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.
Doubtless every person has seen the trunk mystery, the effect of which is as follows: A trunk, mounted upon four legs, is brought out on the stage and proven to be empty by turning it all the way around to show that there is nothing on the back, whereupon pieces of plate glass are placed along the back, sides, and front, the trunk is closed and given a swift turn and then opened, when to the amazement of all, a lady steps out appearing to come from nowhere. The secret of this trick is very simple, and the trunk can be made up very cheaply.
Ill: A Shelf and Panel Set at Right Angles to Form a Place at the Back for the Assistant to Conceal Herself, No Matter Which Way the Trunk is Turned to Face the Audience
In the back of the trunk there is a movable panel with a shelf exactly the same size as the panel attached to its bottom, forming a right angle, the corner of which is hinged to the bottom of the trunk. The back panel can be turned in until it rests on the bottom of the trunk and, when this is done, the shelf part rises and takes its place, making the back of the trunk appear solid.
When the trunk brought out upon the stage, the assistant is crouching on the shelf. The trunk can then be shown empty. This is all very simple until the trunk is turned around when it takes skill not to give the trick away. As soon as the performer starts to turn the trunk around, the assistant shifts her weight on the panel, thus causing it to fall inward and bring the shelf up to make the back appear solid. The assistant is now in the trunk, and the back can be shown clear of any apparatus. When the trunk is turned to the front again, the lady repeats the previous operation in the opposite direction, thus bringing her body to the back of the trunk again.
To make the trick appear more difficult, glass plates are made to insert in the ends, front and back of the trunk. In making the trunk, have the back the same size as the bottom. Fit the piece of glass for the back into a light frame, similar to a window frame. This frame is hinged to the bottom of the trunk and is 1/2 in. smaller all around than the back of the trunk, so that the two pieces of glass can be put in the ends and also allow the back frame and glass to fall flush in the bottom of the trunk. A few rubber bumpers are fastened in the bottom of the trunk to catch the glass without noise as it falls. The best way to work this is for the performer to let the frame down with his right hand while he is closing up the front with his left.
As soon as the trunk is closed, the assistant again shifts her weight to cause the panel to fall in and then the trunk can be turned to show the back, or whirled around and turned to the front again, then opened up, whereupon the assistant steps out, bows to the audience, and leaves the stage.