534 



DETAILS OF CONSTRUCTION. 



think, very much depended the equality 

 of one of its surfaces at least. When the 

 glass was sufficiently heated, the edges 

 were gradually separated until it lay flat 

 on the floor of the oven; the workman 

 then flattened the upper surface by pass- 

 ing a wooden block attached to a long 

 handle over it backwards and forwards, 

 until it was flattened to his mind. From 

 this it will readily be seen how very 

 liable the glass was to be of an uneven 

 surface ; and if the glass got heated so 

 much as to become softened, it came out 

 of the oven in very considerable ridges. 

 The least unevenness in the floor of the 

 oven would cause either concave or con- 

 vex undulations, which is the ground- 

 work of one species of lenses ; while the 

 ridges produce those which run in paral- 

 lel lines, on account of their greater 

 thickness, in a way somewhat analogous 

 to that described by Mr Spencer of 

 Bowood, who had recourse to painting 

 his overlaps on the outside, as he found 

 the leaves under the laps to be burnt in 

 continuous lines exactly corresponding 

 with the laps. This also agrees with the 

 observations of Mr Worall in " Garden- 

 ers' Chronicle," who says — " I have four 

 more or less distinctly scorched parallel 

 lines running horizontally from west to 

 east (action p. m.) and separated by dis- 

 tances of 3 to 5 inches, and these all the 

 effects of one square, and nothing more 

 than slight transverse undulation, in 

 conjunction with latitudinal cunifor- 

 mity, appears. This line begins at the 

 side, and runs the whole length of the 

 square." All this is now changed, and 

 in place of the bottom of the kiln be- 

 ing used for the purpose of flattening, 

 polished sandstone, and latterly polished 

 plates of glass, have been used for that 

 purpose ; and for the purpose of dimin- 

 ishing the chance of injury to the glass 

 in removing it from the flattening plate 

 to 'the annealing pit, these plates are 

 placed on frames mounted on wheels, and 

 travelling on iron rails to the place where 

 they are piled on edge to be annealed ; 

 and " cockling " has in consequence 

 almost disappeared. Although it still 

 takes place sometimes from minute par- 

 ticles of dust getting under the glass, that 

 fault, for all practical purposes, may be 

 considered extinct ; but the radical de- 

 fect of sheet glass — the want of parallel- 



ism in the two surfaces, which gives it a 

 blurred appearance even in very good 

 qualities — still remains, and, we are 

 afraid, never can be remedied, because it 

 is a necessary consequence of the method 

 of manufacture. 



Hartley's patent rough-plate glass, one- 

 eighth of an inch in thickness, and weigh- 

 ing nearly 30 ounces to the superficial 

 foot, has been recommended for conserva- 

 tory and hothouse roofs, and is said to be 

 exempt from the bad effects complained 

 of in the common sheet glass. This glass 

 is strongly recommended by Dr Lindley, 

 who says — " This glass is prepared by 

 rolling, which destroys transparency with- 

 out diminishing translucency. It is slightly 

 rough on the surface, which has the 

 important effect of dispersing the sun's 

 rays instead of concentrating them. The 

 roughness, however, renders it less agree- 

 able to the eye, and would make it objec- 

 tionable for the perpendicular sides of 

 glass houses. It renders a shade super- 

 fluous in summer ; and we do not expect 

 that it will intercept any material amount 

 of light in winter. We believe, indeed, 

 that light passes through it as freely, 

 though not so directly, as through trans- 

 parent glass. The mere fact of its render- 

 ing a ' shade ' — one of the worst of the 

 gardener's nuisances — unnecessary, gives 

 the ' patent rough-plate glass ' a great 

 value in our eyes." 



As to its fitness for houses to be early 

 forced, its utility is not as yet fully con- 

 firmed ; and it is even probable that of it 

 there may also be good and bad qualities, 

 as well as of the various kinds of sheet 

 glass. How far it may be beneficial for 

 the roofs of glass houses and pits during 

 winter, when all the light we can possibly 

 obtain in 'a dark and cloudy climate is 

 required, is also as yet not quite fully 

 determined ; but during the heat of 

 summer, in consequence of the subdued 

 light passing through it, we have always 

 found plants thrive under it better than 

 under that which is more transparent. 

 It may be procured not only in large 

 pieces, but also of great thickness and 

 strength, and has been used in plates 

 5J feet long by 4 feet in breadth, and 

 f ths of an inch in thickness, in the gar- 

 den of Josiah Wilson, Esq., Stamford 

 Hill, where it has given the greatest satis- 

 faction. The roof upon which it is used 



