STEAM INSTALLATION 197 



There are several different systems but 

 they may all be grouped roughly into three 

 classes: (i) Those in which a vacuum is 

 created by means of a pump or other me- 

 chanical device; (2) those in which the air 

 is expelled by raising the steam to a relative- 

 ly high pressure, and then preventing it from 

 returning by some form of automatic mer- 

 cury seal, and (3) those in which a constant, 

 though slight, vacuum or tendency to vac- 

 uum is maintained, by connecting the sys- 

 tem with the chimney and utilizing the "pull" 

 of the draft. 



These systems are now being rapidly in- 

 stalled in public buildings and dwellings, and 

 no doubt will be found more satisfactory 

 than steam for greenhouses. In addition to 

 the advantages given above it is claimed for 

 these systems that they are more economical 

 of fuel than are either steam or hot water, 

 that the circulation is better and surer, and 

 also that there is no trouble arising in long 

 runs from water of condensation. 



Arrangement of Boilers. In the common 

 gravity system of steam heating the boilers 

 must be below the level of all mains and 

 coils. When they cannot be so located, 



