576 Application of Hot Water in Heating Hot-houses. 



4th. If a boiler have sufficient surface to receive the 

 effect of the fire, and the whole apparatus contains as much 

 water as will convey the heat from the fire to the heating 

 surface in the time corresponding to its velocity, its capacity 

 need not be further increased, except as a reservoir of heat, 

 to act when the fire ceases to burn. Fig. 5. 



5thly. Where heat is required only during the action of 

 the fire, a large surface in proportion to its capacity may be 

 used with advantage to give off heat over the descending 

 pipe, fig. 6 and fig. 7 ; cooling in this manner will increase 

 the velocity. 



Gthly. 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. 



7thly. In a close boiler a pipe may, at any distance from 

 the boiler, rise to any height and descend again ; but, it must 

 neither rise twice, nor dip after leaving the boiler ; where it 

 is necessary to raise it, there should be an open pipe inserted 

 at each extremity of the height of the rise ; advantage has been 

 taken of this circumstance to avoid doorways, fig, 8. 



8thly. A certain quantity of motion would be obtained by 

 a single horizontal pipe between any points except the 

 bottoms of the vessels ; but, the nearer to the surface the 

 more motion will be obtained ; and, with one pipe, there 

 must be a double current in it. 



9thly. The retarding effect of friction is directly as the 



