Abstract: A method for observing a moving web having printed thereon a regularly repeated pattern and control features along the web, comprising mounting two photoelectric cells a fixed distance apart in the direction of movement of the web, the photoelectric cells being adapted simultaneously to scan identical data in different locations on said web, the cells being connected in an electrical circuit which is balanced when they observe identical data and which produces an output when they observe non-identical data due to one cell but not the other observing a control feature.

Abstract: A method of and apparatus for producing continuous surface elements, e.g. for so-called air supported shelters, according to which a tubular foil, preferably of plastic material, is wound in several layers between two drums spaced from each other by a distance substantially equaling half the width of the surface element or integral multiples thereof. Immediately prior to winding the tubular foil between the drums, adhesive is applied to the foil so that the outward side of the tubular foil on a preceding winding adheres along a greater or less path to the inward side of the tubular foil in the following winding.