Mr Hammond and some others asserted that a continuous or vertical rise is a hindrance to the circulation, and is the cause of repeated failures in the working of hot-water apparatus. I repeat that I can point to many apparatus, fitted up by my own firm and others, where nearly the whole, as it were, is a return-pipe, while there are others where nearly the whole piping is a flow-pipe, both systems working admirably. There are one or two things in Messrs Hamiltons' letter to which I wish to refer, although I am not particularly alluded to. It must not be concluded that because the houses nearest the boiler heat easiest it establishes any principle. I can conceive several possible explanations of this; and if Messrs Hamilton were to call in a properly-qualified engineer, I have no doubt their apparatus could be made to work quite satisfactorily with very little expense, - at least I know one who would be quite prepared to undertake to do so on the safe principle of "No cure, no pay." If Messrs Hamilton, however, state the quantity of piping correctly at 8500 feet, I beg to point out to them that the "Climax" boiler of Messrs Hartley and Sugden (I presume "Barr and Sugden" is a slip - there is no such firm of boiler-makers), of the largest size - viz., 5 feet - is only given out to heat, ap-proximately, 4000 feet; and where there are so many circulations, I certainly would never think of loading them above 2500 each.

Any amount of failures, through causes probably never suspected, will never prove that, given two 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 assertion that hot-water engineers never recognised the desirability of this form under any circumstances? I asserted in my last letter that, as far as concerned 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 extremely 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 floors 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 apparatus - 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 circumstances 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 circulation.

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 Hammond, as well as others, in assuming that the motive power in a hot-water apparatus is not the pressure of the cold water in the return, but some mysterious quality imparted to the water by the heat which makes it fly away from the earth. It is an error to suppose that while water at, say, 60° presses downwards towards its centre of gravity, the same water at, say, 212° has any tendency to fly upwards: there is no such tendency: the all-pervading and ever-present law of gravitation applies to all substances in nature in exactly the same way according to their densities, and no substance has the slightest tendency to rise of itself, whatever its temperature.

A cubic foot of water in its solid form, and the same in the form of vapour, is attracted in equal degrees. The law is that "every particle of matter attracts every other particle by a force that decreases as the square of the distance increases, and increases as the square of the distance decreases." I trust no one will think this does not apply to the hot-water apparatus. A proper consideration and understanding of these fundamental laws saves a world of trouble in discussing the matter. What I said about a lighter fluid forcing a heavier uphill really does not need further illustration, but to any doubter I say again, "Come and see." Come and see water at 80° forcing water at 60° uphill. [But why not let the water at 60° go down-hill? - Ed].

Atmospheric air is lighter than water, bulk for bulk, but nevertheless it raises water 34 feet. Atmospheric air is lighter than mercury, but it raises it 13 inches.

1 feel highly honoured indeed by the decision of the stoke-hole affair being left in your (Editor's) hands and mine. We have none of us anything to gain by upholding either false theory or practice, and when called upon I shall be delighted to give a fair and unbiassed opinion on the matter.

A. D. Makenzie.

2 Grove Terrace, Edinburgh.

[We have devoted so much space to this important question, that we must now close this discussion for the present. - Ed].