112 GLASS. 



a quarter of an inch, but where the panes or pieces of glass are 

 not above five inches wide, one eighth of an inch is sufficient. 

 Half an inch in roof-sashes, unless they are placed at an angle 

 of not less than 45, is almost sure to produce breakage, except- 

 ing the temperature within be kept sufficiently high to prevent 

 the water retained between the panes from freezing. 



Broad laps are objectionable, also, on other accounts ; for the 

 broader the lap the sooner it fills with earthy matter, forming 

 an opaque space, and these spaces are so numerous as to have 

 a very considerable effect upon the transparency of the roof, 

 which is injurious by excluding the light, and is also unsightly 

 in appearance. It may be puttied, but its opacity is the same, 

 and its appearance no better than if filled with dirt. Where 

 the lap is not more than one fourth of an inch, it may be puttied 

 without any very disagreeable effect, but if the glass be per- 

 fectly smooth in the edges, puttying is useless, and the glass is 

 better without it. 



The most approved practice as to the laps, whether in roofs 

 or common sashes, is, to make the breadth of the lap equal to 

 the thickness of the glass, leaving it entirely without putty. 

 But it is extremely difficult to get glaziers to attend to this, and 

 it can only be obtained by employing good workmen, and keep- 

 ing strict supervision over the work. This is not only the most 

 elegant of all modes of glazing, but the safest for the glass, 

 which, as we have observed, is seldom broken by any other nat- 

 ural means but the expansion of frozen water retained between 

 the laps. This mode is also by far the easiest to repair, and is 

 more durable than any method of filling the laps with putty, or 

 with lead. 



There are various other modes of glazing, as the lead and 

 oopper-lap methods, which, however, are so very objectionable 

 as to be unworthy of occupying space in our description. The 

 methods of shield glazing are equally objectionable, and little 

 used. Curvilinear glazing has been used somewhat extensively, 

 and is, in the opinion of some men of undoubted skill, superior 

 to the other methods already spoken of. 



Curvilinear lap-glazing appears preferable to the square mode, 

 for various reasons, one of which is, that the curve has a ten- 



