34 



SCIENTIFIC NEWS. 



[April 1st, 1887. 



the water, as very little will suffice to stop it altogether. 

 Care should be taken to avoid unnecessary dips in the 

 piping, especially such as are shown in Fig 2. They not 

 only cause increased friction, but owing to the density of 

 the water being increased as it travels further from the 

 boiler, it follows that the column of water b will be denser 



and heavier than a, and consequently have a tendency to 

 make the water flow back to the boiler, from b to a, instead 

 of from a to b. It is often necessary to dip the pipes under 

 doorways and other structural obstacles, but this should be 

 avoided as much as possible. There are also cases in which 



it is necessary to dip part of the piping below the level of 

 the bottom of the boiler, and when provision of this kind 

 has to be made it will be best to carry up the vertical flow 

 pife to a higher level, as the longer the ascending and 



descending pipes are respectively, the greater will be the 

 difference in the temperature of the water they contain, and 

 the greater will be the inequality of the pressure. This 

 will increase the power of circulation, and assist the flow 

 through the pipes which are below the level of the boiler. 



It is also important that air vents should be provided in all 

 positions where it is probable that air will accumulate. 

 Air being much lighter than water, the vents should be 

 made in the tops of the pipes, and if not self-acting they 

 should be opened frequently, for if air be allowed to accumu- 

 late it will greatly impede, and may altogether stop, the 



Fig. 10. 



circulation of the water. When the boiler and pipes are 

 first charged with water care should be taken to drive off 

 the air, but this will not suffice. Cold water holds air in 

 solution in greater quantity than does hot water, and when 

 the water put into the boiler is heated, much of the air 

 leaves the boiler and rises to the highest points it can reach 



in the pipes ; at these points there should be vents. So 

 far as the circulation is concerned, it can be relied on for 

 any height to which it is practicable to carry up a column 

 of water. The practical limit will be one of construction, 

 for, as is well known, the pressure of water on each square 

 inch increases about half a pound for every foot of vertical 



Fig. 12. 



height. In the usual way of heating a glass-house the pipes 

 are very httle above the boiler, and the system is essentially 

 a low-pressure one. Where the pipes are required at higher 

 levels, it is simply a question of rightly proportioning the 

 strength of the boiler and pipes, and of having good joints. 

 Unhke steam, hot water has very little expansive force, and 



