34 
ON THE RELATIVE MERITS OF IRON AND WOOD 
ROOFS, FOR STOVES, GREENHOUSES, &c., &c. 
BY J. THOMSON, 
LANDSCAPE GARDENER, NURSERYMAN, AND HOTHOUSE Dl^SlGNER, BEULAH SPA, 
CROYDON, SURREY. 
With Practical Observations and Calculations on the Consumption of Fuel and Breakage of 
Glass under both kinds of roofs ^ made during fifteen years' practice as Gardener to the Duke 
of Northumberland at Sion House^ Kew^ and elsewhere : with an account of several 
years' observations made on heating Forcing and other Houses with common Flaes^ 
Steam Boilers^ and with six of the most approved systems of heating by Hot Water. 
Also a description of the Economic Egg-shaped Boiler^ the invention of the writer^ 
which has given general satisfaction by the efficacy and simplicity of its construction^ 
and its economy of Fuel and Labour. 
Sir, — Knowing that a great diversity of opinion exists as to what description 
of materials are of most avail in the construction of roofs for stoves, greenhouses, 
and other buildings intended for the culture of fruits, and of tropical and other 
plants ; and having had for years the management of the extensive ranges of 
glass at Sion House and other places where the lights and rafters were constructed 
both of cast-iron and wood ; I am induced to submit to your consideration the 
result of my practical observations relative to the two descriptions of houses 
under consideration, as a guide to persons who may be inclined to build, but are 
unacquainted with the injurious consequences of ill-constructed hothouses for 
horticultural purposes. Having had fifteen years'* practical experience, and the 
management during that long period of above three thousand running feet of 
glass designed for the culture of fruits and plants, I am enabled to speak with some 
decision on the subject : and there are, I doubt not, hundreds of practical gardeners 
who will confirm the truth of the following observations, and agree with me in the 
decided conviction I entertain of the superiority of wood to iron. I feel fully 
justified indeed in saying, that when the merits of wood and the demerits of iron are 
fully ascertained, the erroneous prejudice in favour of the latter will cease to exist 
in the minds of all candid men who are practically acquainted with the properties 
of the two materials. Every person possessing even a slight knowledge of the 
expansion and contraction of all metallic substances, may form some idea of the 
inevitable expansion of a large iron-roofed house on a hot summer s day, and of its 
unquestionable contraction during a night of severe frost. So powerful have I 
known the action of the sun's rays to prove in expanding the iron rafters and lights 
of a large roof on a hot day, that I have found the strength of two and sometimes 
three men insufiicient to force down the sliding lights for the admission of air. In 
fully equal proportions have I witnessed the contraction of the metal during the 
intensity of winter^ when so large have been the apertures between the rafters and 
