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(I) the pathway, and (c) the lowest point excavated : 

 the same course may be adopted if the soil, though not 

 naturally so dry as this, can be rendered so by thorough 

 drainage ; but when the ground does not admit of perfect 

 drainage, the structure must be sufficiently elevated to 

 avoid the risk of injury from the dampness of the locality. 

 The angle of elevation is not, as it is sometimes asserted 

 to be, a point of indifference, though mathematical accu- 

 racy is certainly by no means required : in the annexed 

 engraving, the angle of the roof is about 55, this pro- 

 vides for the admission of the sun's rays in the winter 

 months, when his position is comparatively low in the 

 horizon, to a much greater extent than could take place 

 if a more ordinary slope were adopted. A still more 

 elevated pitch would doubtless effect this object in a still 

 more perfect manner ; but would not be equally applica- 

 ble to the requirements from a permanent structure, 

 which would be wanted for summer as well as winter use. 

 A reference to the sketch, will at once shew the general 

 nature of the internal arrangements. There should be a 

 tank (d) supported by brick piers (p) in which a circula- 

 tion of heated water would supply a genial warmth to the 

 soil above, and to the roots of the plants growing in the 

 soil ; this tank should be heated by a small boiler, conve- 

 niently placed with reference to adjacent arrangements ; 

 a series of iron pipes (e) attached to the same boiler, 

 would supply the requisite heat to the atmosphere. It 

 may perhaps be thought that the application of the gutter 

 system of heating would in this case be preferable ; but 

 as there would be a perfect command of moisture, as 

 will be explained further on, it is desirable to have dry 

 heat also, under controul, and this can be better effected 

 by means of the pipes than by adopting the gutter plan 

 of heating. I cannot in this place forbear protesting 

 against the limited surface of piping generally employed 

 in heating plant structures ; what is thought to be just 



