A Philadelphia apprentice asks:

"Will you tell me what is the trouble with the job shown in this sketch? A horizontal 1½ - inch iron pipe A A runs from the pump to the lower tank, with a branch B leading off to the upper tank and a branch C acting as an overflow from the higher to the lower tank. The difference in elevation between the tanks is 15 feet and they are about 300 feet apart. When the stop-cock D is closed and the pump started up, water refuses to overflow from the higher to the lower tank, but when the cock D is open the pump forces water through all right."

[It is probable that air collects in the short horizontal overflow pipe. The whole difficulty can undoubtedly be removed by putting in a T instead of the elbow E, and running a piece of pipe with an open end about a foot above the top of the tank. The reason the water does not flow through the pipe C is that you cannot force water into a space already occupied by air, as in this pipe C. You must arrange for the air to escape, which it could not do with the bend at E as shown in your sketch.]