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.
A second time Mr Makenzie says he will show "that increasing the difference of the temperature between the water as it leaves the boiler and in the returning column acts exactly in the same manner as increasing the height." But in place of showing this he gives a long list of isolated quotations from other men's writings; and as proof for or against anything, isolated quotations are of very little value. I would here ask Mr Makenzie if he thinks that the authors whom he has quoted have reached the acme of perfection in their theoretical reasoning on the question of heating by means of hot water, and that no further inquiry on the subject is necessary? If he thinks so, I don't think so; and that I am right the number of unsatisfactory heating hot-water apparatuses that are at work in every county in the kingdom is a proof.
I now come to notice where Mr Makenzie says that my way of increasing the motive power "is a most improper way, for two considerations. First, the quantity of piping must be settled by consideration apart from the circulation; and second, because by increasing the quantity the friction increases in greater proportion than the power, consequently increasing the amount of pipe past a certain point will prevent circulation altogether." Now in reference to the first consideration, why my way of increasing the motive power is "a most improper way," I would ask Mr Makenzie, why must the question of circulation be left out of consideration when deciding the quantity of piping requisite to heat a range of plant-houses? If the quantity to do so "must be settled by consideration apart from circulation," then according to the second consideration the quantity of piping might be such as would "prevent circulation altogether".
The question of circulation should be the first consideration. If the water circulates properly through the apparatus, minor difficulties in connection with its erection are easily got over. Now with regard to the second consideration. It becomes important when considered in connection with another statement of Mr Makenzie's. The second consideration says - "by increasing the quantity " (of piping) "the friction increases in a greater proportion than the power." The statement referred to is a quotation from Ganot's Physics, and the conclusions that Mr Makenzie draws therefrom, and reads as follows: "The pressure exerted by a liquid in virtue of its weight (or gravity) on any portion of the liquid, or on the sides of the vessel in which it is contained, depends on the depth and density of the liquid, but is independent of the shape of the vessel and of the quantity of the liquid." This quotation, Mr Makenzie says, "proves that the expenditure of power is the same in raising water by pressure in a vertical direction as at an angle;" and that it also proves what he proved already, " that the pressure depends on the depth or height and the density of the liquid, so that it makes no difference to the motive power, seeing the friction in both cases must be the same." Now according to this consideration the motive power is limited in its action by friction.
According to the statement, the same amount of motive power that would cause circulation in apparatus of, say, 500 feet of piping with the highest elevation - say, 5 feet immediately above the point on which the fire acts - would cause as good a circulation in an apparatus of, say, 5000 feet of piping having the same elevation attained by a slow gradient, but situated 500 feet from the point on which the fire acts. Let it be observed that " the height is the same," and the motive power the same in the two apparatuses here indicated. But surely the heated volumes of water will meet with more friction in attaining 5 feet of elevation by travelling up a slow gradient of 500 feet in length, than it would meet with by ascending vertically to the same height. The motive power that is the immediate cause of the water circulating in the hot-water apparatus, and the power that is the cause of water finding its levels by gravitation or pressure, are totally different. The power (heat) which is the immediate cause of circulation in the hot-water apparatus is an imparted power. The power by which water rises to a common level in a pipe or pipes is inherent in the water itself.
In the heating apparatus the water receives the power to ascend at one point, while at all other points of the apparatus it is parting with that power, and increasing its power to descend. Hence the heated volumes of water should reach the highest point of the apparatus before they part with any of the power by which they reached that point. Now about " 10 or 100 miles" of piping being sufficient to drive the water through the apparatus at a speed only equalled by the electric spark. I have to say that Mr Makenzie must know - unless, like that other engineer, "he knows very little of what he is talking about" - that the difference of the specific gravity of the water as it leaves the boiler and returns thereto is limited. At a temperature of 39 1/2° water attains it greatest specific gravity, and its least at 212°. These temperatures represent the greatest difference that can occur in the specific gravity of water, and therefore increasing the surface of piping beyond what would be sufficient to cool the water from 212° to 39 1/2° would in no way increase the motive power.
On the other hand, if the rate at which the water moves in the pipes is determined by the difference in elevation between the highest and lowest points in the apparatus, then the water could be made to circulate at almost lightning speed: we could have stock-holes any depth.
I now notice where Mr Makenzie tells us "that a lighter fluid, bulk for bulk, can force a heavier uphill, every mercurial barometer, every pump and chimney in the country, proves." I would ask Mr Makenzie as a special favour to explain in what way does the mercurial barometer show "that a lighter fluid, bulk for bulk, can force a heavier uphill." In one of the tubes of the barometer there is a vacuum at the top. In the hot water there is no vacuum at any point. The feed-cistern prevents that. Then how does the action of the common pump illustrate " that a lighter fluid, bulk for bulk, can force a heavier uphill"? When raising water by means of the common pump, the atmospheric pressure is withdrawn from one end of the tube, while it acts with its normal force at the other. Nothing analogous to this occurs in causing the water to circulate in the hot-water apparatus.
Now about the chimney illustration, "that a lighter fluid, bulk for bulk, can force a heavier uphill." The heavier fluid in most cases stands aside and permits its lighter brother to pass upwards. Sometimes, however, the heavier fluid refuses to stand aside, the consequence being a smoky room. I have now only to say that, as requested by Mr Makenzie in 'The Gardener' for August, I will undertake to heat a range of houses as indicated by him without sinking the bottom of the boiler more than one foot below the floor-level on which the pipes are laid. And further, that the apparatus will work as well with the boiler at this level as if the top of the boiler was sunk below the floor-level. The question of whether the apparatus works as well the one way as the other I would leave to the decision of the Editor of ' The Gardener' and Mr A. D. Makenzie.
Brayton Hall, Aug. 12, 1879.