216 REPORT— 1879. 



It is an accepted proposition of the stream line theory that the stream 

 lines surronnding a submerged body are similar in character at all speeds, 

 the speed of stream at any one point in tbe system bearing at all speeds 

 the same proportion to the speed of the submerged body. We may as- 

 sume then that as with a submerged body so also with a ship moving at 

 the surface of water, the speed of stream at any given point with refer- 

 ence to the ship (except in the purely frictional wake perhaps) will be 

 proportional to the ship's speed, at all speeds up to that at which sensible 

 waves commence to be formed. This would, moreover, continue to be 

 the case at higher speeds could the water surface be forcibly kept level 

 by a water deck (for instance) surrounding the ship in all directions, at 

 the level of her water-line. 



Put then a pressure log where you will, its ' rate ' will under the sup- 

 posed conditions be constant for all speeds. 



In the actual case of a ship without the imaginary water deck, the 

 ' rate' will be in the same manner constant at all low speeds, and will at 

 higher speeds vary with varying speed only in virtue of the introduction 

 of the new set of fluid motions appropriate to the wave system which 

 begins to accompany the ship at higher speeds, and which essentially varies 

 in its character with varying speed. 



The predominant characteristics of the wave system which thus comes 

 into play at the higher speeds may be roughly noted, so far as pertinent 

 to the pressure log question, as- follows : — ■ 



(1) The wave system may be divided into two distinct series, the 

 transverse and the diverging, in the former of which the line of crest is 

 nearly square to the line of motion, in the latter trailing backwards at an 

 angle of forty or fifty degrees. 



(2) The waves (of the diverging series particularly) are comparatively 

 short along the line of crest, and die away gently into the level water a& 

 the ends. 



(3) Each series of waves is a continuous series, which, though it has 

 an abrupt commencement at the bow of the ship, 1, has no definite termi- 

 nation, but extends away backwards wave behind wave, the waves only 

 very gradually diminishing in height as they lengthen along the crest. 



(4) In the transverse series the waves are placed directly one behind 

 the other, or nearly so, so that in a ship with very long parallel sides the 

 crests of the waves may be seen in cross section against the side repeated 

 one after the other. In the diverging series, on the contrary, a line drawn 

 from the highest point of each wave crest to the highest point of the next, 

 and from that to the next, and so on, intersects the lines of the crests at 

 an angle much sharper than the angle contained between the lines of the 

 crests and the line of motion of the vessel. The consequence of this 

 arrangement is that none of the diverging series of waves touch the side 

 of the ship at all, with the exception of the first member of the series^ 

 which has its highest point near the stem of the ship (and in some cases 

 the second member also in a very small degree). 



(5) The system of proper local motions of the water composing the 

 waves probably resembles that recognised as appropriate to ocean waves, 



1 I am speaking here only of waves originating at the entrance of the ship, and 

 which are the only ones very important in reference to the pressure log question ; but 

 it is worth noting that a very similar set of waves, of both transverse and diverging 

 character, originate at the run of the ship also, the two sets of transverse waves (i.e. 

 the bow set and the stern set) becoming fused into one joint series. 



