188 HOT-WATER BOILERS AND PIPES. 



distance through which the water circulates, but to the time of 

 transit. If, then, by raising the pipes vertically, the difference 

 between the temperature of the flow and return pipes be in- 

 creased, it appears to be the most practical method of increasing 

 the velocity of motion. The increased velocity, therefore, is 

 indicative of increased power, and in a hot-water apparatus it is 

 the velocity of circulation which enables it to overcome any 

 extraordinary obstructions. 



Neither the principle nor the practice of an apparatus is in 

 the least affected by having an additional number of pipes lead- 

 ing out of, or into, the boiler ; the effect is the same, whether 

 there be more flows than return pipes, or, conversely, more return 

 than flow pipes. 



4. Level of Pipes. Some persons have supposed that if the 

 pipes be inclined so as to allow a gradual fall to the boiler in its 

 return, additional power is gained. This appears very plausible, 

 particularly with regard to some forms of apparatus, but the 

 principle is entirely erroneous. This error appears to arise from 

 treating the subject as a simple question of hydraulics, instead 

 of a compound result of hydrodynamics. If the question were 

 only as regards a fluid of uniform temperature, then the greatest 

 effect would be obtained by using an inclined pipe; but the 

 water in the pipes we are now treating of, is of varying density 

 and temperature, which very materially alters the results. 



Contrary to the ideas of some persons, the circulation of the 

 water first takes place in the lower pipe ; in consequence of the 

 water in the boiler becoming lighter by the absorption of heat, 

 the column of water in the return-pipe, being of greater density, 

 forces its way into the boiler, when the water in the upper pipe 

 falls into its place. Now, suppose the distance between the 

 entrance of the return-pipe and that of the flow-pipe be 12 inches. 

 This distance is neither increased nor diminished by any incli- 

 nation of the return-pipe towards the boiler, the effective pressure 

 being in both cases the same. 



Discarding the erroneous hypothesis that the motion of the 

 water commences in the upper pipe instead of the lower one, 

 and the motion commences at the entrance of the lower pipe into 



