470 THE GARDENER. [Oct. 



heated essential to rapid circulation ?" hi answer to this question, I 

 agree with Mr Hammond that it is not essential to circulation, but in 

 my opinion is most convenient in general practice. What I contend 

 is, that elevation is essential to rapid circulation, but I differ with him 

 when he says that in place of it being essential the continuous rise is a 

 hindrance to circulation. I hold that it makes not the least difference 

 whether the rise be vertical or gradual, if the height and difference 

 between the mean temperatures or densities of the ascending and 

 descending volumes of water be the same. If Mr Hammond can 

 prove that the circulation will be less rapid under these circumstances, 

 I will submit that a gradual rise hinders circulation ; but if he fails to 

 prove that, as I predict he will, then of what avail is all Mr Hammond's 

 talk about an imaginary return current hindering circulation in a gra- 

 dient rise 1 [Is it imaginary ? — Ed.] Lastly, he asks, " Is it necessary 

 or essential to rapid circulation that the top of the boiler should be 

 below the main body of flow and return pipes whether there is only 

 one house to heat or a range of houses ? " In answer to that question, 

 I may say that that would depend on the height of the boiler and what 

 Mr Hammond may consider a " rapid circulation." If I understand 

 Mr Hammond to mean what he stated in the February number, that 

 the circulation will be as rapid with the bottom of the boiler one foot 

 below the level of the return as if it were several feet deeper, then I 

 must again differ with him. I maintain that the circulation in hot- 

 water pipes increases in proportion to the difference of weight or pres- 

 sure between the ascending and descending volumes of water on the 

 base. Now this difference of pressure is increased in two ways : by 

 raising the elevation, and also, to a limited extent, by increasing the 

 difference between the mean temperatures of the two volumes of 

 water. Mr Hammond thinks that the circulation will be as rapid 

 with the bottom of the boiler one foot below the level of the return 

 as if it were several feet deeper. Now it is plain that the water before 

 entering the boiler is at its lowest temperature, and consequently at its 

 greatest density. And as it has been already stated that the pressure 

 of a liquid on a given base is measured by the vertical height or depth 

 and density, independent of the shape of the vessel or the quantity of 

 water which it may contain, hence it is evident that the greater the 

 fall before entering at the bottom of the boiler, the greater is the dif- 

 ference of pressure and gain of power to counterbalance the lighter 

 volume in the flow. This will appear more plain when we consider 

 that it is only the ascending and descending volumes, equal to the 

 vertical height, that press on the base, and therefore act on circulation. 

 Now, as an illustration of the fact that elevation increases circulation, 

 we will take for example an apparatus with an elevation of 4 feet : we 

 will suppose the mean temperature of the ascending column of the flow 

 is at the temperature of 98° Fahr., and that of the descending column 

 of the return at 62° ; the pressure on the base of the water in the flow 



