The stone lintel, with its necessarily limited span, laid down shear strength and bending stress as the criteria of design. Movement of the beam under load and temperature change were of no practical significance. When spans were increased with the invention of arch construction they had to be made of relatively small pieces. In consequence, thermal movement took place by a geometrical change of shape in the structure, with the small displacements allowed for by dry jointing the stones of the arch and loose filling in the spandrels. When one thinks that such principles were in use for arch construction right up to the nineteenth century, it is realized that the problem of the mechanical engineering treatments required for movement is relatively new.