COVERING GLASS ROOFS. 



551 



thought of by self-styled hothouse- 

 builders, who, in too many cases, show as 

 slight an acquaintance with the rules of 

 carpentry as they do of the principles 

 upon which hothouse-building should be 

 based. Indeed, looking at many specimens 

 of their handiwork, one would think 

 their ideas were stereotyped to error. 

 When the parapet walls are built of brick 

 or rubble stone, inch-iron dowels should 

 be built in them, at 10 feet distances, let 

 into the wall not less than 12 inches, and 

 upwards into the wall-plate at least to 

 the extent of half their thickness : by 

 this means the superstructure and the 

 walls will be firmly knit together. If the 

 parapets are of ashlar, the dowels should 

 be sunk 6 inches into the top course. 

 Top wall-plates in lean-to houses have 

 hitherto been little attended to ; they 

 have in general been merely a 4 or 6 

 inch square batten, built into the solid 

 wall at top, and surrounded with bricks 

 or stones and mortar. To these the top 

 ends of the rafters have been nailed or 

 otherwise fastened. 



The ridge beams of span-roofed houses 

 may be regarded as wall-plates, as they 

 answer a similar purpose. Fig. 787 



Fig. 787. 



shows an improved form, with its coping, 

 and the manner in which the top ends 

 of the rafters are secured to it. Such 

 ridge beams are only to be used where 

 the roof is composed of rafters and sashes, 

 the top ones of which are to slide up and 

 down for ventilation. Their form is, 

 however, different when ventilation is 

 effected at that part, and has been else- 

 where explained. 



§ 5. — COVERING THE ROOFS OF GLASS 

 HOUSES AND PITS, FOR THE EXCLU- 

 SION OF COLD OR THE RETENTION OF 

 HEAT. 



The advantages arising from covering 

 glass roofs appear to have been to a cer- 

 tain extent known, and partially acted 

 upon, perhaps as early as the beginning 



of the last century. At all events, speak- 

 ing from our own personal knowledge, we 

 have seen hothouses covered during win- 

 ter fifty years ago ; and, judging from the 

 very complete manner in which the ope- 

 ration was performed, the arrangements 

 preconcerted to carry this into effect leave 

 no doubt on our mind that hothouses 

 were furnished with shutters almost from 

 the period of their introduction into Scot- 

 land. This fact is hot, we presume, un- 

 known to reading gardeners, even although 

 much younger than ourselves, as the case 

 has been published long ago, and not only 

 described and detailed, but also figured 

 in a very copious manner — for example, in 

 "Treatise on several Improvements recent- 

 ly made in Hothouses," by the late J. C. 

 Loudon, published in Edinburgh in 1805. 



We are led to these remarks, as we find, 

 within this year or two, coverings recom- 

 mended as if quite a new feature in horti- 

 culture, while scarcely one of the plans 

 recommended is half so good, efficient, 

 and businesslike as those we have alluded 

 to as being in full operation more than 

 half a century ago. Modern writers re- 

 commend covering the exterior of the 

 house with canvass screens, not one of 

 whom, that we have met with, appearing 

 to have thought how much more effectu- 

 ally frost could be excluded and heat 

 retained by internal coverings or screens. 

 Undoubtedly there are cases — such as low 

 pits, &c. — where this mode could not very 

 conveniently be adopted, but in many 

 others internal coverings could be em- 

 ployed with perfect ease. Night covering 

 on the outside of hothouses is very desir- 

 able during winter, and, if judiciously 

 applied, prevents the excessive dryness in 

 the atmosphere which is so injurious to 

 plants, and can scarcely be avoided when 

 strong fires are applied to resist the cold 

 from without. The temperature of the 

 glass in the roof of a hothouse at night, 

 when fully exposed, is exactly the mean 

 of the external and internal air ; and con- 

 sequently, in very cold weather, and when 

 there is moisture floating in the internal 

 atmosphere, it becomes a great condenser, 

 and dries the air of the house much faster 

 than it would otherwise be. This an outer 

 covering prevents in a great measure, as 

 well as economises fuel, and lessens the 

 evil of too much fire heat. 



Continental cultivators calculate that 



