422 THE GARDENER. [Sept. 



the two columns acts exactly in the same manner." Increased height 

 gives increased pressure on the bottom and sides of the boiler, and all 

 other parts of the apparatus. But pressure has no more to do with 

 causing circulation as it takes place in the hot-water apparatus, than 

 Mr Makenzie or I have to do with causing the magnetic needle to 

 point northwards. If the water escaped at any point of the apparatus, 

 then the higher the supply above the outlet the more rapid would be 

 the efflux ; but the water does not escape at any point of the apparatus, 

 — it circulates therein ; and unless there is a difference between the 

 specific gravity of the water in the two columns — no matter what may 

 be the vertical height of the latter above the boiler — circulation will 

 not take place, — which is a plain proof that the only motive power at 

 work in the hot-water apparatus is the difference of the specific gravity 

 of the water at different points of the apparatus ; and the only way by 

 which this motive power can be increased is by increasing the differ- 

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

 the highest point, and returns thereto at the lowest point of the 

 apparatus. A second time Mr Makenzie says he will show " that in- 

 creasing the difference of the temperature between the water as it 

 leaves the boiler and in the returning column acts exactly in the same 

 manner as increasing the height." But in place of showing this he 

 gives a long list of isolated quotations from other men's writings ; 

 and as proof for or against anything, isolated quotations are of very 

 little value. I would here ask Mr Makenzie if he thinks that the 

 authors whom he has quoted have reached the acme of perfection in 

 their theoretical reasoning on the question of heating by means of hot 

 water, and that no further inquiry on the subject is necessary ? If he 

 thinks so, I don't think so ; and that I am right the number of un- 

 satisfactory heating hot-water apparatuses that are at work in every 

 county in the kingdom is a proof. 



I now come to notice where Mr Makenzie says that my way of 

 increasing the motive power " is a most improper way, for two con- 

 siderations. First, the quantity of piping must be settled by con- 

 sideration apart from the circulation ; and second, because by increas- 

 ing the quantity the friction increases in greater proportion than the 

 power, consequently increasing the amount of pipe past a certain point 

 will prevent circulation altogether." Now in reference to the first 

 consideration, why my way of increasing the motive power is "a 

 most improper way," I would ask Mr Makenzie, why must the 

 question of circulation be left out of consideration when deciding 

 the quantity of piping requisite to heat a range of plant-houses 1 If 

 the quantity to do so " must be settled by consideration apart from 

 circulation," then according to the second consideration the quantity 

 of piping might be such as would " prevent circulation altogether." 



The question of circulation should be the first consideration. If the 



