Mr Hammond wishes, in his last, to confine the discussion to two points. Respecting the first, he gives some reasons why he says no; but I fail to see an explanation in any of his letters as to how more than two compartments or houses can be heated as he proposes, with bottom of boiler only 1 foot below the pipes. He admits that outside paths cannot be crossed, but is silent as to inside paths. Will he kindly answer Mr Makenzie's question, p. 231, as to inside paths?

Glass houses are, or should be, arranged so that plants may be grown successfully, with the greatest economy of labour possible, not to put hot-water apparatus in; and a system which will not admit of doors and paths being placed where required is at once condemned, even if the water does circulate the best in that form. Then, again, with a continuous fall from boiler, the placing of pipes under stages and in other confined places is more awkward, as they fall from the point where they enter the house to the far end, and then continue to fall in the return, thus taking up twice the height compared with pipes that rise to the far end and fall in the return - a fact alone which would often entail great trouble in a long house. It appears to me that Mr Hammond has left out one very important fact in his theory - I say theory, because I find no statement that he has succeeded in heating a range of houses on his plan; if he has, I shall be only too glad to make the journey north to see the apparatus at work, if he will kindly permit me - and that is, air.

Water, when heated in the boiler, gives off air in greater or less proportions; and that it continues to do so after it leaves the boiler, or that it at least carries the air with it for some distance, is proved by the fact that, in a large apparatus having air-cocks instead of open air-pipes, air is found to accumulate at all the cocks, even those most distant, and not at that only where it would the most easily go in leaving the boiler. Now the retarding of the circulation by friction is so much greater where the water has to pass under confined air than when passing under the upper surface of pipe, that I have known it stopped on one side of a house where there was a slight inequality (often not a quarter of an inch) in the rise of the pipes, which allowed the air to accumulate; on removing the inequality, the water at once circulated properly. Yet Mr Hammond advocates an arrangement of pipes in which every particle of air which passes beyond the highest point above the boiler has to fight its way back against the flow of the water to that point before it can escape.

Has he ever watched the air under the ice covering a stream? I have; and the way in which a large bubble, detached by the stream from air accumulated at a high point, was carried down, struggled part of the way back to be carried down again, until it often took it minutes to regain its place, taught me that it could not be wise to expend the force of the flow of hot water in contending with the inclination of air to reach the highest point. Why not allow the air to go the same way with the water? He will say, "Why not then let the water rise the whole way until it is about to re-enter the boiler if you are correct?" I would, if the houses were built for the pipes, and not the pipes for the houses. Careful experiments were made near Derby by a gentleman in heating a large factory where it was possible to arrange the pipes either to rise to the highest point at first or at last; he tried both, and proved to his satisfaction at least, that the circulation was quicker, and that less coal was required to maintain the same heat in the room, with the latter arrangement.

When the hot water leaves a boiler it must be replaced with other water out of the return-pipe, and the quicker it is replaced the quicker will be the circulation. Does Mr Hammond intend to state that water in the return-pipe will fail to fill the vacuum caused in boiler by the rise of the heated water in the same time, if it have only an inclination of 1 foot in 100, as if it had 10 feet to fall in the same length 1 If so, all rivers would flow at the same speed. In conclusion, I should be like Mr Makenzie, only too pleased to hear of a system which would give us a quick circulation through a large system of pipes without a deep stoke-hole; but I fail to find any evidence in either the arguments or statements of Mr Hammond to prove that he has made the discovery. Henry J. Pearson.

Beeston, Notts.__________________

I daresay both you and most of your readers are heartily tired of this discussion. Although the subject is a most important one, it has been well ventilated, and I think may now be safely left in the hands of those whose business or pleasure lead them to carry into practice the various forms of apparatus recommended.

Allow me very shortly to notice one or two points brought forward in the several communications which appeared in your last. In reply to one statement of Mr Stevens's, in reference to "pressure or gravitation," if he will think the matter out a little further, he will find that this pressure or gravitation - which he evidently thinks has nothing to do with the circulation of the water - has not only to do with it, but is the sole and only cause of the upward motion in the flow-pipe. I tried to make this plain in my last, and if I have failed it is not because such is not the case, but because of my inability to put the matter in as clear a light as I should have wished. This same inability to simplify a somewhat abstruse subject is no doubt the cause of Mr Stevens's not comprehending my reference to friction. His remarks about it having been proved by "Albion" and others that a continuous ascent is not necessary, are, to say the least of it, misleading. From the very first I admitted that there is no necessity for the pipes having a continuous ascent; indeed, I do not recollect that any of your correspondents took up this position, - the opposite is the case.