214 BEPOET— 1879. 



more consistent with the supposition of a variation of rate at different 

 speeds from about - 91 to "93. The impossibility of making a more definite 

 statement than this is an example of the impossibility of ' rating ' a log 

 very correctly for different speeds, by runs on the measured mile in a 

 tideway, except by means of a great number of runs. 



2. The instrument did not appear very sensitive to the effect of varia- 

 tions in distance of pressure aperture from the ship's side, at any rate 

 wben such distance exceeded one foot. 



3. The pressure hole in the side of the plain tube B gave little if at 

 all less height of column than that in the disc tube A. (This agrees 

 with proposition (3), page 257, in the 1874 Report.) 



4. The ' zero ' holes in the ends of tubes A and B both gave the same 

 height of column when tube B was, not flash with the ship's side, but 

 projecting about 2 inches ; the column given by tube B when flush with 

 the side being the greater of the two by perhaps 2 per cent, of the whole 

 head due to the speed of the ship at the time. There was no means of 

 testing which of these two ' zero ' holes (or whether either) gave the 

 correct zero of pressure. As speed increased both sank somewhat, rela- 

 tively to a fixed point in the ship, but whether more or less than the 

 water outside the ship was not ascertained. I am myself doubtful whether 

 the excess of pressure given by the zero hole in tube B, when flush with 

 the ship's side, relatively to that given by the zero hole of the disc-tube 

 A, is a genuine excess of pressure on the former due to some action of 

 the frictional eddies, or a genuine defect of pressure on the latter due to 

 the tube being possibly not dead square to the side (the disc consequently 

 moving somewhat obliquely through the water). An error of squareness 

 of the tube to the ship's side of 1° (pointing sternward) would, perhaps, 

 have been competent to produce the observed effect. 



Now, as to the information afforded by these results on the points 

 suggested by the 1874 Report, as needing solution. 



The treatment of the subject in that Report suggests a division of the 

 question into two, namely (1), the operation of the pressure log regarded 

 simply as a measure of its own speed in reference to the water it passes 

 through (the foremost difficulty here involved being that of establishing 

 a 'working zero ' for the instrument) ; (2), in what way its operation as 

 an independent measure of the ship's speed through the water is affected 

 by the motions impressed by the passage of the ship on the fluid she dis- 

 places ; in other words, by the difference between the speed of the in- 

 strument through the water it meets with and the speed of the ship in 

 reference to the surrounding ocean. 



The Report described a series of experiments which dealt only with 

 question No. (1), and stated that the investigation of question No. (2) 

 might perhaps be introduced as a part of the methodical series of expe- 

 riments on resistance of ships which Mr. Froude was conducting at 

 Torquay. The investigation would certainly be of great value in refer- 

 ence to the general question of resistance of ships ; but the pressure of 

 other work has, I regret to say, prevented its being undertaken. 



Of the results obtained in the Iris, and which I have above enume- 

 rated, Nos. (3) and (4) are pertinent only to question No. (1), namely, 

 the performance of the instrument regarded as a measure of its own 

 speed through the water it meets with. I think they show that a hole 

 iu the side of a tube, as much as two diameters distant from its end, will 

 burnish as good a measure of pressure due to the speed of flow past it 



