ON INSTRUMENTS FOR MEASURING THE SPEED OF SHIPS. 259 
out of the lower part of the hollow or chamber in the nozzle, so that 
any bubbles of air which may enter the pressure-pipe become im-- 
pounded in the upper part of the hollow, instead of rising in the 
pressure-pipe. 
jf f. The pressure-pipe. The pipe here shown is the largest of those used, 
G9: 
hh. 
and it is in the lowest possible position. The range of vertical adjust- 
ment is indicated by dotted lines. 
A disk 16 inches in diameter, divided to degrees, and, by a vernier, giving 
tenths of degrees, fixed to the lower of the two collars in which the 
arbor slides—-the collar, namely, in which the arbor is clamped so as to 
define its level. The collar, with the divided disk attached to it, can 
be clamped in any required circumferential position, so as to secure the 
pressure-hole in the required position relatively to the line of motion. 
The glass index-tube, forming a connexion between the pressure-pipe 
and the vacuum-chamber, and provided with scale for reading the. level 
at which the water stands. 
jj. The vacuum-chamber. The required degree of exhaustion is pro- 
kek. 
duced in it by the descending leg of a siphon. It is connected at the 
top with the external air by a vertical india-rubber pipe, and with the 
siphon by a horizontal one, either of which can at pleasure be closed 
air-tight by a clamp. 
The siphon, consisting of a water-chamber and a descending pipe. 
The lower end of this pipe is turned upwards, and is closed by a cork 
while the siphon-chamber is being charged with water through an 
aperture with screwed stopper at the top. When the chamber is fully 
charged, the cork is removed and the water descends, raising the column 
on the other side above the top of the glass: tube. The india-rubber 
connexion with the vacuum-chamber is then closed, and air is admitted 
to the latter through the india-rubber pipe at the top, until the water 
assumes a convenient zero-level. The vacuum-chamber is effectually 
‘jacketed ” with paraffine, so that changes of atmospheric temperature 
do not rapidly affect its interior. 
tl. A plane surface or deck (of thin board, 14 x 19 inches) for restraining 
the surface of the water, so as to prevent the formation of waves and 
the consequent dissipation of pressure, and give additional stiffness to 
the pipe and the arbor which carries it. The deck is securely bracketed 
to a pair of transverse bars, carried by vertical slides which are attached 
to the side-frame of the’truck, and which are firmly clamped when the 
deck is brought to the required level. The brackets which carry the 
deck can be adjusted on transverse bars, and are finally clamped to them 
(like the saddle of the rest on the bed of the lathe) when the deck has 
been duly adjusted to the pipe. The drawing shows the deck as fixed 
at its working immersion. 
As the hole in the deck is necessarily large enough to admit the 
largest pipe, and as it is convenient that the fit should be easy while 
the adjustments are being made, each pipe is provided with a detached 
stout plate through which it slides with a close fit,. and which by a 
suitable arrangement is firmly clamped to the deck and blocked by 
wedges on all sides so as to support the pipe effectually, and, more- 
- over, prevent the admission of air behind the pipe, which at high speeds 
would affect the negative pressure in the rear. To exclude the air with 
still greater certainty, a “wall” of tin encloses the sides and rear of 
the tube above the plate (acting as a water-trap), so that the hole through 
82 
