248 THE GARDENER. [June 



If " it is not the case that " engineers and gardeners are agreed upon, 

 as being essential to rapid circulation of the water in the pipes, " that 

 the boiler should be sunk " below the level of both flow and return 

 pipes, what does Mr Makenzie mean by the following assertion : 

 " Those having practical experience know that where it is practicable 

 to place the boiler say 10 or 12 feet below the floor on which the 

 pipes are laid, there is a very much more rapid circulation than where 

 there is only say 3 feet of difference between the bottom of return and 

 top of flow pipes." It is evident from the latter quotation that Mr 

 Makenzie is agreed that sinking the boiler below the main body of the 

 pipes is essential to rapid circulation. And to prove " conclusively the 

 great value of having a deep stokehole," as being essential to rapid cir- 

 culation, he quotes Mr Kinnear Clarke's theoretical tables for finding 

 the velocity at which the water circulates in a hot-water apparatus. 

 According to these tables — as given by Mr Makenzie, — by increasing 

 the main height from 10 feet to 20 feet, the water will circulate at an 

 additional speed of 40 feet per minute. Mr Kinnear Clarke, however, 

 adds an important qualification to his theoretical conclusions on this 

 matter by saying, " The velocities due are not attained. The actual 

 velocities are in some cases not more than a half, or even a ninth, of 

 the velocities due to gravity." But supposing the velocities due to 

 gravity were attained, it would not prove " conclusively the great value 

 of having depth of stokehole." Elevation of the flows could be got 

 without having recourse to Paddy's plan of elevating the roof of his 

 dwelling by sinking the floor. This elevation theory, with the view of 

 increasing the rate of circulation, is based upon the fact that the higher 

 the point from which a body falls vertically, the more rapid is its 

 motion as it nears the earth. Now a fluid of less specific gravity 

 cannot fall through one of greater ; and in a properly adjusted heating 

 apparatus each volume of water, from the highest point to the lowest, 

 in the circuit of the pipes, is relatively lighter than the volume below it, 

 and therefore the velocity does not increase on the journey from the 

 highest to the lowest point, like a body falling through space, but is 

 the same at all points of the apparatus ; and therefore sinking the bot- 

 tom of the boiler more than a foot or so below the floor on which the 

 pipes are laid is a mistake, if it is done for the purpose of accelerating 

 the circulation of the water in the apparatus. The proper way of at- 

 taining the latter is not by increasing the vertical height between the 

 lowest and highest point of the apparatus, but by increasing the differ- 

 ence between the temperature of the water as it leaves the boiler at 

 the highest point and enters at the lowest ; and the way to do this is 

 by causing the water to flow over a larger surface of piping in the 

 houses to be heated. 



Then in reference to my objection to giving the flow-pipes a continuous 

 ascent from the top of the boiler to the furthest points to which they extend 

 in the various compartments to be heated, Mr Makenzie says, " This arrange- 

 ment of the pipes is often carried out, not because it is considered essential for 



