ON INSTRUMENTS FOR MEASURING THE SPEED OF SHIPS. 263 
are at once obvious: one of the most important of these is that which 
relates to the effect experienced by a pressure-tube when arranged as a log, 
from the stream-line disturbances which the passage of a ship’s hull intro- 
duces into the relative speeds of the water surrounding the various parts of 
the hull. 
It is hoped that this latter investigation, and perhaps all the others that 
are required, may be introduced as part of the series of experiments on 
the forms of ships which I am conducting here for the Admiralty, since the 
two subjects are inherently and closely related to each other, But the 
introduction of the experiments now reported has under present circumstances 
been, in effect, an interruption ; and though the interruption was permitted, 
it has been carried to the full limits of the permission. 
Incomplete as the experiments are, they tend, I fear, to confirm rather 
than to dissipate the difficulties which have to be overcome before the pressure- 
log can be accepted as supplying the greatly desired object, an independent 
and self-justifying measure of a ship’s speed. 
The inventors whose plans have been before the Committee have, I believe, 
felt the difficulties forcibly. Mr. Berthon* and Mr. Napier have indeed 
expressed their belief that it was unsurmounted, perhaps unsurmountable. 
The foremost of the difficulties to be overcome is that of finding a self- 
justifying zero of the pressure-scale. 
This, primd facie, might haye been supplied by either of three condi- 
tions :— 
1) The determination of the position of neutral pressure. 
(2) The determination of the position of maximum negative pressure, and 
_ the ratio of this to the maximum positive pressure. 
(3) The determination of the ratio of the negative pressure, in the region 
: of tolerably uniform negative pressure in the rear of the tube, to 
the maximum positive pressure. 
With regard to the former of these conditions, the present experiments 
show, I think, conclusively that the position of the neutral point is governed 
by conditions which it is difficult to count on with certainty ; or if this diffi- 
culty be surmounted at all, it only can be by much laborious investigation : 
there remains the circumstance that the neutral point is placed exactly where 
the pressure is changing with maximum rapidity in terms of angle of posi- 
tion; so that any small error in taking account of the governing conditions 
will produce the greatest relative amount of error in the working zero from 
which the pressures are counted. 
Thus the very elegant and instructive proposition as to the existence of 
this neutral point at a little over 40° from the line of motion, which 
Mr. Berthon discoyered and determined with approximate exactness, and 
announced long before the promulgation of the doctrine of stream-lines had 
shown that such a point should exist nearly in that position, appears to 
involve special difficulty in its utilization as the basis of a pressure zero. 
And difficulties hardly less serious in amount attach themselves to the 
determination of the two other conditions which have been referred to, though 
it is no doubt true that subsequent examination may determine with 
sufficient exactness the conditions which govern the relation of the negative 
pressure in the rear of the tube, to the positive pressure in front of it, in 
such a manner that the causes of uncertain variation may be excluded, and 
* Mr, Berthon has since informed me that I have rather overstated his opinion on this 
point. 
