This section is from the book "Questions And Answers On The Practice And Theory Of Sanitary Plumbing", by R. M. Starbuck. Also available from Amazon: Questions and Answers on the Practice and Theory of Sanitary Plumbing.
The washout water closet, the washdown, syphon, syphon jet, washdown-syphonic action water closets, and lowdown styles.
It should be well trapped; the trap should have a good seal; all surfaces exposed to soil should be well scoured; the flush should be abundant, and noiseless, without undue waste of water. The trap seal should not be hidden, and no mechanical devices should be used with the exception of the tank, in the action of the water closet.
In order that there may be no direct communication between the water closet and the house drinking water, also that a better flush may be supplied.
An explanation will be found under "Syphonage."
Dead ends that are neither supplied with ventilation nor water, are avoided. Sections of pipe that in the old closets between trap and bowl, were unprotected, in the new closets are covered with water; a better flush is secured, better ventilation, and a greater degree of cleanliness, with less odor.
The best plan is to use a brass flange to which the lead bend is soldered, using a rubber gasket between the flange and the base of the closet. In some cities it is allowable to flange the lead bend off on the floor, and to use a ring of putty between the closet and the lead for the joint.
 
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