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.
When cooking certain foods the ordinary cover on a vessel confines too much steam, while if no cover at all is used, too much will escape, hence a cover which is provided with a vent is very desirable. The cover attachment shown in the illustration is evidently well adapted for service on such occasions and can be easily made as follows:
Ill: The Small Triangular Surface Provided with Holes For Releasing Surplus Steam is Covered with a Cap
Lay out a small triangle on some level part of the cover and punch several holes inside of the lines. Cut A cap, A, from a piece of tin to cover the holes. A small projection on the edge of the cap serves to swing it one way or the other, as needed, and it is fastened loosely to the cover with a soft-copper rivet, so that it may be easily opened.