128 
HEATING HOT-HOUSES BY HOT-WATER. 
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. 
17 
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 circum¬ 
stance to avoid doorways. (Fig. 19.) 
8thlv. A certain quantity of motion would be obtained by a 
single horizontal pipe between any joints except the bottoms of 
vessels; but, the nearer to the surface, the more motion will be ob¬ 
tained ; and, with one pipe, there must be a double current in it. 
