This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Notwithstanding the editorial note at the end of the papers on the above subject in the September issue of 'The Gardener,' I ask room for the following remarks. I would not have made this request if Mr Makenzie in his last paper had not attributed to me assertions which I have not asserted, and assumptions which I have not assumed, at anytime during the controversy.
I have not at any point in the discussion "asserted that a vertical rise is a hindrance to the circulation, and is the cause of repeated failures in the working of hot-water apparatus." On the contrary, I assert that the highest point of the apparatus should be reached by a vertical pipe out of the top of the boiler; and that when the heated volumes of water have to reach the highest point by travelling up a slow gradient of hundreds of feet in length, the result is a return current in the flows, which is a hindrance to circulation, and the cause of an immense amount of heat being wasted in the stoke-hole.
Mr Makenzie admits "that there is no necessity for the pipes having a continuous ascent." Then why is it that in 999 cases out of 1000 they are fixed in this way % And why does Mr Makenzie advise us to have our stoke-holes as deep as practicable % There being no necessity for a continuous ascent of the piping, there is no necessity for a deep stoke-hole.
I have not, at any time during the discussion, asked Mr Makenzie "if the upper strata in the flow-pipe travels faster than the lower strata." This is what I asked - Does the upper and hotter stratum travel faster than the under and colder stratum on the downhill journey? And if not, why not? And I again, for the third and last time, respectfully ask the same question. I have not, at any time during the discussion, assumed that the mere fact of water becoming hotter "makes it fly away from the earth." The heated water would remain in the boiler if colder water did not come in contact with it. The colder being of greater density than the hotter, moves towards the lowest place; and the hotter being of less density than the colder, moves towards the highest place. The movements of both are movements in obedience to the law of gravitation, in accordance with their respective specific gravities or densities. This fact I pointed out in 'The Gardener' of February last; and what I have been contending for since is, that the apparatus should be fitted up in such a way that at all points thereof the volume of water therein will move of its own accord in the right direction, in obedience to the law of gravitation, and not have to be forced by the succeeding, or drawn by the preceding, volume in that direction.
Mr Makenzie invites us to "come and see water at 80° forcing water at 60° uphill." Now, suppose we did see water at 80° forcing water at 60° uphill, we would only have an ocular demonstration that the apparatus in which this took place was fitted up on a wrong principle. We would be looking at an apparatus in which the colder water was floating or forcing the hotter water up out of the boiler into the flow-pipe ; but once the hotter gets into the flow-pipe, a change takes place, and the hotter forces the colder uphill. Could anything be more condemnatory of the practice of having the piping fixed with a continuous ascent than that, when so fixed, the process of circulation takes place as here described 1 If it is right both in theory and practice that the colder water should force the hotter up out of the boiler, it is right that the colder should force the hotter uphill at all points of the apparatus. The law is, that when two fluids of different densities are in immediate contact with one another, that of greater density descends and takes the lowest place, and that of lesser density ascends and takes the highest.
And therefore, if the hotter water lifts the colder at any point of the apparatus, it is a proof that the method on which the latter is fitted up tends more to hinder than to facilitate the process of circulation.
Mr Makenzie tells us that " atmospheric air is lighter than water bulk for bulk." This fact I think is pretty generally understood by most people. But atmospheric air cannot force water bulk for bulk uphill. It takes a column of atmospheric air somewhere about forty-five miles high to balance a column of water somewhere about thirty-five feet high. And a column of atmospheric air of the above height could not force the water uphill one inch unless the pressure of the atmosphere was removed by a mechanical or other contrivance from the upper end of the tube in which the water is wanted to rise.
The "suction-pump" theory of circulation will not work alike at all points of the apparatus, nor will the", "strata" theory. We therefore conclude that neither is the right theory. The latter works alike at all points, and "does not take up a fragment of the subject and forget all the rest".
I agree with Mr Henry J. Pearson that "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." But if Mr Pearson thinks that the method of erecting the hot-water apparatus recommended by me would be prejudicial to the growth of plants, or interfere with the economical working of the houses to which it was applied, he is mistaken in his conclusions. The same facilities for having "doors and paths where required" are offered by the method I advocate as are offered by the method in general practice. And by adopting the former we get over the necessity for those "unmitigated nuisances," as Mr Makenzie truly terms them - deep stokeholes, and for dipping and rising again of the pipes. When "dips" occur, they render the circulation uncertain and extremely difficult.