all times. A refinement was the guide frame gas holder, where the heavy columns were replaced by a lighter and more extensive framework. Vertical girders (standards) were intersected by horizontal girders and cross-braced. This could be bolted onto an underground or above-ground tank. The Cutler patented guide frame dispensed with the horizontal girders using diagonal triangulated framing instead. Cable-guided gas holders, invented by Pease in 1880, had a limited use, but were useful on unstable ground where the rigid systems could buckle and jam the lift. Spiral-guided gas holders were built in the UK from 1890 until 1983. These have no frame, and each lift is guided by the one below, rotating as it goes up as dictated by helical runners. Both telescoping types use the manometric property of water to provide a seal. The whole tank floats in a circular or annular water reservoir, held up by the roughly constant pressure of a varying volume of gas, the pressure determined by the weight of the structure, and the water providing the seal for the gas within the moving walls. Besides storing the gas, the tank's design serves to establish the pressure of the gas system. With telescoping (multiple-lift) tanks, the innermost tank has an approximately lip around the outside of the bottom edge, called a cup, which picks up water as it rises above the reservoir water level. This immediately engages a downward lip on the inner rim of the next outer lift, called a dip or grip, and as this grip sinks into the cup, it preserves the water seal as the inner tank continues to rise until the grip grounds on the cup, whereupon further injection of gas will start to raise that lift as well. Holders were built with as many as four lifts. An extra flying lift could be retrofitted into column or frame gas holders. This was an additional inner tank that extended above the standards, when the infrastructure would support the extra shear forces and weight. Though not exclusively, spiral guides were used. Dry-seal-type gas holder Dry-seal gas holders have a static cylindrical shell, within which a piston rises and falls. As it moves, a grease seal, tar/oil seal or a sealing membrane which is rolled out and in from the piston keeps the gas from escaping. The MAN (Maschinenfabrik Augsburg-Nürnberg AG) was introduced in 1915: it was polygonal and used a tar/oil