ON INSTRUMENTS FOR MEASURING THE SPEED OF SHIPS. 213 



The object of the application of a pressure log to the ship was to aid 

 in the determination of the speed during the steam trials ; the special 

 object of the use of two tubes and other principal peculiarities of the 

 arrangement just described was to investigate, if opportunity should 

 serve : (1) the effect of distance of pressure hole from the ship's side (in 

 virtue of difference of position in regard to the frictional wake) ; (2) the 

 extent to which, if at all, the indication given by a pressure hole not far 

 from the end of a plain tube such as tube B, falls short of that given by 

 one in the side of a tube fitted with an end plate or disc, such as tube A ; 

 (3) the relative goodness of the measures of general pressure of water 

 (to wive the zero of the scale of pressure due to speed) respectively 

 afforded by the hole in the end of the disc-tube A when projected far from 

 the ship's side, and that in the end of the plain tube B when drawn in 

 flush with the ship's side. (That some such arrangement as either of 

 these might fulfil the desideratum of a ' working zero ' was suggested in 

 Mr. Froude's Report.) The general method in which the two tubes were 

 to be utilised for the above objects, was to retain one of the two unaltered, 

 as a check on variations of speed or trim of ship, while the desired 

 variations of condition were successively introduced into the other, and 

 the effect noted. 



Experiments of this kind could not however be carried on during the 

 runs on the mile, and as a fact circumstances did not admit of the 

 investigation of the points above described except in a- very hasty and 

 cursory manner during a preliminary run. During the runs on the mile 

 the disc tube alone was used, retained unaltered, at a projection of 15 

 inches (to the outer surface of the disc), the side hole being of course used 

 for the pressure, the end hole for the zero. The trials of the ship in 

 which the log was used were made on two different days, there being on 

 each day 8 runs on the mile, namely, 4 at 16 knots, 2 at 12 knots, and 2 

 at 8 knots. 1 During each run over the mile, the zero of the scale by 

 which the height of column was read off, was held level with the surface 

 of water in the glass communicating with the ' zero ' hole, the height of 

 column in the pressure tube being noted by the scale every 12 seconds; 



The speed of the ship through the wa.ter in any series of measured 

 mile runs in tidal water has of course- to be inferred from the recorded 

 times occupied in running the mile, by the aid (explicit or implicit) of 

 some assumptions in respect to the character of the variations of tide that 

 may have been occurring throughout the series of runs. The method of 

 eliminating tidal errors adopted by Mr. Froude in analysing the results of 

 the measured mile runs of the Iris would take too long to explain here ; 

 suffice it to say that it is clearly as accurate a method as can be found, 

 since it utilises to the full all the obtainable facts. 



The information yielded by the experiments generally may be stated 

 as follows. 



1 . Comparison of speed theoretically appropriate to height of column 

 indicated by log, with speed of ship through water estimated from time 

 running the mile, in the manner above referred to, assigns about "925 

 to 1, as the average ' rate ' of the log (or ratio of speed of ship to 

 apparent speed bv log). The results are not inconsistent with the 

 supposition that the ' rate ' was uniformly -925 at all speeds ; but are 



1 Four runs were also taken on each day at full speed (about I8i knots), but the 

 log could not be used in these. 



