ON INSTRUMENTS FOR MEASURING THE SrEED OF SHIPS. 217 



namely, a forward motion under the crest and a backward motion under 

 the trough, tbough these local motions clearly cannot be supposed in the 

 leading members of the system to penetrate the water so deeply as in 

 properly formed ocean waves. 



(G) The transverse waves are necessarily much longer (measured 

 normally to the line of crest) than the diverging waves, and their proper- 

 motions therefore probably penetrate much the more deeply of the two. 

 (In ocean waves the wave motion at a depth equal to about one-ninth the 

 wave depth is held to be about half that at the surface.) 



(7) The diverging wave system is well marked at moderate speeds, at 

 which the transverse system scarcely appears. When, however, the speed 

 of the ship becomes high in comparison with her length of entrance, the 

 transverse system comes rapidly into promiuence. 



(8) The lengths of the waves (that is, their distance apart measured 

 normally to the ci'est line) vary as the square of the speed, and as the 

 leading wave crest remains always at the bow of the ship, change of speed 

 changes considerably the fore and aft positions of the subsequent members 

 of the transverse series relatively to fixed points in the side of the ship, 

 this rate of change of position increasing with distance sternwards. 



The general effect of the wave system upon the 'rate ' of a pressure 

 log fixed, near the water surface, amidships in a ship with long parallel 

 side will, then, be somewhat as follows. At moderate speeds at which 

 the transverse wave series does not come into play, the effect of the wave 

 system on the log will be nil, and the : rate ' will be the constant ' rate ' 

 due to the stream line motion at its position. As the speed increases and 

 the transverse waves appear, and lengthen out with increasing speed, the 

 log indication will be alternately increased and diminished relatively to 

 the true speed, according as the speed reached brings trough or crest 

 over it. This kind of result would clearly be almost the most inconvenient 

 imaginable, 1 and though I have supposed an extreme case, the evil would 

 in most cases partake of the character I have sketched. 



The most objectionable feature in this supposed result is clearly due 

 to the change of position of the wave features relatively to the logs which 

 accompanies change of speed, and the magnitude of this evil is clearly 

 lessened generally speaking by putting the log as far forward as possible, 

 and therefore closer to the bow wave, the stationary or datum point of the 

 system, but by so doing it will be rendered more subject to irregularities 

 in its action due to pitching of the vessel. T am inclined to think that 

 underneath the position of the first wave-trough at a little under the full 

 speed of the ship would generally give about the best result, and I con- 

 sider this position (independently of the pitching question) preferable to 

 the back slope of the bow wave, for although the absolute effect of the 

 wave on the indication of the log would be more in the former position 

 than in the latter (in fact on the midslope it would be nil), the change of 

 wave position due to change of speed would produce a more changing 

 effect (and cause a more rapidly changing ' rate ') in the latter position 

 than the former. 



It is of course advisable to put the log as deep as possible, and indeed 



1 It did not indeed appear noticeably in the Iris, although the log was, as has 

 been stated, amidships and quite close to the water surface; but she is a ship with 

 nothing resembling parallel sides, and makes no transverse waves of importance 

 until she comes to making a single wave from end to end, with the trough amid- 

 ships. 



