258 REPORT—1874, 
(1) A covered tank or water-space, 278 feet long in all, about 228 feet 
of this being available for the run. The water is 36 feet wide at 
the surface and 10 feet deep. 
(2) A railway suspended from the framed roof, dead straight and dead 
level, at a height of 19 inches above the water, the space between 
the rails being quite clear, and the rails being traversed by an end- 
less wire rope. 
(3) A small double-cylinder engine to drive the truck, fitted with a special 
governor, and capable of assigning to the truck a series of definite 
steady speeds (if required, indeed, any definite steady speed) between 
100 feet per minute (about 1 knot) and 900 feet per minute (or about 
9 knots). 
The above-named elements are also the fundamental parts of the apparatus 
used in the experiments which I am carrying out for the Admiralty in the 
investigation of the resistances of ship-models of various forms at various 
speeds. 
For the purpose of the present experiments, there was attached to the 
truck an additional apparatus, represented in Plate XIII. 
It may be serviceable to observe at starting that, with a view to many 
(perhaps sufficiently obvious) points of convenience, the principle adopted 
in the arrangement of the pressure-gauge is one in virtue of which it might 
be termed a “ sympiezometer ”—the variations of pressure to be recorded 
being, however, not those of the atmosphere, but those of the pressure of 
the water on the open end of the instrument, that is to say, on the pressure- 
hole. It is true that were the pressure of the atmosphere to vary during any 
individual “run,” that variation would enter into the result; but this is 
a condition which, because of its inevitably infinitesimal character, may be 
safely left out of the account. 
The following references will assist in explaining the arrangement. 
Fig. 1 (Plate XIII). 
A A, A’ A’, Longitudinal timbers of the truck-frame. 
B. Transverse timber of truck-frame. 
aa, A stout standard, bolted to the main cross bar. 
bb, A shallow headstock (as it may be called) like that of a lathe, securely 
screwed to the foot of aa. 
ec, A vertical cylindrical steel arbor, which is capable of sliding vertically 
through a pair of collars which revolve (without endways-motion) in 
the bearings afforded by the headstock. The arbor can be clamped to 
the lower of these collars by a pinching-screw at any level which its 
length permits—that is to say, with a travel of 10 inches. 
dd. A sort of “chuck” or screwed hollow nozzle, to which the various 
pressure-pipes used in the experiments are fixed by a union collar, so as 
to be thus carried concentrically by the arbor. As the first step in filling 
the system with water, the air which this chuck contains is wholly 
exhausted by a mouth-pipe which leads out of the highest part of the 
interior. 
ee, An india-rubber pipe which conveys the water to the indicating part of 
the apparatus. This pipe is long enough to allow the arbor to be ad- 
jJusted vertically (so as to vary the depth of immersion of the pressure- 
hole) and circumferentially (so as to allow the hole to be presented in 
any required direction relative to the line of motion). The pipe leads 
