166 GREENHOUSE CONSTRUCTION AND HEATING. 
as being always in action, while the latter have to be 
opened to allow the confined air, etc., to escape, and 
then closed again. An air-pipe usually consists of a 
small brass union screwed into a hole drilled in the top 
of the pipe, at the highest point, with a few feet of iron — 
or compo. tubing of small diameter attached, the 3in. 
or $in. size being generally employed. The top of this 
pipe, which should be carried up in a corner or other 
convenient place, and secured by holdfasts, must be rather 
higher than the top of the feed-cistern, so as to prevent 
any overflow when the latter is full. 
The position of the feed-cistern is not a matter of any 
great importance, but it is usually, and best, connected 
witn the return pipe close to the boiler. It ought to be 
elevated well above the highest point in the pipes, anything 
from 2ft. to 10ft. above them being suitable. It may 
be fixed on a high staging, or shelf, in any convenient 
place, either inside the house, or in the stoke-hole (if 
this is covered in), but not out in the open air, as sometimes 
seen, where it is liable to become frozen in severe weather. 
It should contain at least two or three gallons to each 
100ft. run of 4in. piping, to allow for the expansion 
and contraction of the water as it becomes heated and 
cooled again. ‘The connecting pipe ought not to be less 
than #in. diameter even for a small apparatus, and for 
a large one should be from lin. to 2in. in diameter. 
When fixing large and powerful boilers, especially those 
of the tubular types, which heat quickly and create a 
rapid and powerful current in the pipes, it is at once 
usual and highly advisable to place a vertical stand-pipe, 
several feet in height, and open at the top, on the flow 
