156 GREENHOUSE CONSTRUCTION AND HEATING. 
heating than in comparatively small and simple ones. I 
have known a “ flow” to be fixed thus: [”~ LT Li 
(in order to avoid the pipes crossing the pathways), but 
though there was a circulation, it was very slow and 
uncertain, and the result very unsatisfactory. Such ‘dips’ 
in a return would be somewhat less injurious, and a ‘' rise” 
or arch at some point in a ‘‘ flow’’—as over a doorway 
or the like—would interfere very slightly with the current, 
though the fewer of such irregularities in level there are the 
better. 
There are a good many different ways of arranging or 
disposing the pipes in a house, differing more or less 
according to the size and character of the structure, their 
number, the temperature required to be maintained, the 
lie of the ground, and so forth. In all such systems of 
heating the skill of the engineer will be displayed in the 
adoption of the method that is at once the best and simplest 
possible under the peculiar circumstances of the case. All 
unnecessary complications must be studiously avoided, and 
the more simple and straightforward the arrangement 
throughout, and the fewer angles, elbows, changes of level, 
etc., the better and more economical will the whole 
prove in working. If the levels are carefully adjusted, 
and the natural tendency of the heated water to fly to the 
highest point is duly considered, the matter will usually be 
found by no means a difficult one, and a satisfactory result 
easily arrived at. The different descriptions of boilers 
in use are now somewhat numerous, and the usual sizes 
of piping are the 2in., 3in., and 4in., but both will be 
treated of more fully farther on. 
The two most simple arrangements are the plain circuit 
