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.
Therefore, whatever the velocity may be in an apparatus at 10 feet of height, this is increased by the difference between 96.6 and 136.2; or, in other words, by increasing the height from 10 to 20 feet, the motive power, whatever it is, is increased nearly 42 per cent. Mr Hammond, after proving conclusively - to his own satisfaction at least - that the vertical height has nothing to do with the motive power, says the way to increase the velocity is by increasing the difference between the temperature of the water as it leaves the boiler at the highest point and enters at the lowest. This is exactly equivalent to saying, that although he is certain 2 and 2 does not make 4, he is quite sure 1 and 3 does. I will show that increasing the vertical height and increasing the difference between the temperatures of the two columns acts exactly in the same maimer, and exactly in conformity with Mr Clarke's figures.
But if, as Mr Hammond says, the proper way to increase the velocity of the circulation is "by causing the water to flow over a larger surface of piping in the houses to be heated," then the more piping on an apparatus the quicker the circulation; and if, in an apparatus of say 3000 feet, it takes an hour and a half from the time the fire is lighted until all the water in the apparatus has passed through the boiler, by increasing the quantity to 6000 feet, will the water make the circuit in less time - say an hour and a quarter? If increasing the quantity of piping determines the rate of speed at which the water moves, then it would be possible to put on piping - say 10 or 100 miles - sufficient to drive the water through the apparatus at a speed equalled only by the electric spark. I have no doubt they will. I may here- state that, as any one may see who takes the trouble to read my former letter, when I referred to Mr Hammond not mastering the subject, I referred not to the whole subject of heating with hot water, as Mi-Hammond tries to make it appear, but to that particular branch of it which deals with the motive power.
I will now 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 maimer as increasing the heights.
I will again quote Mr Clarke; page 484, he says: "Motive power of water in circulation through heating pipes. The ascensional force is measured by the difference in weight of the two columns of water of the same height ascending and descending to and from the boiler. The difference of weight is ascertained from the difference of the average temperature of the columns from which the respective densities are deducted by the aid of table No. 109, page 339." He then goes on to give the formula which I quoted. Mr Hood in his work, page 18, says: "The higher we make the ascending and descending pipes, the more rapid is the circulation of the water;" "because, as motion is obtained in consequence of the difference in weight of the ascending and descending columns of water" (and this difference in weight is owing to different densities; or, as Mi-Hammond says, the difference of their specific gravities), "the greater the height of these columns the greater must be the difference in their weight, and therefore must be the force and velocity of motion." Again, on page 28, Mr Hood says: "There are two ways by which the amount of the motive power may be increased, - one, by allowing the water to cool a greater number of degrees between the time of its leaving the boiler and the period of its return through the descending pipe;" "the other, by increasing the vertical height of the ascending and descending column." " The effect produced by these, are precisely similar, for by doubling the difference of temperature between the flow and return the same increase in power is obtained as by doubling the vertical height." Mr Thomlinson, in his ' Treatise on Warming and Ventilating,' adopts and endorses Mr Hood's calculations, pages 133 to 138. Mr Deuchar's ' Garden Architect: Treatise on the Construction of Hothouses,' page 187, says: "When the motive power, therefore, is not of sufficient strength, the increase of the height of the column ascending from the boiler must be depended on for any additional motive power." Mi-Hammond says that the proper way is to increase the quantity of piping in order that the water may be colder when it returns to the boiler.
No doubt this is one way, but 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 a greater proportion than the power, and consequently increasing the amount of pipe past a certain point will prevent circulation altogether. I am sure I need no elaborate argument to prove this to the intelligent readers of ' The Gardener.'
Now, as to the alleged return current in flow-pipe. Mr Hammond has no misgivings upon this point. He asserts that a return current goes on continually in the flow-pipe, and to stem this current all the efforts of his genius are directed. His manner of proving the existence of this current is curious: it is, that water of less specific gravity cannot force that of a greater uphill. If a lighter fluid cannot force a heavier uphill, there never can be a forward motion at all in hot-water pipes with a rise to far end from boiler, nor can there ever be any circulation in any apparatus, even on his own principle; for, suppose there is 4 feet of a vertical pipe before the continuous descent begins, how can the lighter and hotter water get up this pipe except it force the colder and heavier water before it, for the water at the very top of the boiler must be hotter and lighter than at a point 4 feet away? That a lighter fluid, bulk for bulk, can force a heavier uphill, every mercurial barometer, every pump and chimney in the country proves.
All circumstances connected with the case show conclusively that this back current can only exist for a few minutes after the fire is kindled, and is, as Mr Hammond says, not worth talking about.