424 THE GARDENER. [Sept. 



" he knows very little of what he is talking about"— that the difference 

 of the specific gravity of the water as it leaves the boiler and returns 

 thereto is limited. At a temperature of 39i° water attains it greatest 

 specific gravity, and its least at 212°. These temperatures represent 

 the greatest difference that can occur in the specific gravity of water, 

 and therefore increasing the surface of piping beyond what would be 

 sufficient to cool the water from 212° to 39^° would in no way increase 

 the motive power. On the other hand, if the rate at which the water 

 moves in the pipes is determined by the difference in elevation be- 

 tween the highest and lowest points in the apparatus, then the water 

 could be made to circulate at almost lightning speed : we could have 

 stock-holes any depth. 



I now notice where Mr Makenzie tells us "that a lighter fluid, 

 bulk for bulk, can force a heavier uphill, every mercurial barometer, 

 every pump and chimney in the country, proves." I would ask Mr 

 Makenzie as a special favour to explain in what way does the mer- 

 curial barometer show " that a lighter fluid, bulk for bulk, can force a 

 heavier uphill." In one of the tubes of the barometer there is a 

 vacuum at the top. In the hot water there is no vacuum at any 

 point. The feed-cistern prevents that. Then how does the action of 

 the common pump illustrate " that a lighter fluid, bulk for bulk, can 

 force a heavier uphill " 1 When raising water by means of the common 

 pump, the atmospheric pressure is withdrawn from one end of the 

 tube, while it acts with its normal force at the other. Nothing 

 analogous to this occurs in causing the water to circulate in the hot- 

 water apparatus. 



Now about the chimney illustration, " that a lighter fluid, bulk for 

 bulk, can force a heavier uphill." The heavier fluid in most cases 

 stands aside and permits its lighter brother to pass upwards. Some- 

 times, however, the heavier fluid refuses to stand aside, the conse- 

 quence being a smoky room. I have now only to say that, as requested 

 by Mr Makenzie in ' The Gardener ' for August, I will undertake to 

 heat a range of houses as indicated by him without sinking the bottom 

 of the boiler more than one foot below the floor-level on which the pipes 

 are laid. And further, that the apparatus will work as well with the 

 boiler at this level as if the top of the boiler was sunk below the floor- 

 level. The question of whether the apparatus works as well the one 

 way as the other I would leave to the decision of the Editor of ' The 

 Gardener ' and Mr A. D. Makenzie. 



J. Hammond. 



Brayton Hall, Aug. 12, 1879. 



-s->J^£J<r-M!5— 



