Page:The Development of Navies During the Last Half-Century.djvu/277

 having surpassed the highest spead attained by the 'Lightning,' the first torpedo boat supplied to the service — of which craft more by-and-by. There was nothing resembling her in the navy with reference to the proportion of midship section to length, the extreme fineness of her entrance and run, and the ratio of her enormous horse power to displacement. It is doubtful if any trials have been more fraught with valuable instruction to marine engineers than those of the 'Iris.' At first she was a great disappointment to all who had in any way been connected with her. It had been calculated that with 7000 horse power she ought to realise something like a speed of 17.5 knots, but with 7500 horse power she only managed 16.6 knots, the revolutions of the engines being ninety-one.

Space does not permit of any detailed description of the numerous experiments which followed, but it is sufficient to say that the screw propellers were found to be vastly too large in diameter, and their friction in the water was thus excessive, that their pitch was increased, and that the 'Iris,' with 7735 horse power, attained a mean speed of 18.572 knots, fully a knot more than the constructive department at the Admiralty ever expected to get out of her. The 'Mercury,' her sister ship, built on the same lines, under an adjoining shed at the same dockyard, and engined by the same firm, was even more successful, attaining a mean speed of 18.87 knots per hour, and this in 1879. It is hardly to be wondered at that the fame of such unparalleled achievements spread all over Europe and excited the