This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
This funnel-filling filter automatically prevents the solution from running over if the filtering is slow or the filter substance becomes dogged. The upper inverted bottle holds the solution to be filtered, the cork being fitted with a glass tube as shown, and when in use the cork is forced into the neck of the bottle so that no air can enter between it and the glass. The support for holding the bottles has two brackets, one to fit the neck of the upper bottle and the other used as a shelf for the receiving bottle. In operation, the solution runs from the upper bottle into the funnel, holding the filter paper, but it cannot fill the funnel completely, because the end of the glass tube is lower than the edge of the funnel, and as soon as the liquid in the funnel covers the end of the tube, all inflow of air into the upper bottle is stopped, and, thereby, further flow of the solution into the funnel prevented, until enough has filtered through to uncover the end of the tube and thus permit air to again enter the upper bottle. - Contributed by G. Simons, Chicago.