ON INSTRUMENTS FOR MEASURING THE SPEED OF SHIPS. 257 
z; inch diameter is about 43°; and that of a tube 3 inch diameter, with 
a hole 4, inch diameter, is about 45°°5. 
The position of the neutral point was also in these experiments sensibly 
affected by some unknown condition, dependent apparently upon the degree 
of projection of the tube into the fluid, and which I think may possibly have 
been of the nature of a vibration of the tube. Of this I will here merely 
say further that it prevented a precise determination of the degree to which 
the neutral angle is affected by close proximity of the hole to the end of the 
tube. 
(3) The maximum positive pressure (which was obtained, of course, with 
the hole pointing directly in the line of motion) falls slightly short of that 
theoretically due to the speed, and is apparently unaffected either by the size 
of the tube or of the hole init. It appears also to be unaffected by the 
above-mentioned unknown condition, being practically identical under all 
conditions, except when the hole approaches close to the end of the tube 
(within, say, a distance equal to the diameter of the tube), in which case the 
pressure is found to diminish. 
(4) For some distance on either side of the neutral point the pressure 
decreases nearly uniformly, with uniform increments in the angular departure 
of the hole from the line of motion. The rate of decrease is about -04 of the 
maximum positive pressure for every degree of angle. At angles of more 
than 50° the column was always unsteady, and it was impossible to obtain 
accurate measures of it; but the observations show consistently a maxi- 
mum of negative pressure at somewhere about 70°, and then a decrease 
of between one third and one half of the maximum negative pressure 
between 70° and 90°. From 90° to 180° the negative pressure remains about 
uniform*. 
The amounts of these negative pressures, besides being, as already men- 
tioned, rather indefinite in consequence of the fluctuations of the column, 
are sensibly affected by the unknown condition already referred to, and 
therefore it is impossible to speak positively as to their absolute amount. 
(5) A hole in the stopped end, instead of in the side, of the pressure- 
tube (the tube being set as in the experiment for side pressure) gives a con- 
siderable negative pressure, varying in amount according to the position of 
the hole in the disk which closes the end of the tube. In the case tried, 
the tube was 1-1 inch external diameter, the hole was ,4, diameter, and 
was eccentric in the disk by about half the radius of the tube. It was 
tried at a speed of 6 feet per second, corresponding with a direct pressure of 
-56 foot; and the negative pressure recorded when the hole was nearest the 
forward edge was ‘64 foot. When it was 180° from this position (7. ¢. nearest 
to the rearward edge) the negative pressure was ‘29 foot; and this appeared 
to be the position of minimum negative pressure. The maximum negative 
pressure observed was 67 foot, and was at 45° from the foremost position. 
At 90° it was -64 foot, and at 135° was -41 foot. 
I proceed to describe the principal features of the apparatus, and the mode 
of trying the experiments. 
The fundamental parts are as follows :— 
* The diagram, Plate XIV., shows the pressure for all angles between 0° and 180? under 
three of the different conditions tried. The curves thus presented, between 0° and the 
neutral angle, somewhat resemble curves of sines. ‘The degree of resemblance is indicated 
by the companion lines shown in fainter dots, and which are true curves of sines. It may 
be observed that the wider the neutral angle the greater is the departure from the com- 
panion curve. 
1874. 8 
