Owing to the peculiar structure of glass, and its liability to fly or collapse when exposed to sudden changes of temperature, a process of annealing becomes necessary in order to produce a more equal distribution of the tensions throughout the structure of the glass; otherwise, glassware of any thickness would be in such a state of tension as to be extremely liable to fracture when passing through any sudden change in the atmospheric temperature, especially in frosty weather. In this state it is useless or dangerous for general purposes. On this account most glasswares undergo a form of annealing at some time during the process of their manufacture. And in the case of certain goods, such as table glass, lamp glasses, optical glass, etc., special care and time are devoted to this process of annealing. Often in the case of improperly annealed glass, instances are known where its unhomogeneous structure has suddenly given way as the result of derangements set up by internal tension. Friction, or rough handling whilst cleaning, at the ordinary temperature of the atmosphere, is sufficient to cause a rupture. Therefore annealing cannot be too carefully attended to. For annealing the glass manufacturer uses a lehr, which is an arched tunnel with a fully exposed opening at the exit end and partially closed at the entrance end, where the goods are introduced. The lehr is heated at the entrance end to a temperature of about 350° Cent., which temperature is gradually diminished towards the exit end, which is quite cool. The hot end, or entrance, should be constantly at a temperature just short of the actual deformation or softening point of the glass introduced; usually the entrance is in a position near, or convenient to, the glass furnace around which the glass blowers make the goods. In old-fashioned works coal-fired lehrs are used, but they are very unsatisfactory and difficult to regulate. The heat of the lehrs in modern works is maintained and regulated by a series of gas burners situated under the floor of the tunnel or lehr. Along this floor are placed iron trays linked up with each other to form a continually travelling track, which gradually moves towards the cold end of the lehr; these trays are operated by a mechanical jack and gears. As each tray of goods comes out of the cooler end of the lehr, they are taken off and conveyed to the warehouses for cleaning and packing, and the empty tray is sent back to the entrance end to be linked up and refilled again with fresh goods.