Q. Will you explain the theory of Rogers Field's syphon? The obscure point is the pneumatic action involved, by which the sealing of the long leg of the syphon enables a very small stream to charge it, while, were it not so sealed, the liquid would merely dribble down the long leg, and the air would not be expelled. The books give no adequate explanation.

A. The dipping of the lower leg or outlet of the syphon in water facilitates the expulsion of the air in this way: the water, as it falls through the tube, carries with it by friction particles of air, just as a stream poured from a pitcher into a basin will carry bubbles down under the surface of the water into which it falls. The sealing of the end of the tube prevents any of the air from getting back after once being carried out, so that a continuation of the process for a few minutes generally takes out air enough to set the syphon in action: Any contrivance by which the water may be made to fall into the basin at the foot of the outlet-limb freely - i. e., without dribbling down the sides of the tube - will hasten the expulsion of the air by mixing air and water together on the way down. For this reason pure water works better than sewage, for the impurities of the latter soon collect upon any points or edges from which the water may be made to pour clear of the side of the tube, and by forming a lip leads the water back to the side of the tube again. For this reason it is difficult to make the annular syphons work on a small scale with sewage. The free admission of the air to the syphon after the tank is emptied is essential to the success of the device. If not so admitted - i. e., if both ends remain sealed, with the syphon full of water - the tank will not fill again, but the water will pass out through the syphon in driblets, as it comes in. In order to secure the emptying of the syphon, Mr. Field provides either a small syphon to discharge the lower basin after the tank is emptied, or a V-shaped notch in the edge of this basin to allow the water-seal to be broken by draining the water below the end of the outlet-limb. The seal is restored again as soon as the flow becomes enough to fill the notch. In this point, as we understand it, consists Mr. Field's invention, on which the patent for his curved pipe-syphon depends. The choking of this notch or lower syphon by the impurities of sewage forms the chief impediment to the general use of the invention, for it often ceases its automatic action when used for sewage, unless occasionally looked after to clear the rubbish from this point.