As soon as the supply basin surface falls, the opening of the aeration tube becomes uncovered and air is drawn into the syphon. But until the surface has fallen some centimetres the supply of air is not sufficient to cause the syphon action to stop completely, and thus the escape varies gradually from the maximum discharge to zero as the water surface falls a few centimetres till it reaches its original level. In certain cases it is possible to do without the aeration tube, especially when the fall used in the syphon is not great and when it is possible to arrange matters so that the velocity of the water flowing past in front of the syphon is small. The syphon with a width of 3 metres escapes 8 cubic metres per sec. of water. APPENDIX G. GAUGES. (See Chap. III., Arts. 2 and 3; also see _Hydraulics_, Chap. VIII., Art. 5, and Appendix H.) 1. The gauge should be placed on that bank and facing in that direction which enables it to be most conveniently read by the gauge reader and by officials passing the place. 2. The gauge should be of enamelled iron secured by copper screws to a post of squared and seasoned wood which is either driven beforehand[64] into the channel or spiked to a masonry work. Even in the deepest channel a long enough post can be arranged for. A masonry pillar is not necessary. The post may be rectangular in cross-section, with upstream and downstream edges cut sharp. This prevents, or greatly reduces, the heaping up of water at the upstream side and the formation of a hollow downstream. If the “Ward” gauge of two vertical planks is used, the planks should meet at an acute angle, not a right angle, and not be wider than 7 inches each. [64] Driving after the gauge is attached may loosen or break the screws. 3. The top of the gauge should be slightly above the highest probable water-level. The post should extend up to the top of the gauge. 4. If ever the graded bed of the channel is altered the zero of the gauge should be altered. There may be some risk of confusion at first, but it can be avoided by exercising due care and making notes. The levels of the old and new zeros should be recorded. 5. A gauge at a distance from the bank is objectionable. It collects jungle, cannot be properly read, and is liable to be damaged by floating logs or boats. A gauge should be as near as possible to one bank or the other. If the bank is vertical, the gauge should be quite close to it. If, owing to silt deposit, the gauge is dry at low supply, the deposit can be removed by the gauge reader.