To bring about the accomplishment of a project so extraordinarily broad involves engineering and mechanical conceptions of a high order, and, as we have seen, these have been brought to bear on the subject by Edison, together with an intimate knowledge of compounded materials.

The main features of this invention are easily comprehensible with the aid of the following diagrammatic sectional sketch:

It should be first understood that the above sketch is in broad outline, without elaboration, merely to illustrate the working principle; and while the upright structure on the right is intended to represent a set of molds in position to form a three-story house, with cellar, no regular details of such a building (such as windows, doors, stairways, etc.) are here shown, as they would only tend to complicate an explanation.

It will be noted that there are really two sets of molds, an inside and an outside set, leaving a space between them throughout. Although not shown in the sketch, there is in practice a number of bolts passing through these two sets of molds at various places to hold them together in their relative positions. In the open space between the molds there are placed steel rods for the purpose of reinforcement; while all through the entire structure provision is made for water and steam pipes, gas-pipes and electric-light wires being placed in appropriate positions as the molds are assembled.