For a conoidal or bell-mouthed orifice 1 cm. diameter the effect of temperature was greater:—
| Temperature F. | C. |
| 190° | 0.987 |
| 130° | 0.974 |
| 60° | 0.942 |
an increase in velocity of discharge of 4% when the temperature increased 130°.
J. G. Mair repeated these experiments on a much larger scale (Proc. Inst. Civ. Eng. lxxxiv.). For a sharp-edged orifice 21⁄2 in. diameter, with a head of 1.75 ft., the coefficient was 0.604 at 57° and 0.607 at 179° F., a very small difference. With a conoidal orifice the coefficient was 0.961 at 55° and 0.98l at 170° F. The corresponding coefficients of resistance are 0.0828 and 0.0391, showing that the resistance decreases to about half at the higher temperature.
§ 27. Fire Hose Nozzles.—Experiments have been made by J. R. Freeman on the coefficient of discharge from smooth cone nozzles used for fire purposes. The coefficient was found to be 0.983 for 3⁄4-in. nozzle; 0.982 for 7⁄8 in.; 0.972 for 1 in.; 0.976 for 11⁄8 in.; and 0.971 for 11⁄4 in. The nozzles were fixed on a taper play-pipe, and the coefficient includes the resistance of this pipe (Amer. Soc. Civ. Eng. xxi., 1889). Other forms of nozzle were tried such as ring nozzles for which the coefficient was smaller.
IV. THEORY OF THE STEADY MOTION OF FLUIDS.
§ 28. The general equation of the steady motion of a fluid given under Hydrodynamics furnishes immediately three results as to the distribution of pressure in a stream which may here be assumed.
(a) If the motion is rectilinear and uniform, the variation of pressure is the same as in a fluid at rest. In a stream flowing in an open channel, for instance, when the effect of eddies produced by the roughness of the sides is neglected, the pressure at each point is simply the hydrostatic pressure due to the depth below the free surface.
(b) If the velocity of the fluid is very small, the distribution of pressure is approximately the same as in a fluid at rest.
(c) If the fluid molecules take precisely the accelerations which they would have if independent and submitted only to the external forces, the pressure is uniform. Thus in a jet falling freely in the air the pressure throughout any cross section is uniform and equal to the atmospheric pressure.