312 FRUIT CULTURE UNDER GLASS. 



tirely unnecessary to form deep, damp stokeholes, 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, in- 

 creases itt specific gravity as it speeds in its way back to the 

 boiler again, and a downhUl 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 dif- 

 ference when the pipes went the whole length of the house 

 on an incline, or on a dead level all the way round tUl 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 sud- 

 denly, and most especially that at any point they do not dip 

 below the level of the return-opening into the boUer. I have 

 had the working of apparatus where pipes, descending perpen- 

 dicularly, crossed under a walk and rose again perpendicularly 

 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 arrange- 

 ment 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. 



There is another error frequently committed in arranging 

 the route of the water. Suppose, for instance, a boiler fixed 

 at one end of a house of, say, 80 or 1 00 feet long, as part of th e 

 work allotted to it. As in the case of span-roofed houses, it 

 may be desirable to have three or four rows of pipes aU 

 round the house. Now it is not uncommon to find two rows 

 called the flow-pipes taken all round the house to near the 

 boUer, and there to start back with other two on the same 



