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.
About 10 days after setting the incubator one may easily start another hatch by placing more eggs on top of the incubator in the following manner: Make a pad about 1 in. thick of any cotton material and place it on top of the incubator. Cut four pieces of boards, 1 by 4 in., and fit them around the top of the incubator. Nail them together as in making the sides and ends of a box. Pad the inside of this frame about 1 in. thick and tack it on top of the incubator, being careful that none of the material comes too close to the lamp. Place the eggs inside of this tray and cover them with a pad about 3 in. thick. Turn the eggs the same as those on the inside. When the first hatch comes out, place the eggs kept on top in the incubator after having cleaned it with a solution of carbolic acid. - Contributed by Hattie J. Day.