314 



SCIENCE OF GARDENING. 



Part II. 



is impelled by manual labor, or clock or jack machinery, and has been successfully used 

 for ventilating public rooms and churches. 



1600. Soil, it must be obvious, is perfectly within the control of art, which, in fact, 

 can far surpass nature, when increased dimensions of the parts of plants and improved 

 quality of fruit are objects. 



1601. Water is equally at our command with soil : it may be made to pass through the 

 house in a surface rill ; or under the soil in subterraneous channels ; may be retained 

 in a cistern or basin; or introduced in tubes, either to throw' up innumerable jets from 

 the floor, or pour them down from the roof to serve as rain. It may be supplied 

 directly to the roots of plants, without wetting their leaves, in the manner of irrigation ; 

 be stagnated round them, as in natural marshes, or made to ascend as vapor from 

 steam-pipes, by pouring it on flues or hot bodies, or even watering the floor or interior 

 surface of the house. Having ascended and filled the air, it parts with its caloric, and is 

 precipitated on the plants in the form of dew. 



Subsect. 2. Forms of Hot-house Roofs. 



1602. T/ie general form and appearance of the roofs of hot -houses, was, till very lately, 

 that of a glazed shed or lean-to ; differing only in the display of lighter or heavier frame- 

 work or sashes. But Sir George Mackenzie's paper on this subject, and his plan and 

 elevation of a semi-dome (Hort. Trans, vol. ii. p. 175.), have materially altered the 

 opinion of scientific gardeners. Knight made the first observations on this figure. Sir 

 George Mackenzie's plan for forcing-houses, he observes, is extremely interesting ; but 

 contains "some defects which cannot be obviated without deviating from the spherical to 

 the spheroidal form, which Sir George states to be objectionable, on account of the 

 great nicety requisite in the workmanship. On making a few trials, to ascertain the 

 varieties of forms which might be given to forcing-houses, by taking different segments 

 of a sphere, I, however, soon became perfectly satisfied that forcing-houses, of excellent 

 forms, for almost every purpose, and of any convenient extent, might be constructed 

 without deviating from the spherical form ; and I am now perfectly confident, that such 

 houses will be erected and kept in repair at less expense, will possess the important 

 advantage of admitting greatly more light, and will be found much more durable than 

 such as are constructed according to any of the forms which have been hitherto 

 recommended. By employing a small segment (Jig. 251. b, c) of a large sphere 

 (Jig. 251. a, a), as low and as wide a forcing-house as can be wanted for any purpose, 

 may be readily obtained. Instead of the half of a hemisphere of thirty feet diameter, 

 let the half of one of fifty feet (a, a) be chosen, and from the base of this, cut off thirty- 

 five degrees (b, b), and from the summit fifteen degrees (c, c) ; and the following pro- 

 portions for a forcing-house (Jig. 251. b, c) will be given. Its height (including 

 eighteen inches of upright opaque front, opening as shutters,) will be twelve feet ; its 

 width in the centre fourteen feet, and its length very nearly forty feet ; and there are 

 very few purposes for which a house, constructed according to some of the intermediate 

 forms, between that above mentioned and the acuminated semi-dome, will not be found 



251 



b L. 



extremely well adapted." A few observations on Sir G. 

 the improvements on it, proposed by Knight, were made by 

 Neill (Edin. Encyc. art. Hort.) and the next in order by us in 

 Remarks on the Construction of Hot-houses, &c . 1 8 1 7. A year be- 

 fore (1816) we had invented a wrought-iron sash-bar, the section 

 of which (fig. 252. a) is not more than half an inch wide, 

 and a half bar (fig. 252. b) equally light (a specimen of both of 

 which was presented to the Hort. Soc. in May 1816.) ; and in 

 1818 we completed a considerable erection of glass roofs at 

 Bayswater (Jig. 253.), on the curvilinear principle, the first, we 

 believe, attempted in Britain. The object of such a junction 

 of different curvatures in the Bayswater example is to show, 



Mackenzie's plan, 



252 



and 



