6i4 THE POPULAR SCIENCE MONTHLY. 



men"' ends with the production of the rough plate. Its next 

 guardians bear the name of " grinders." These confer upon the 

 glass the property of transparency. The grinding is accomplished 

 by means of rotary grinding machines, the abrading material 

 being common river-sand, dredged from the Alleghany. Several 

 million bushels are annually required for this purpose. The 

 plates are firmly fixed on rotary platforms by means of plaster of 

 Paris, and rotating disks are so arranged that they cover the entire 

 surface of the plate at each rotation of the platform. Small jets 

 of water keep the grinding-sand constantly wet. But such treat- 

 ment only removes the rough exterior ; the smoothing is accom- 

 plished by means of emery, finer and finer grades being used as 

 the process proceeds. The final polish is given by means of rouge 

 (carefully calcined sulphate of iron), which leaves the glass per- 

 fectly smooth and ready for use. 



Many doubtless remember the time — not so very long distant 

 — when such a thing as American plate glass was totally un- 

 known. It all came from France. But we have discovered — much 

 to our satisfaction — that quite as good plate glass can be made at 

 home as can be brought from across the water. Some, not as cau- 

 tious as ourselves, say that the home product is the superior. 

 Certainly the demand for it increases about as rapidly as new 

 factories can be built to supply it. The joint product of the two 

 Creighton plants is nearly two hundred and fifty thousand square 

 feet per month, or about seventy acres of plate glass a year ! It 

 takes some eleven hundred hands to turn out such a product as 

 this, and its value is reckoned in the hundred thousands. Nat- 

 ural gas is used everywhere throughout both works, displacing, 

 perhaps six thousand bushels of coal daily. Among other duties, 

 it supplies steam for engines of probably not less than three 

 thousand aggregate horse-power. The new factory, some miles to 

 the east of Creighton, will have a capacity, when completed, of 

 three hundred thousand square feet of glass a month. We should 

 hesitate to introduce so many figures, remembering the general 

 aversion to statistics ; but they will present, better than anything 

 else, a just conception of the magnitude of the operations con- 

 nected with a large factory, and will perhaps dispel the notion — 

 if such exist — that we are still largely dependent upon French 

 dexterity for our supply of plate glass. 



Such, in brief, is an outline of the three processes by which a 

 pane of glass may be produced. Each day it becomes more per- 

 fect, until now there seems little further to hope for, unless it be 

 that the glass might lose some of its readiness to break into pieces 

 on the least provocation. Our windows are already as large as we 

 care to have them, and so clear that, every once in a while, some 

 unlucky soul ignores the fact that the window has any glass in it. 



