9G0 
MR. O. REYNOLDS ON THE MOTION OF WATER AND OF 
The tubes were fixed to stands with carefully graduated scales behind them, so that 
the height of the mercury or carbon in each limb could be read. It had been antici¬ 
pated that readings taken in this way would be sufficient. But it turned out to be 
desirable to read variations of level of the smallness of t ooo tli of an inch or -j^th of a 
millimetre. 
A species of cathetometer was used. This had been constructed for my experiments 
on Thermal Transpiration, and would read the position of the division surface of two 
fluids to TAooobh inch (Phil. Trans. 1879, p. 789). 
The water was carefully brought into direct connexion with the fluid in the gauge, 
the indiarubber connexions facilitating the removal of all air. 
27. Means adopted in measuring the discharge .—For many reasons it was very 
desirable to measure the rate of discharge in as short a time as possible. 
For this purpose a species of orifice or weir gauge was constructed, consisting of a 
vertical tin cylinder two feet deep, having a flat bottom, being open at the top, with a 
diaphragm consisting of many thicknesses of fine wire gauze about two inches from 
the bottom ; a tube connected the bottom with a vertical glass tube, the height of water 
in which showed the pressure of water on the bottom of the gauze; behind this tube 
was a scale divided so that the divisions were as the square roots of the height. 
Through the thin tin bottom were drilled six holes, one an eighth of an inch diameter, 
one a quarter of an inch, and four of half an inch. 
These holes were closed by corks so that any one or any combination could be used. 
The combinations used were : 
Gauge No. 1. 
No. 2. 
No. 3. 
No. 4. 
No. 5. 
The -g- inch hole alone. 
The \ inch hole alone. 
A inch hole alone. 
Two | inch holes. 
Four inch holes. 
According to experience, the velocity with which water flows from a still vessel 
through a round hole in a thin horizontal plate is very nearly proportional to the area 
of the hole and the square root of the pressure, so that with any particular hole the 
relative quantities of water discharged would be read oft' at the variable height 
gauge. The accuracy of the gauge, as well as the absolute values of the readings, 
was checked by comparing the readings on the gauge with the time taken to fill 
vessels of known capacity. In this way coefficients for each one of the combinations 
1, 2, 3, 4, 5 were obtained as follows :— 
