It has been estimated that, on the average, slabs of plate-glass lose one-third of their original weight in the grinding and polishing processes, and it is obvious that the erosion of this great weight of glass must absorb a great amount of mechanical energy, while the cost of the plant and upkeep is proportionately great. Every factor that tends to diminish either the total weight of glass to be removed per square yard of finished plate, or reduces the cost of removal, must be of the utmost importance in this manufacture. The flatness of the plates as they leave the annealing kiln has already been referred to, and the reason why the processes of grinding and polishing have formed the subject for innumerable patents will now be apparent. The very large expansion of the use of plate-glass in modern building construction, together with the steady reduction in the prices of plate, are evidence of the success that has attended the efforts of inventors and manufacturers in this direction. At the present time, plate-glass is manufactured in very large sheets, measuring up to 26 ft. in length by 14 ft. in width, and in thickness varying from 3/16th of an inch up to 1½ in., or more, for special purposes. At the same time the quality of the glass is far higher to-day than it was at earlier times. This high quality chiefly results from more careful choice of raw materials and greater freedom from the defects arising during the melting and refining processes, while a rigid process of inspection is applied to the glass as it comes from the polishing machines. For this purpose the sheets are examined in a darkened room by the aid of a lamp placed in such a way that its oblique rays reveal every minute imperfection of the glass; these imperfections are marked with chalk, and the plate is subsequently cut up so as to avoid the defects that have thus been detected.