By Sir George Stewart Mackenzie. 217 



set up, let such a mixture be made into a mortar with water, 

 and two coatings laid on, each half an inch thick. Let the 

 first one become dry without fire, and if any cracks appear, fill 

 them up with the same composition. When the repairs are 

 dry, (and many will not be required if the above proportions 

 be used, and with materials well mixed) then put on a gentle 

 fire for a few hours, and when the flue is again cold, fill up any 

 cracks that may appear, and lay on the other coating of half 

 an inch thick ; allow it to dry gradually, and fill up cracks, 

 as before. After this second coating is dry, the fire for heating 

 the house may be lighted. No pains should be taken to make 

 the surface smooth, for the rougher it is, the more heat will 

 radiate from it, and it will be better if the surface is rendered 

 black, or of a dark colour. The unshaded part of the figure 

 shews the clay and sand laid on. 



At first sight it is obvious that this form of flues occupies 

 as little space as possible ; and when it is desired to clean it, 

 any portion of it is easily removed, and as easily replaced. 

 Cast iron is a very cheap and durable material, and readily 

 procured; and there are few places where clay and sand, 

 sufficiently good for the purpose of covering it, cannot be 

 obtained. 



It may be considered whether it would not be of advantage 

 to have steam-pipes made of thin cast-iron, coated with sand 

 and clay. 



I am, Sir, 

 your faithful and obedient Servant, 



George Stewart Mackenzie. 



Edinburgh, 

 eth September, 1821. 



