ON INSTRUMENTS FOR MEASURING THE SPEED OF SHIPS. 215 



-as can be wished ; and that a small hole in the closed end of another 

 tube, flush with the ship's side, will furnish at any rate a very tolerable 

 working zero. 



The results I have numbered (1) and (2) are, so far as they go, per- 

 tinent to question No. (2), namely, the effect on the indications of the 

 log, produced by the motions caused by the ship in the surrounding 

 water. The information they afford is, however, very slight, as I will 

 proceed to show. 



The motions caused by the ship in the surrounding' water are two- 

 fold, namely, the frictional wake, and the stream line (or quasi- stream 

 line) motions. On the frictional wake part of the ques ion, the experi- 

 ments which were intended to be made in the Iris (as stated in paragraph 4, 

 above), on the effect on the pressure column of distance of the pressure 

 hole from the ship's side, would have given most important information, 

 had the trials of the ship permitted of their being properly carried out. The 

 hasty observations actually made, though sufficient to show that the pres- 

 sure hole, at a distance of a foot from the side, is clear of the extreme 

 ardency of the frictional wake, do not inform us how far it is necessary 

 that the pressure hole should be from the ship's side (at any given dis- 

 tance from the bow of the ship), in order for it to be altogether clear of 

 the wake. Yet if it is not altogether clear of it, the instrument cannot 

 be a permanently satisfactory measure of the speed of a ship which has 

 to remain long afloat, because any fouling of the ship and conseqnent in- 

 crease in skin friction, must increase the speed of frictional wake at given 

 speeds and given distance from the ship's side, and consequently diminish 

 the pressure indicated by the log at given speed. 



With reference to the more complicated question of the effect of the 

 stream line or quasi-stream line motions upon the ' rate ' of a pressure 

 Jog, the measurement of the ' rate ' (as by these experiments) in the 

 special case of the Iris, with the log in the single position in which it was 

 tried, is of small general value. The experiments which Mr. Froude 

 contemplated making in reference to this point, referred to in the 1874 

 Report, were to be of the nature of the application of a pressure log 

 to a great variety of models of ships in a great variety of positions. I 

 do not think that Mr. Froude expected that the information so ob- 

 tained would do more than enable us to so place the log in any given 

 ship, that its 'rate' should be approximately predeterminable and so far 

 uniform for all speeds, that it could be ' rated ' with sufficient accuracy 

 for ordinary sea-going purposes by a few runs at one speed on the mea- 

 sured mile. 



In the absence of these special experiments, our present knowledge of 

 the phenomena attendant on the passage of a ship through water may be 

 brought to bear advantageously on the question of the right place in the 

 ship for a pressure log ; and as I have had the advantage of frequently 

 discussing the subject with Mr. Froude, and had other opportunities for 

 its special study, in connection with our experiments on Resistance, I may 

 be permitted perhaps to add a few words in reference to it. 



At low speeds in smooth water, when the surface of the water sur- 

 rounding a ship is visibly quite undisturbed by waves cansed by her 

 movement, it cannot be doubted that the motions taking place at various 

 points in the surrounding water are simply those which would take place 

 in the same positions relatively to a symmetrical submerged body the 

 lower half of which was similar to the immersed hull of the ship. 



