214 GREENHOUSE CONSTRUCTION AND HEATING. 
Screw-down ‘angle-valves” (Fig. 157) are frequently 
employed in connecting a set of heating pipes on to a main 
running on a lower level, and for such positions are very 
suitable, as well as being slightly less costly than the 
ordinary ‘‘ straight ” valves. 
Coils.—It should perhaps have been mentioned 
previously that a ‘‘coil” of heating pipes is really a congre- 
gation of parallel pipes arranged either horizontally or 
vertically, for affording a large amount of heating surface 
Fia. 158. 
ina small space. Hence they are more suitable for heating 
churches, schools, and similar buildings than horticultural 
structures. Coils are made with from three to thirty or 
more lengths of piping, these being usually arranged in a 
single or double row (vertical), or in the case of a vertical 
coil in a circular or square form. The ends of the pipes 
are fixed and carried in suitable cast iron ends, with a 
hollow water-way, one of which is connected with the flow 
from the boiler, and the other with the return. 
A horizontal coil is shown in Fig. 158, and a vertical 
(circular) one in Fig. 159. Ornamental covers of perforated 
