The syphon must touch the liquid in the (upper) reservoir (the source, where the liquid comes from) in order for it to work. This allows atmospheric pressure to push the liquid up the tube. The top of the tube is where the lowest pressure is, so the liquid will move towards that area in the same way as a barometer. The top of a syphon is the same as the height of a barometer, because they work in the same way. The lower pressure is caused by the liquid falling on the downhill side. A syphon can raise water to a height of 25 feet above the source elevation. New discoveries in Syphon Pump Metering allows for dispensing liquid above the source. United States Patent #5,358,000 was the first to teach this method.