It is not likely that anything new can be added to what is already known on the subject of hot-water circulation - the whole matter has again been made plain by your correspondent, Mr J. Inglis. The heat from the fire rarefying the water, disturbs its equilibrium, and the warmed particles through their buoyancy, like a cork, are forced to ascend, and the cold water, from its greater density, descends to fill its place. It is quite easy to see, however, that your able correspondent, Mr Hammond, is right so far. For a time a counter-current must be produced in the same pipe, the heated water flowing along the upper curve of the pipe, and the cold returning along the under side, like water in the eave-gutter. One's hand placed first on the upper surface and then on the under side of the pipe, will at once prove the truth of this; and this fact may appear to retard the circulation for a short time, supposing a fire just lighted under a cold boiler. This appears to be all Mr Hammond contends for, although his facts may not quite justify his conclusions.

The illustration of the siphon is used to explain the movement of the water, and though it does not explain all, it is true as far as it goes: the balance of weight in favour of the cold water running down a perpendicular pipe or an inclined plane must accelerate the circulation, if it does not originate it. A very curious instance of the effect of the cold water in deciding the course of the circulation once took place in our experience. It was in this wise: a boiler with three distinct sets of pipes attached was just out of the hands of the mechanics, and all concerned were impatient to see the apparatus tested. One set of pipes was all but on a level with the boiler, another set was some 5 feet and the third set was 10 feet above the boiler. Each set was laid nearly level, but with a perpendicular dip where attached to the boiler. It is with the 10-feet set we have to record our experience more particularly. A good fire was first made to heat the flues and dry the boiler outside, then water was slowly poured into the apparatus down the flow-pipe of the 10-feet set, when very soon the warm water was circulating in the lowest set of pipes, but as the filling proceeded, these again grew cold, and the middle set became warm.

The filling went on as described down the flow-pipe of the upper set into the boiler, when the warm water gradually rose up the return pipe, flowed round, and finally arrived by that route into the flow, where the cold water was being run off at a small expansion-box over the boiler. The filling finished, to our temporary astonishment the circulation continued the reverse way, the warm water rising in the return pipe and returning to the boiler down the flow-pipe. This was to all appearance reversing the order of nature. We have somewhere read that the laws of nature are the same all the world over, except in Ireland; but the above incident took place in England, where the laws of mechanics and those relating to liquids work harmoniously, and consequently those of nature could not really be reversed this time: the water was only wrong in taking the wrong course, the pipes being new and coming from Scotland. After logically settling this point, the whole on reflection was explained by the fact of there being 10 feet of perpendicular pipe, which contained continuously the coldest and consequently the heaviest portion of the water, and outbalanced the warm water rising in the return.

We suffered much mental uneasiness in having the laws of nature thus apparently treated so contemptuously by our new apparatus; so after a few weeks' action, and no change likely to be voluntarily effected in its behaviour, we stopped the fire and cooled the water, and started afresh, this time with full pipes and a proper and orderly course of procedure on the part of the water. The share which the principle of the siphon has in the circulation of hot water was thus very excellently and accidentally illustrated. The boiler, from the nature of the position, was not sunk in the earth, although so much below the upper set of pipes. But after all, the fire is the great mover of the water, as the sun moves the winds: and as an eminent horticultural writer some years ago lucidly ventilated and illustrated the theory of hot-water circulation by showing the effects of the sun in originating the Gulf Stream, whereby the climate of these fortunate islands was maintained at a forcing-house temperature; and as the water is made to move in a system of pipes independently of the auxiliary principle of the siphon; - so, after all, the sinking of boilers in deep stoke-holes may not be an expedient of such necessity, if some other auxiliary can be substituted for the siphon.

As the cold water, from its density, has a drawing power down an inclined plane, so hot water, from its rarity, may be made to exercise a lifting power, like a balloon, or like smoke up a chimney.

We have known more than one heating-apparatus erected with the view of taking advantage of this effect of heated water, the motive power on the fire being above the level of the return, in consequence of the difficulty of sinking the boiler. I may say the whole of the pipes connected with the boiler were return-pipes, with just this exception that the heated water rose at once to a considerable height above the boiler, 9 feet at least, where it was discharged into an expansion-box, and thence distributed to the various houses by other pipes opening into the expansion-box - the return-pipes, so to speak, entering level into the bottom of the boiler. The apparatus worked satisfactorily: the gardener was a well-known prize-taker at the district shows. By this plan several difficulties of doors and passages were overcome, which could not have been effected on the siphon principle. One other apparatus we remember, on a large scale on the same principle, fixed by Messrs Weeks. There really seems no reason why of necessity boilers should be fixed below the level of all the pipes: we should, of the two, prefer the siphon principle, of course, as being the most compact and most efficient in application, the law of gravitation being antagonistic to the other; but where difficulties of drainage or deep excavation or approach are to be contended against, the lifting principle might be adopted with advantage.

It is well understood in an apparatus on the siphon principle, if any part of the return-pipe dips below the bottom of the boiler, there the coldest water will lie, and act as an effectual plug against the circulation; but by the use of the perpendicular pipe discharging the heated water at once into an elevated expansion-box, and thence being distributed, there seems no reason why the boiler should not be actually above the level of the return pipes, for by this means the boiler is made simply an enlarged gland or portion of the pipes. It once was our duty to stoke a boiler which was no boiler at all, but only a spiral coil of 4-inch pipes, the return being of course the bottom of the coil. The water, in its course through the various houses and apartments heated, had to pass through many ups and downs, at one place through a succession of pipes placed vertical and parallel, and connected by siphon-bends top and bottom. The apparatus worked perfectly, but great attention was required that no air should accumulate at any of the bends, as if it did, the circulation was impeded at once. Water was supplied from a ball-tap at a considerable elevation above the highest point of the pipes.

Heating by hot water has so generally been carried out on the siphon principle, that public opinion has become settled that there is no other mode whatever. The fact that it is the most convenient, and can be applied in all but very exceptional cases, has had a tendency so to settle the matter; but we agree with the editor in saying that the whole subject is worthy of discussion, to our mutual gratification and instruction. The Squire's Gardener.