"The extra expense, also, of erecting metallic-roofed houses was a consideration. In 1833 I contemplated building a new range of hot-houses; and being desirous of knowing how much they would cost, if erected of metal, a plan of the range was prepared and sent to Birmingham, and another to Sheffield, with a desire to be furnished with estimates for that purpose. The estimate from Birmingham was 1,800_l._; and the other, from Sheffield, was 1,850_l._ These appeared to me such enormous sums, that I at once set about calculating how much the range would cost if built of wood under my own inspection; and the result was, that I was able to complete the whole range, including masonry (which was omitted in the metal estimates), for less than 500_l._ "Besides the extra cost of metallic roofs, we must add the extreme heat of such houses in hot weather, and their coldness in times of frost; the liability to breakage of glass from expansion and contraction of the metal; the very limited duration of the smaller portions, as sash-bars, from corrosion, by exposure to the alternations of heat, cold, and moisture, inseparable from gardening operations, and which could only be prevented by making use of the expensive material, COPPER; and the difficulty, when compared with wood, of repairing any damages, as a wooden roof could at any time be set to rights by a common carpenter. These different items formed in my mind so many objections to its use, and the same disadvantages soon became generally apparent. "It was now thought advisable by some parties that, in order to obviate the many disadvantages in the use of metal, the rafters and frame-work of the sashes ought to be made of wood, and the sash-bars of metal. This plan certainly presented more advantages than the other, yet it was quite obvious that materials so incongruous could never give satisfaction; and accordingly, in a few years, as I had anticipated, the rage for these structures gradually subsided, and the use of wood again became resorted to by most persons, as the best material for horticultural purposes. [Illustration: COMMON MODE OF GLAZING ROOFS.] "In the construction of glass-houses requiring much light, there always appeared to me one important objection, which no person seemed to have taken up or obviated; it was this. In plain lean-to or shed roofs, the morning and evening sun, which is on many accounts of the greatest importance in forcing fruits, presented its direct rays at a low angle, and, consequently, very obliquely to the glass. At those periods most of the rays of light and heat were obstructed by the position of the glass and heavy rafters, so that a considerable portion of time was lost both morning and evening; it consequently became evident that a system by which the glass would be more at right angles to the morning and evening rays of the sun would obviate the difficulty, and remove the obstruction to rays of light entering the house at an early and late hour of the day.