[23] From experimental data obtained by Mr Froude, this correction can be made with certainty. The reasons for it may be explained as follows:—If an extremely thin short plane is drawn through the water it meets a certain resistance due entirely to surface-friction; that is, supposing the plane to be thin enough to eliminate wave-making and eddy-making. If the length of the plane is doubled while the depth is kept the same, the resistance at the same speed is not, as might at first appear to be the case, doubled accordingly. Owing to the friction of (say) the first half of the plane, the water is made to partake of the motion of the plane, so that the second half of the length, rubbing not against stationary water, but against water partially moving in its own direction, does not experience so much resistance from it. Adding a third equal length, it would have less surface friction than the second, and so on to infinity. [24] See papers by Mr Mansel, enumerated in list at end of chapter. [25] For description of apparatus, see Trans. Inst. Mechanical Engineers, 1877. [26] A body which shortly afterwards joined with a kindred society in forming the “Institution of Engineers and Shipbuilders in Scotland,” hereafter noticed. [27] Following the methods laid down in the Treatise on Shipbuilding, edited by Prof. Rankine, Mr John Inglis, Pointhouse, instituted calculations in 1873 of the longitudinal strains of two steamers built by his firm, the form of the waves being assumed trochoidal. The result of these calculations—which, under Mr Inglis’ directions, were got out by Mr G. L. Watson, subsequently distinguished as a yacht designer, and then in the employ of Messrs Inglis—appeared in the form of curves of hogging moments in _Engineering_ for 1st May, 1874. Mr Inglis found that entering upon the work of calculation had a very decided effect in giving him clearer ideas of how distribution of weight and buoyancy affected the structure of a vessel. [28] The substance of this paper is contained in a series of three articles on the Strength and Strains of Ships given in “Naval Science” (vol. i. and ii., 1872-3), a high-class journal ably edited by Sir E. J. Reed, but unfortunately abandoned after the fourth year of publication. [29] It should be stated that under certain circumstances of lading and support the value assigned by Mr John for the maximum bending moment may be exceeded in merchant vessels, and that in some special classes of ships—particularly light-draught vessels in certain circumstances of lading and support—the sagging moment may prove of most consequence. Instances are indeed on record of light-draught vessels giving way completely under the excessive sagging strain brought upon them at sea.