474 THE GARDENER. [Oct. 



circulations of hot-water pipes, one 20 feet and the other 4 feet above the boiler 

 (everything else being equal), the lower one should heat first. It must not be 

 supposed for one moment that the "theory" is that, under all circumstances, 

 no matter where placed— no matter how far away, no matter what friction or 

 what boiler power — the highest must heat first. This is a mere burlesque on 

 the subject. What is correct in theory must be correct in practice ; but theory 

 does not take up a fragment of the subject and forget all the rest. 



I observe that Mr Hammond asks if the upper strata in the flow-pipe travels 

 faster than the lower strata. I answer, it does. Any one who doubts this 

 I invite to examine for himself by calling on me here, where he will see an 

 apparatus with certain parts of the 4" pipe made entirely of glass, showing the 

 motion very plainly at the various points : nothing can be plainer and more 

 convincing. 



The only other point of Mr Hammond's letter calling for comment is his 

 quotations from Hood. I am perfectly well aware Hood recommends the 

 form of apparatus advocated by Mr Hammond. What then comes of the asser- 

 tion that hot-water engineers never recognised the desirability of this form 

 under any circumstances ? I asserted in my last letter that, as far as con- 

 cerned myself, my practice is to be entirely guided by the circumstances of 

 the case. Hood recommends this for diametrically opposite reasons to Mr 

 Hammond's; for, immediately following the quotation from page 169 of the last 

 edition, he says : " This" (the reversal of the circulation) "arises from the ex- 

 tremely rapid motion of the water in vertical pipes, by which means the whole 

 of the heated water passes directly to the highest level, without delivering any 

 to the lower horizontal branches." Mr Hammond asks me if I think Hood 

 and the other authors quoted have reached the "acme of perfection." I 

 answer no; but when I find all men of science agreeing upon the existence and 

 operation of certain laws, and when, moreover, their reasoning in support of 

 these appear to myself to be as plain as that "two and two make four," or 

 that "the whole is equal to all the parts," really Mr Hammond must excuse 

 me when I prefer to stick to my own views when in such good company. 

 That 1 don't follow Hood in his practical application in this matter is because 

 I have repeatedly proved to my own satisfaction — and I am quite prepared to 

 prove to the satisfaction of any other person— that where there are several 

 iloors of buildings to heat in the manner referred to by Hood, quoted by Mr 

 Hammond, it is not only possible to have a good circulation with the flow 

 and return of about equal length in what may be called the ordinary manner, 

 but as good a circulation may be had with nearly the whole of the piping a 

 flow pipe, with a direct and vertical return. I can point to several places, 

 both here in Edinburgh and elsewhere, fitted up with the flow winding through 

 the various flats, then a direct and vertical return to the boiler, and the ap- 

 paratus — some in operation for ten years — giving the very highest satisfaction, 

 and no such thing as a reversal of the circulation takes place. In these cir- 

 cumstances I think I am quite justified in coming to the conclusion that it is 

 no necessary consequence of a heating apparatus with a winding flow and a 

 vertical return that there should be any reversal or obstruction of the circula- 

 tion. Whatever may cause this reversal in certain cases, it is clear that it is 

 not this form of the apparatus, or it must of necessity take place in every case 

 where the vertical return exists. 



In conclusion, allow me again to point out the mistake made by Mr Ham- 

 mond, as well as others, in assuming that the motive power in a hot-water ap- 

 paratus is not the pressure of the cold water in the return, but some mysteri- 



