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Proceedings of the Poyal Society of Edinburgh. [Sess. 
innermost one of hg. 4 are for the same head D. The areas under this 
second series of curves are given in the lower part of Table I., and have 
been plotted in fig. 5 as a curve of discharge for the orifice ; this curve lias 
been prolonged downwards by adding that of fig. 3. The change in 
curvature when the orifice is just completely filled is very noticeable. A 
curve of actual discharges for an aperture 2J inches in diameter, correspond- 
ing with the experimental results given in Table II., is shown also on fig. 5. 
DISCHARGE 
Fig. 5. 
The head could not be carried above 4*49 inches because this represents the 
full supply of water to the apparatus. 
When the water level is in the neighbourhood of the top of the orifice 
there is some irregularity in the flow ; sometimes the hole runs full, and at 
other times there is a small air-space at the top, the surface conditions being 
unstable. It may be well to point out here, though the effect is of most 
importance when the orifice is not completely drowned, that the height of 
the water at the vertical plane of the aperture itself is always less than the 
“ still water head ” measured at some distance away from the point of dis- 
