102 APPLICATION OF HOT WATER IN HEATING HOT-HOUSES. 
not be further increased, except as a reservoir of heat, to act when the fire ceases 
to burn, as fig. 3. 
Fig. 3. 
5thly. Where heat is required only during the action of fire, a large surface in 
proportion to its capacity may be used with advantage, to give off heat over the 
descending pipe; figures 4 and 5, cooling in this manner, will increase the 
velocity. 
6thly. The aperture of the upper pipe should not be more than about an inch 
below the surface of the water, or as much as prevents its drawing air in an open 
boiler ; but the lower it is below that, the less effect will be obtained ; the lower 
pipe should enter the boiler where it has least tendency to cool, and check the 
fluid rising to the upper pipe from the fire surface. 
Fig. 4. 
Fig. 5. 
