THE discussion on the above subject is increasing in force. If it goes on much longer, 'The Gardener' will be in danger of exploding, unless provided with a safety-valve. In the issue for April two of your correspondents bring forward - the one an equalised mixed forcing, the other a push - and - pull theory of circulation; and in the May issue Mr A. D. Makenzie furnishes a strata theory.

The latter gentleman says that my paper in the February issue "only brings forward the ghost of an old friend with a new face," and that the system of fitting up a hot-water apparatus therein advocated " has been familiar to gardeners and hot-water engineers for a generation." Now, I am not going to dispute this. It may be all true, for anything I know to the contrary. I only know that if the method "has been familiar to hot-water engineers for a generation," they have kept the matter to themselves, and, as a rule, carried out a different one. Mr Makenzie disposes of several of the statements contained in my paper by simply calling them grievous and serious mistakes, errors, etc, etc. It is not surprising, however, that he adopted this hurried manner, seeing that he was only dealing with "the ghost of an old friend." Indeed it would be unfair to expect him to examine it minutely. Ghosts, whether of friends or foes, are said to be uncanny; and none of us would like to have more to do with them than we possibly could help.

I fancy, however, that unless more logical reasons are furnished for the incorrectness of the statements which I have made in reference to the subject under consideration than those supplied by Mr Makenzie, that "the ghost of an old friend with a new face " will be an apparition of frequent occurrence to hot-water engineers.

If "it is not the case that" engineers and gardeners are agreed upon, as being essential to rapid circulation of the water in the pipes," that the boiler should be sunk " below the level of both flow and return pipes, what does Mr Makenzie mean by the following assertion: "Those having practical experience know that where it is practicable to place the boiler say 10 or 12 feet below the floor on which the pipes are laid, there is a very much more rapid circulation than where there is only say 3 feet of difference between the bottom of return and top of flow pipes." It is evident from the latter quotation that Mr Makenzie is agreed that sinking the boiler below the main body of the pipes is essential to rapid circulation. And to prove "conclusively the great value of having a deep stokehole," as being essential to rapid circulation, he quotes Mr Kinnear Clarke's theoretical tables for finding the velocity at which the water circulates in a hot-water apparatus. According to these tables - as given by Mr Makenzie, - by increasing the main height from 10 feet to 20 feet, the water will circulate at an additional speed of 40 feet per minute.

Mr Kinnear Clarke, however, adds an important qualification to his theoretical conclusions on this matter by saying, " The velocities due are not attained. The actual velocities are in some cases not more than a half, or even a ninth, of the velocities due to gravity." But supposing the velocities due to gravity were attained, it would not prove "conclusively the great value of having depth of stokehole." Elevation of the flows could be got without having recourse to Paddy's plan of elevating the roof of his dwelling by sinking the floor. This elevation theory, with the view of increasing the rate of circulation, is based upon the fact that the higher the point from which a body falls vertically, the more rapid is its motion as it nears the earth. Now a fluid of less specific gravity cannot fall through one of greater; and in a properly adjusted heating apparatus each volume of water, from the highest point to the lowest, in the circuit of the pipes, is relatively lighter than the volume below it, and therefore the velocity does not increase on the journey from the highest to the lowest point, like a body falling through space, but is the same at all points of the apparatus; and therefore sinking the bottom of the boiler more than a foot or so below the floor on which the pipes are laid is a mistake, if it is done for the purpose of accelerating the circulation of the water in the apparatus.

The proper way of attaining the latter is not by increasing the vertical height between the lowest and highest point of the apparatus, but by increasing the difference between the temperature of the water as it leaves the boiler at the highest point and enters at the lowest; and the way to do this is by causing the water to flow over a larger surface of piping in the houses to be heated.

Then in reference to my objection to giving the flow-pipes a continuous ascent from the top of the boiler to the furthest points to which they extend in the various compartments to be heated, Mr Makenzie says, "This arrangement of the pipes is often carried out, not because it is considered essential for the rapid circulation of the water, but because, all things considered, it is in most cases the best arrangement for other reasons".

Now local circumstances may necessitate the sinking of the boiler below the level of both flow and return pipes, but there is no reason why the flows on entering the building to be heated should not be carried to their highest point at once, and then commence a descending course. And I maintain that this way of fixing the pipes not only "works fairly well," but that it is the right way of fixing them; and that it can be carried out more advantageously in an extensive system than "where there is only one or at most two houses to be heated".

Next, Mr Makenzie says it is an error to suppose that one particle of water cannot transmit heat to an adjoining particle; but he supplies no proof of its being so. In a question of the kind under discussion, assertions are useless, unless backed by some means of proving their correctness, and Mr Makenzie supplies no way by which it can be proved that one particle of water is able to transmit heat to an adjoining particle. All experiments with the view of testing this go to prove that "practically the particles of which water is composed are unable to transmit heat to each other by conduction.

To prove this in a rough way, take sheets of equal dimensions of the following minerals, that is, "iron, stone, lead, and ice," and hold them one at a time on the palm of the hand in front of a brisk fire: the heat emitted by the fire will be transmitted to the hand by the above minerals in accordance with their respective conducting power, the surface of the ice exposed to the action of the fire will receive as much heat from the latter as the "iron, stone, or lead;" but because of the inability of the particles of water to transmit heat by conduction, the palm of the hand is not warmed, but, on the contrary, is exposed to a temperature at the freezing-point so long as the thinnest film of ice intervenes between it and the source of heat. Yes, " water is water," and for any practical purpose has "no power to transmit any heat by conduction." Then about the return current in the flow-pipes, Mr Makenzie admits that it takes place in the first instance, but says, "The back motion in the flow-pipe must immediately cease as soon as the average temperature in the flow-pipe becomes higher than the average temperature in the return." Now, if this were correct, "the back motion" would not continue more than a few minutes after the fire was set agoing below the boiler.

As the first volume of heated water that entered the flow would raise the average temperature of the latter above the average temperature of the return, so that back motion would not be worth talking about if it only continued the length of time indicated by Mr Makenzie. It continues, however, and is as constant as the forward motion in all apparatuses that have the flow-pipes laid on a continuous ascent throughout their length. The reason for the continuance of the return current is plain. The point of the apparatus on which the fire acts contains a volume of water of less specific gravity than the lower strata of water in the flow-pipes, and whether we treat the question as one of "pure hydraulics" or as one of "hydrodynamics," it won't alter the fact that water of less specific gravity cannot force that of a greater uphill, and therefore the colder and relatively heavier volumes of water in the under side of the flows will roll back in hotter and relatively lighter volumes in the boiler, "like so many boulders tumbling down the Cumberland Screes," notwithstanding what Mr Makenzie may think to the contrary.

Again, Mr Makenzie says, "The difference of temperature between the upper part of the flow-pipe and the under part has nothing to do with it" (that is, with the back motion), "any further than that the upper strata travel faster than the colder under strata." Now if the colder under stratum did travel uphill, there is no doubt it would do so at a slower pace than the upper and hotter stratum. But when each stratum reaches the highest point and commences the downhill journey in the return-pipes, how do they behave as regards their respective rates of travelling? Does the upper and hotter stratum travel faster than the under and colder stratum on the descending course? and if not, may I ask Mr Makeuzie to explain why not? According to my ideas of circulation, the colder volumes of water should lead in the race from the highest to the lowest point, whereas, according to this strata theory of circulation, the hotter volumes make the circuit of the pipes in less time than the colder. In an apparatus where this takes place the circulation is bad: the heat in the stokehole, where it is not wanted, is excessive; and in the houses, where it would be of use, it is unsteady and unsatisfactory.

In conclusion, I quite agree with Mr Makenzie when he says it is evident that I have not mastered the subject. It is difficult to master any subject, however simple, and heating by hot water is rather a complicated one. The readers of 'The Gardener,' however, will form their own opinions as to whether Mr Makenzie's or my ideas on the question are the more correct. J. Hammond.

Brayton Hall, 12th May 1879.

[We beg to thank several correspondents for their contributions on this subject, and regret that we cannot make room for them this month. - Ed].