Open main menu
This page needs to be proofread.

In a few cases the constants for screw current meters have been

Then for the determination of the constants a and /S in (1), by the method of least squaresa


En2§ .'v - Eninzf

mZn2 - [21:12

mEn'v -211211

m21z2 - [En]2

stream and to check oscillations of the water column. Let the difference of level of a pair of tubes A and B (fig. 145) be taken to be h=kv2/2g, then k may be taken to be a corrective coefficient whose value in well-shaped instruments is very nearly unity. By placing his instrument in front of a boat towed through water Darcy found k=1-034; by placing the instrument in a stream the velocity of which had been ascertained by floats, he found k = I -006; by readings determined by towing them in R. E. Froude's experimental tank in taken in different parts of the section of a canal in whichaknown volume of water was flowing, h


C' D

B i e.. lllffiir


l:f~ li: ' J fl i~ 1

/ .5 nl.. . 11"

-'-1/A Y?


li W



f ?

- xssssv



L, . v " ' "", s- 1 - ' ' // / “, ~ <1 "1 1;;§ ': $"' § / 3.4/§ {¢'Z

e found k=O-993. He believed the first value to be too high in consequence of the disturbance caused

by the boat. The mean of the other two values is almost exactly unity (Recherches hydmuliques, Darcy and Bazin, 1865, p. 63). W. B. Gregory used somewhat differently formed Pitot tubes for which the k = 1 (Am. Soc. Mech. Eng., 1903, 25). T. E. Stanton used a Pitot tube in determining the velocity of an air current, and for his instrument he found k= 1-030 to k= I~032 (“ On the Resistance of Plane Surfaces in a

Current of Air, ” Proc. Inst. Civ. Eng., 1904, 156).

One objection to the Pitot tube in its original form was the great difficulty and inconvenience of reading the height h in the immediate neighbourhood of the stream

surface. This is obviated in the Darcy gauge, which can be removed from the stream to be read.

Fig. 146 shows a Darcy gauge.

It consists of two Pitot tubes

having their mouths at right angles. In the instrument shown, the two tubes, formed of copper in the

lower part, are united into one for strength, and the mouths of the tubes open vertically and horizontally. The upper part of the tubes

is of glass, and they are provided with a brass scale and two verniers b, b. The whole instrument is supported on a vertical rod or small pile AA, the fixing at B permitting the instrument to be adjusted to any height on the rod, and at the same time allowing free rotation, so that it can be held parallel to the current. At c is a two-way cock, which can be opened or closed by cords. If this is shut, the instrument can be lifted out of the stream for reading. The glass tubes are connected at top by a brass fixing, with a stop cock a, and a flexible tube and FIG. 144.

which the resistance of ship models is ascertained. In that case the data are found with exceptional accuracy. § 144. Darcy Gauge or modzjied Pitot Tube.-A very old instrument for measuring velocities, invented by Henri Pitot in 1730 (Hisfoire de l'1-lcadémie des Sciences, 1732, p. 376), consisted simply of a vertical glass tube with a right-angled bend, placed so that its mouth was normal to the direction of flow (fig. 145). The impact of the stream on the mouth of the tube balances a column in the tube, the height of which is approximately h='U2/2g, where v is the velocity

(Vg 51| at the depth x. Placed

li.;;| | l with its mouth parallel % ~:Hi'i l;fL?w ;', 1::; ':, V A: to the stream the water

{ 21.3
§ i;{';§ !» § ii* inside the tube is nearly

“illfi '.l;'1' at the same level as the lt. 5 Il# , ' surface of the stream, V gli" i il | V and turned with the !'**> .li l ' Q mouth down stream, the |§ , 1 [il 1 fluid sinks a depth it .» 1, 1 l Q h' ='v*/2g nearly, though / Q, i' I the tube in that case "" ' '~~ Y interferes with the free A B C flow of the liquid and

FIG 145 sompwhal; modifies éhg

resu t. itot expan e

the mouth of the tube so as to form a funnel or bell mouth. In that case he found by experiment

h = 1 -522/Qg.

But there is more disturbance of the stream. Darcy preferred to make the mouth of the tube very small to avoid interference with the mouthpiece m. The use of this is as follows. If the velocity is required at a point near the surface of the stream, one at least of the water columns would be below the level at which it could be read. It would be in the copper part of the instrument. Suppose then a little air is sucked out by the tube m, and the cock a closed, the two columns will be forced up an amount correspond» ing to the difference between atmospheric pressure and that in the tubes. But the difference of level will remain unaltered. When the velocities to be measured are not very small, this instrument is an admirable one. It requires observation only of a single linear quantity, and does not require any time observation. The law connecting the velocity and the observed height is a rational one, and it is not absolutely necessary to make any experiments on the coefficient of the instrument. If we take 'u=k/ (Qgh), then it appears from Darcy's experiments that for a well-formed instrument k does not sensibly differ from unity. It gives the velocity at a definite point in the stream. The chief difficulty arises from the fact that at any given point in a stream the velocity is not absolutely constant, but varies a little from moment to moment. Darcy in some of his experiments took several readings, and deduced the velocity from the mean of the highest and lowest. § 145. Perradil Hydrodynamometer.-This consists of a frame abcd (fig. 147) placed vertically in the stream, and of a height not less than the stream's depth. The two vertical members of this frame are connected by cross bars, and united above water' by a circular bar, situated in the vertical plane and carrying a horizontal graduated circle ef. This whole system is movable round its axis, being suspended on a pivot at g connected with the fixed support mn. Other horizontal arms serve as guides. The central vertical rod gr forms a torsion rod, being fixed at r to the frame abcd, and,

passing freely upwards through the guides, it carries a horizontal