, small graduations of speed can be easily made. In regard to the general application of turbine machinery to large ships, the conditions appear to be more favorable in the case of the faster class of vessels such as cross-Channel boats, faster passenger vessels, cruisers, and liners; in such vessels the reduction in weight of machinery, as well as economy in the consumption of coal per horse power, are important factors in the case, and in some vessels the absence of vibration, both as regards the comfort of passengers, and in the case of ships of war permitting greater accuracy in sighting of the guns, is a question of first importance. As regards cross-Channel boats, the turbine system presents advantages in speed, absence of vibration, and, owing to the smaller diameter of the propellers, reduced draught. As an instance, a boat of two hundred and seventy feet length, thirty-three feet beam, one thousand tons displacement, and eight feet six inches draught of water could be constructed with spacious accommodation for six hundred passengers, and with machinery developing eighteen thousand horse power; she will have a sea speed of about thirty knots, as compared with the speed of nineteen to twenty-two knots of the present vessels of similar size and accommodation.