A Treatise On The Principles And Practice Of Harbour Engineering

124 HARBOUR ENGINEERING. Section. Fig. 102. —Stevenson’s Wave-stroke Dynamometer. places al. uncertain intervals. The location of the dynamometer may or may not coincide with these places ; in any case, it is a matter of mere hazard and surmise. Yet, imperfectly as they realise ideal conditions, instruments of this class are as yet the only available means of obtaining practical data in regard to the force of waves. Stevenson’s apparatus is perhaps the best known. It is illustrated in Hg. 102. Several im- provements have been contrived since it was first designed, but in principle it consists of a flat disc, perpendicular to which, and behind it, are arranged four rods passing through a firmly fixed cylinder. The disc is set fronting the sea, and when it is struck by a wave the rods are forced back simultaneously through the cylinder, thereby extending a spring connected with the front of the latter. On each rod is a leathern ring, which, prior to movement, is in contact with the back plate of the cylinder. The passage of the rods through this plate is unrestrained ; but the rings cannot pass, and so they are forced along the rods. When the latter resume their original position under the recoil of the spring, the distance travelled by the rings is a measure of the intensity of the blow. An instrument on these lines, but with special features, was constructed a short time back by Messrs W. H. Bailey and Co., Ltd., of Manchester, for use on the coast of Japan. It is illustrated in fig. 103. The principal modihcation consists in placing the instrument on trunnions with a swivel base-plate, so that it may be adjusted both horizontally and vertically to any desired angle. A pencil attached to the index-rod and a revolving drum, enable the record to be kept graphically over a continuons period. The calibration of these instruments is effected in the same way as ordinary spring balances; that is, by the imposition of dead loads. This method is open to the objection already stated, that statical pressure is quite a different thing from dynamical force, and a more appropriate system would be to calibrate by means of falling weights in units of kinetic energy. Yet, even then, there would be the difficulty of the conversion of these last into their statical équivalents. No satisfactory solution has yet been put forward.