352 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



Hot water heating may also be effected by bringing the water to a 

 much higher temperature, say 300 Cent. In this case Perkins's high 

 pressure apparatus is made use of, so called from the inventor of the 

 system. In this the water circulates in pipes which have no communi- 

 cation with the open air, and are built in the thickness of the walls 

 from one story to another, or beneath the floors, as in the low pressure 



Fir,. 242. Hot water heating apparatus. 



system, except that the tubes are constructed of stout iron and joined 

 with great care, and are bent into spirals or coils, K, H, E (Fig. 243), 

 on each floor, above the furnace, where they are subjected to the direct 

 action of the heat, up to the rooms of the upper stories, where they fill 

 certain spaces in the form of fireplaces or stoves, D, G, J, as in -the 

 represented figure. Above the coil in the uppermost story there is 

 an expansion vessel a, containing a certain quantity of air which the 



