374 THE GARDENER. [Aug. 



be possible to put on piping — say 10 or 100 miles — sufficient to drive the water 

 through the apparatus at a speed equalled only by the electric spark. I have 

 no doubt they will. I may here state that, as any one may see who takes the 

 trouble to read my former letter, when I referred to Mr Hammond not mastering 

 the subject, I referred not to the whole subject of heating with hot water, as Mr 

 Hammond tries to make it appear, but to that particular branch of it which 

 deals with the motive power. I will now show that increasing 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 heights. 



I will again quote Mr Clarke ; page 484, he says : " Motive power of water in 

 circulation through heating pipes. The ascensional force is measured by the 

 difference in weight of the two columns of water of the same height ascending 

 and descending to and from the boiler. The difference of weight is ascertained 

 from the difference of the average temperature of the columns from which the 

 respective densities are deducted by the aid of table No. 109, page 339." He then 

 goes on to give the formula which I quoted. Mr Hood in his work, page 18, 

 says : " The higher we make the ascending and descending pipes, the more rapid 

 is the circulation of the water;" "because, as motion is obtained in conse- 

 quence of the difference in weight of the ascending and descending columns of 

 water" (and this difference in weight is owing to different densities; or, as Mr 

 Hammond says, the difference of their specific gravities), " the greater the 

 height of these columns the greater must be the difference in their weight, and 

 therefore must be the force and velocity of motion." Again, on page 28, Mr 

 Hood says : " There are two ways by which the amount of the motive power may be 

 increased, — one, by allowing the water to cool a greater number of degrees between 

 the time of its leaving the boiler and the period of its return through the descend- 

 ing pipe ;" "the other, by increasing the vertical height of the ascending and 

 descending column." " The effect produced by these are precisely similar, for by 

 doubling the difference of temperature between the flow and return the same in- 

 crease in power is obtained as by doubling the vertical height." Mr Thomlinson, 

 in his ' Treatise on Warming and Ventilating,' adopts and endorses Mr Hood's cal- 

 culations, pages 133 to 138. Mr Deuchar's ' Garden Architect : Treatise on the 

 Construction of Hothouses,' page 187, says: "When the motive power, there- 

 fore, is not of sufficient strength, the increase of the height of the column ascend- 

 ing from the boiler must be depended on for any additional motive power." Mi- 

 Hammond says that the proper way is to increase the quantity of piping in order 

 that the water may be colder when it returns to the boiler. No doubt this is 

 one way, but a most improper way, for two considerations : first, the quantity of 

 piping must be settled by consideration apart from the circulation ; and second, 

 because by increasing the quantity the friction increases in a greater proportion 

 than the power, and consequently increasing the amount of pipe past a certain 

 point will prevent circulation altogether. I am sure I need no elaborate argu- 

 ment to prove this to the intelligent readers of ' The Gardener.' 



Now, as to the alleged return current in flow-pipe. Mr Hammond has no 

 misgivings upon this point. He asserts that a return current goes on continu- 

 ally in the flow-pipe, and to stem this current all the efforts of his genius are 

 directed. His manner of proving the existence of this current is curious : it is, 

 that water of less specific gravity cannot force that of a greater uphill. If a 

 lighter fluid cannot force a heavier uphill, there never can be a forward motion 

 at all in hot-water pipes with a rise to far end from boiler, nor can there ever be any 

 circulation in any apparatus, even on his own principle ; for, suppose there is 4 

 feet of a vertical pipe before the continuous descent begins, how can the lighter and 



