218 report— 1879. 



a log near the keel of the ship would be altogether out of the depth of the 

 effect of the divergiug waves, though not of the transverse waves, except 

 perhaps in deep slow ships. 



To rehearse briefly the statements and line of argument of this com- 

 munication. 



(1) The behaviour of the pressure log in H.M.S. Iris showed that a 

 pressure hole in the side of a 1^ inch tube, and 3 inches from the end, 

 gave substantially the same pressure reading as one in the side of a similar 

 tube fitted with a disc at the end, such as that above described as 

 tube A.. 



(2) The Iris pressure log results further show that either a small 

 hole in the end of a tube with a disc such as tube A, when projecting 

 from a ship's side ; or again a small hole in the end of a plain tube, such 

 as tube B, set with the end flush with the ship's side, will furnish a very 

 serviceable ' working zero ' for the pressure column. 



(3) Though the Iris results show that at a distance of 150 feet or so 

 from the bow of a ship a pressure hole more than one foot distant from 

 the ship's side may be accounted as clear from the extreme ardency of 

 the frictional wake, it remains to be tested how far distant from the side 

 a pressure hole need be (at any given distance from the bow) in order to 

 be so far clear from the frictional wake that the instrument may be ac- 

 counted as unaffected in ' rate ' by the degree of cleanness of the skin of 

 the ship. 



(4) There is an absence of direct experimental data generally appli- 

 cable to all cases, as to the effect on the ' rate ' of a pressure log, of the 

 ' stream line' (or quasi-stream line) motions of the water surrounding 

 the ship. Variability of such ' rate ' with vai'yiug speed is clearly a 

 much greater evil than absolute greatness of such ' rate,' as involving 

 great additional difficulty in correctly ascertaining its value for a given 

 ship at all speeds by means of M.M. ' Trials.' Our general knowledge of the 

 fluid conditions essential to a ship's progress through the water are so far 

 of assistance to us, in the absence of proper experimental data, in that 

 it shows pretty clearly (a) that ' stream line ' motions proper, i.e. those due 

 to motion of a submerged body (or a ship at the surface at low speeds) 

 would cause a ' rate ' of log constant for all speeds ; and that varying 

 speed will produce varying ' rate ' only in virtue of the formation of sur- 

 face waves ; (b) that to avoid the variability of ' rate ' with varying speed, 

 introduced by the wave system, the best plan appears to be to fix the log 

 as deep below the surface as possible ; and, in fore and after position, 

 vertically under the position of the first wave-trough at rather less than 

 the full speed of the ship. — I remain, dear sir, yours faithfully, 



R. Edmund Froude. 



