By Alexander Cruikshanks, Esq. 517 



only 2 francs per foot, the expence for a house, 30 feet in length, 

 would have been under £ 13. 



In the plans Nos. 2 and 3 I have supposed the outer case, or 

 boiler, to be of cast iron, as well as the case of the furnace in No. 3, 

 and all the other parts of copper. The whole might be made of 

 cast iron ; but there would be a risk of its cracking in those parts 

 exposed to the fire, an accident to be especially guarded against in 

 heating horticultural buildings. Plate-iron or tin-plate might be 

 employed, either wholly or in part ; but cast iron and copper would 

 perhaps be the preferable materials. With respect to the pipes 

 when made of sheet copper or zinc, or tin-plate, they are better 

 calculated to disperse the heat than those of cast iron commonly 

 used, which are perhaps, except in point of strength, the worst that 

 can be employed : they expose a smaller surface, in proportion to 

 the water they contain, than pipes of any other shape ; and from 

 the thickness of metal, never less than f of an inch, and frequently 

 more, they oppose a direct obstacle to the ready transmission of the 

 heat from the water to the surrounding atmosphere. Thin sheet 

 copper is certainly far superior, and perhaps in the end the most 

 economical. Zinc, though otherwise an excellent material, is not 

 well suited for elliptical pipes ; it becomes so soft at the tempera- 

 ture of boiling water that they soon, the lower ones especially, 

 change their shape and become nearly round. If cast iron be pre- 

 ferred, there is no reason why the cylindrical form should be adhered 

 to ; and probably if elliptical pipes of different sizes, cast as thin as 

 possible, were to be supplied from the foundries where those now 

 in use are cast, they would soon supersede the use of the latter. 



No reservoir has hitherto been attached to the small apparatus I 

 have described ; but on a large scale it would of course be necessary, 

 the quantity of water being small, and there being rto mass of brick- 

 work to act as a reservoir of heat : this purpose however is evidently 

 much better answered by a cistern of water within the house, than 



