OF THE CRITICAL VELOCITY OF WATER WITH TEMPERATURE. 
49 
water, and there is a direct steam connection to the boiler room, so that steam can be 
blown directly into the tank. About 8 feet above the base there is an opening, B, 
in the middle of one side, through which the tube C was inserted, its bell-mouth 
being placed at the centre ; and at suitable distances apart pressure chambers, D, were 
formed and connected up to the U-gauge E. The flow of water was controlled by a 
valve, F, and on the prolongation of the pipe a three-way plug valve, G, was inserted, 
so that the watei could run to waste through the H.) or could be discharged by 
the J^ipc J, into the glass flask K. The handle of this tajj w^as provided wuth a 
flexible brass plate, L, in circuit with a chronograph, so that at the middle of its 
swing a circuit was completed by the contact of the brass strip with the pipe, and a 
recoid was obtained on the drum of the chronograph. This latter instrument was 
furnislied with two pens, marking in 0|3posite directions, one ticking seconds and the 
other operating at the beginning and end of each run. This arrangement tends to 
prevent errors in reading. 
The j^ressure chambers were of a sjiecial design and consisted of three sejiarate 
^iieces, the outer one (A) of which coujiles the jiarts B and C togethei", leaving a 
continuous oiDening, D, which may be of any 
required width. In the jiresent case, the two 
sides forming the slit were separated by an 
interval not more than inch, so as to 
prevent, as far as j^ossible, any interference 
with stream-like flow. The part B is recessed 
to form a iiressure chamber, P, connected to the 
gauge by an opening, E. The parts B and C 
are faced so that when drawn togetlier by the 
coupling A, they form a water-tight joint at F, 
and the ends of the are screwed into 
corresponding recesses in B and C. This form of pressure chamber has several 
advantages. The continuous opening gives an accurate mean value of the j^ressure, 
and it can be faced without any burr ; moreover, it may be readily disconnected for 
insjjectioii. 
The i^ipe was of brass, wdthout seam, and 6 feet in length between the pressure 
chambers; its mean diameter was determined by first weighing emjity and then full 
of mercury. The mean diameter thus determined was 0‘3779 inch. 
21 le Measurement of Pressure. 
The accurate determination of the jiressures at the given sections of the pij^e is a 
matter of considerable difficulty, esjiecially at the very low difierences of head 
required for the accurate determination of the slope of j^ressure at velocities below 
the critical velocity. At the higher joressures, a U-tube containing mercury was 
VOL. cci.—A. 
H 
