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.
Expansion, however, is from the centre, and acts with equal force in all directions, therefore all parts of a steam-boiler are made, or ought to be, equally strong.
"C. M." says the relatively heavier water in the flow "must either be forced or drawn uphill;" and, "to prove that it is not drawn," he points out that "there are generally air-pipes at the highest point of elevation, and before the heavy column in the return would draw up the column in the flow, the air would rush in and fill up its place." This may furnish a reason for the incorrectness of the opinion that circulation is principally due to the relatively heavier column; but I think those who hold this opinion will not consider the reason conclusive that they are wrong; and if " C. M." has no other but this air-pipe proof to offer in support of his statement that the relatively heavier water in the flow-pipes "must be forced uphill" by the relatively lighter water, then I submit he has failed in his effort to prove what he undertook to prove. How could air rush into'an endless tube full of water, and the feed-cistern higher than the highest point of the apparatus'? In a properly-fitted-up hot-water apparatus, neither column pushes, pulls, forces, or draws the other: both columns move contemporaneously; and when either column has to push, pull, force, or draw the other, there is a defect in the fitting-up of the apparatus at some point.
We now come to where "C. M." uses the word "absurdity." It is not quite clear to me whether he intends absurdity to apply to his statement or to the inference I drew therefrom. If to the former, I apologise to him for alluding to it; if to the latter, then I ask him what other inference is it possible to draw from the statement referred to, but that the water suffers no diminution of temperature on the way from the boiler to the highest and farthest point of the structure. Next, "C. M." tells us, "it is a well-known fact that a house situated above the level of the others is the hottest, which proves the value of elevation." I must again differ with "C. M." It does not prove the value of elevation; neither does it prove that a quicker circulation takes place in the higher house. It only proves that the hotter water gets there, probably through the apparatus being fitted up on the equalised, mixed, forcing, or on the strata theory of circulation, by some one who advocates sinking the boiler as far as practicable below the main body of the piping, and whose knowledge of heating by hot water extends only a little beyond the fact "that water is water, a liquid, and not iron, stone, lead, or ice." I must also say that it is not "a well-known fact that a house situated above the level of the others is the hottest." If "C. M." will do me the honour of a visit, I will show him that this fact of his is not amongst those "chiels that winna ding".
The points of practical value to be settled in this discussion are, in my opinion, as follows: Is a continuous rise in the flows, from the top of the boiler to their farthest points of extension in the compartments to be heated, essential to rapid circulation? I say No; and in place of being essential, the continuous rise is a hindrance to circulation. Is it necessary or essential to rapid circulation that the top of the boiler should be below the main body of both the flow and return pipes, whether there is only one house to heat or a range of houses? I again say No, and that any number of compartments in a range can be successfully heated without sinking the bottom of the boiler more than one foot below the level of the lowest floor on which it is necessary to place the pipes in any of the divisions composing the range; and further, that this can be done without a dip (that is, dipping and rising again) in the flow. Now if those who differ with me will keep to the points here indicated, it will save valuable space in ' The Gardener ' at present; and when the long nights come, perhaps the Editor will find room for us to discuss the side-issues of this watery question.
J. Hammond. Brayton Hall, July 11, 1879.
I was sure Mr Hammond would be surprised to hear that hot-water engineers and gardeners were perfectly familiar with the system of laying pipes with a continuous direct fall from the top of the boiler down to the return. He is so surprised, that he can even yet hardly credit the astounding intelligence. This being a matter of fact about which it is unnecessary to have any discussion, I have simply to inform Mr Hammond, that if he chooses to come to Edinburgh, I will show him places fitted up twelve and twenty years ago in the manner referred to; and, moreover, my own firm have during the last ten years fitted up a considerable number of places with the pipes running down all the way from the boiler; and I know other hot-water engineers who have done the same. This will surely settle this point, whatever we may make of the others.
Mr Hammond complains that I pass by his remarks by characterising them as serious and grievous errors, etc, because I cannot refute them; but I have refuted them. His statement about it making no difference to the circulation whether the boiler is 2 or 4 feet below the level of the pipes I characterised as a grievous mistake, and I proved this conclusively by quoting one of the first authorities upon the subject. I was in doubt what Mr Hammond really meant in his original article on this point, and therefore I queried thus: "If by this he means that it makes no difference to the circulation whether it is 2 or 4 feet from the lowest point to the highest," etc. Now, most unfortunately for Mr Hammond, he settled this point in his reply to Mr Inglis in the May number by boldly staking his whole case upon two points. He says if he is wrong here he is wrong altogether, and all that he has said is worthless.
I shall prove that on these two points - viz., the return current and vertical height - he is in error, and consequently all he has said is worthless. In that reply he says, "the rapidity of circulation of the water in the heating apparatus is not measured or determined by the elevation of the flow above the 'point on which the fire acts, but by the difference of the specific gravity of the volume of water in different points of the apparatus." I quoted Mr Kinnear Clarke to show the utter fallacy of this idea. I quoted his very words; but it is clear that Mr Hammond does not understand the figures, otherwise he would never have written as he has done in his last. He says Mr Clarke qualifies his conclusions; but I say he does not - not in the slightest degree; and, moreover, Mr Clarke could not possibly qualify this law - he has merely formulated what is admitted by all scientific men since the days of Torricelliana Pascal. Let it be observed that the point here is not the rate the water moves at, which is determined by local circumstances; but whether by increasing the vertical height the motive power is increased also; for if only a half or a ninth of the absolute power due to gravity is available when the height is 20 feet, only the half or a ninth is available - other things being equal - at 10 feet.