316 FRUIT CULTURE UNDER GLASS. 



fixed at a very much lower level than the pipes ; and also to 

 the necessity of laying all the flow-pipes on the incline the 

 whole length of the house to be heated. The importance, 

 too, of having the return - pipes on a considerable decline, 

 has, in my opinion, been very much over-estimated. It is 

 entirely unnecessary to form deep, damp stoke-holes, in order 

 to sink the boiler to a level much below the main body of 

 the pipes, as is so very frequently met with. And as to 

 having the pipes running at an incline, after starting from 

 so high a level, I consider it entirely unnecessary. Indeed, 

 one of the most efficient heating apparatus I ever super- 

 intended, started from about a foot above the level of the 

 boiler, and ran down a gradual decline into the boiler. 

 Immediately the water enters a hothouse it begins to part 

 with the heat absorbed from the fire, gets colder, increases 

 in specific gravity as it speeds in its way back to the boiler 

 again, and a downhill career is most natural to it as soon 

 as it leaves the highest point of action, where its heat is the 

 greatest. Practically I have never found much difference 

 when the pipes went the whole length of the house on an 

 incline, or on a dead level all the way round till it came 

 near to, and dropped into, the return-opening of the boiler. 

 Indeed there is little fear of a good circulation, provided 

 the pipes do not at any point descend and rise suddenly, 

 and most especially that at any point they do not dip 

 below the level of the return-opening into the boiler. I 

 have had the working of apparatus where pipes, descending 

 perpendicularly, crossed under a walk and rose again per- 

 pendicularly to heat another range of 80 feet of glass ; but 

 at none of the points were the pipes lower than two feet 

 above the level of the return-opening into the boiler. This 

 undesirable arrangement worked pretty well until hard firing- 

 became necessary; then the water was thrown out in plunges 

 at the supply cistern. Such an arrangement should always 

 be avoided. 



