Book III. GLAZING OF HOT-HOUSE ROOFS. 321 



1630. The copper lap {fig. 265. c) is the invention of D. Stewart, and its origin may be 

 recognised in the ess-shaped shred of lead introduced by glaziers between newly glazed 

 panes, to retain them in their places {fig. 265. d). The lap is drawn through graduated 

 moulds till at last it is brought into the shape of the letter ess compressed. It adds 

 greatly to the strength of glazing, by giving each pane a solid firm bearing on the upper 

 and lower edges, and by preventing water from lodging between the panes. Where the 

 sashes are flat, however, it occasions droppings of condensed water on the plants, against 

 which there is a general prejudice among gardeners ; and it has been alleged, that the 

 drip from copper becomes in a few years poisonous from the partial oxidation of the 

 metal. In steep roofs, however, this objection does not hold, and there remains in such 

 cases only the objection of the opacity produced by the lap. It has been used in the 

 large conservatories at East Sheen and Woodlands ; but appears to us much too opaque 

 for hot-house roofs, and only adapted for sky-lights in common buildings. If so much 

 light can be spared as is lost by these laps, it were better to increase the number of sash- 

 bars, by which the panes would be smaller, and consequently stronger and less expensive, 

 and no metallic lap would be wanting. It is now entirely or nearly out of use. 



1631. Fragment glazing {fig. 265. b). This is the primitive mode adopted by nur- 

 serymen and market-gardeners, before it was supposed that the productions raised under 

 glass would pay for any thing better. In steep roofs it answers nearly as well as any 

 other mode in respect to keeping out rain and air, but as a somewhat greater lap is re- 

 quired in these crooked or undulated pieces of glass, a flat roof is liable to be covered by 

 dark lines, formed by the lodging of earthy matter in the laps or interstices. Where the 

 bars are not placed more than six or seven inches asunder, centre from centre, this method 

 is much more economical than any other ; and is therefore useful for such country -nur- 

 serymen or market-gardeners as have not, like the nurserymen of London, the opportu- 

 nity of purchasing the hot-houses of decayed gentlemen or bankrupts ; and consequently 

 are obliged to build and construct every part ab origine. 



1632. In rhomboidal glazing {fig. 265. c), the panes are in the form of rhomboids, the 

 advantage of which is, that the water runs rapidly to the lower angle, and passes off both 

 inside and outside along the bar ; and what is retained by capillary attraction, is alleged 

 to be so small as not to have the power of breaking the glass. 



1633. Perforated shield glazing {fig. 265. d). This is a supposed improvement on the last 

 described mode, which it would be, were it not that by the perforation in the upper part 

 of the shield as it is called, the dexter and sinister chiefs are liable to be broken off; and 

 by the prolonged acumination of its base, it is rendered obnoxious to the same casualty 

 in the nombril point. 



1634. Entire shield glazing {fig. 265. e). This plan has been used by Butler, a London 

 hot-house builder ; but it does not seem either to merit or obtain general adoption. It 

 is difficult, indeed, to conceive what are the arguments in its favor beyond that of 

 strength, with a very great loss of light, which may surely be better obtained by Stewart's 

 lap. 



1635. Curvilinear lap glazing {fig. 265. /). This mode appears, unless on very flat 

 roofs, preferable to the common square mode, because the curve has a tendency to 

 conduct the water to the centre of the pane. If the lap is broad, however, the globule 

 retained there by attraction is situated precisely in the point where it is calculated to do 

 most mischief, being in fact as a power on the end of two levers. When the lap is not 

 more than one sixteenth of an inch, no evil of this sort can happen ; it also happens less 

 frequently for the first few years after puttying the lap, and leaving a small opening in 

 the centre for the water to escape. In time, however, according as the house has been 

 used, the putty begins to decay, it becomes saturated with water, and during frost, when- 

 ever the temperature of the house is inadequate to prevent this water from freezing, the 

 panes are certain of being broken. It can hardly be too often impressed on the mind of 

 the gardener, that puttying or otherwise filling up the lap is in no case requisite, if care 

 be taken in the glazing to use flat glass, and never to let the lap exceed one fourth, or 

 fall short of one sixteenth of an inch. This is now rendered the more easily practicable 

 since the invention of a variety of glass called patent crown glass, and which, purchased 

 in panes fit for hot-houses, is hardly more expensive than the other. It may be added, 

 that taking all circumstances into consideration, and especially that of repairs, die common 

 rectangular pane of a small size is, according to common consent, decidedly the best. 



1636. Reversed curvilinear glazing {fig. 265. g) is a method of throwing the water of 

 condensation to the bars, so as to carry it off by their means, and, if possible, prevent it 

 from dropping in the bouse. 



1637. Anomalous surfaces can only be glazed by throwing the panes into triangles 

 and by no other manner, unless by annealing and bending the glass, because three i?the 

 greatest number of points that will touch a globular surface in one plane. By adopting 

 triangular panes the most singular-shaped roofs may be glazed as perfectly as the simplest 

 iorms of surface. r 



