INVESTIGATION OF FLOW OF WATER IN UNIFORM CHANNELS. 359 
of the surface was observable. In the bottom of the tank was 
fixed a stuffing-box through which passed an iron overflow pipe, 
2” in diameter, filed to a sharp edge at the top, and discharging 
through a canvas hose into a drain below. In each experiment 
the pipe was so arranged, that, with the necessary head on the 
orifice, the water flowed over the overflow under a head of about 
5or6mm. A slight change in the level of the water surface in 
the tank, due to a change of pressure in the service main, has, of 
course, a vastly greater effect on the discharge by the overflow — 
than on the discharge through the orifice, and in this way a very 
convenient automatic regulation of the head is obtained. Any 
variation in the head was found to take place very slowly, rarely 
amounting to 2 mm., and being as a rule very much less. The 
following table, representing the times and the heads observed 
during the course of experiment 7, Series II., and which was not 
exceptional in any way, will serve to illustrate this point. 
Table [. 
Time. | Head. Time. | Head. Time. | Head. Time | Head. 
hrs. min. em. hrs. min. em, hrs. min.| cm. hrs. min.| cm. 
2 48) 82°55 2 50| 82°54 3 04 | 82°53 8 20 | 82°55 
2 3 08 | 82°54 3 21 | 83°55 
2 47) 82-52 2 55 | 82°54 8 12 | 82°55 8. 38 | 82°56 
3 3 
These heads were observed by means of a gauge-glass about 
12 mm. in diameter, and an attached boxwood millimetre scale, 
fixed to the front of the tank. The zero of the scale was above 
the centres of the orifices by the amounts shown in the table below, 
so that the observed heads have to be increased by these amounts, 
in order to obtain the height of the water surface above the 
centres of the orifices. There is probably in addition a small error 
due to capillary action in the gauge-glass, but as the absolute 
eads are not required in the investigation, this is a matter of no 
moment. There are four orifices by one or more of which the 
water may be drawn from the tank, but the largest of the four 
was not required. When not in use the orifices were closed by 
