The second compartment is cooler still; and to show the difference between the first and third, we may say that during last winter, when the heat in the former was above 40°, the frost was in the latter, which is the highest point to which the water flows from the boilers, and to which it has a direct, straight, and uninterrupted run. We may add that all our houses are well supplied with pipes, having been put up for forcing Pines and Cucumbers.

The position of affairs stated above is exceedingly annoying. If we increase our fires in order to obtain the required heat in the highest houses, those on the lowest levels are literally roasting. Surely there must be something theoretically wrong here. Either the engineering skill brought to bear in their arrangement was sorely deficient, or there are exceptionable cases in nature's irresistible laws; for we are unfortunately in possession of an instance in which hot water persistently refuses to run freely up-hill according to the rules and regulations laid down by the most scientific hot-water engineers, and we do not think any amount of figures or elaborate calculations would have much effect in inducing the water to alter its ways.

Joseph Hamilton & Son.

Wellington Place, Carlisle.

In the May issue I suggested that it would be well, if the discussion on heating by hot-water continued, to provide ' The Gardener' with a safety-valve; and I hoped Mr Makenzie would have supplied one on the "strata" principle. As yet he has not done so; neither has he, in his last paper, alluded to a stratum of any kind, hot, cold, or of average temperature; and I begin to fear that the "strata" theory of circulation has exploded, and that of average temperature sunk to zero.

I am glad, however, judging from the length of his last paper, to observe that he has not suffered any bodily injury through the occurrence, and the strata valve may yet take a place as an auxiliary in the mercurial barometrical chimney-pump theory of circulation, as propounded by him in ' The Gardener' for August.

I do not think the average-temperature point worth warming up again; but I know Mr Makenzie will pardon me if I again ask him Does the upper and hotter stratum travel faster than the under and colder stratum on the down-hill journey 1 and if not, why not?

Mr Makenzie says he has refuted my statements, and that I am "wrong fundamently on almost every point." I have no doubt he thinks so; but I would bint, that possibly be is the only intelligent reader of ' The Gardener; who holds this opinion. From the first I contended that the highest point of the apparatus should be as near the boiler as possible, and not as far from the boiler as it can be got as is the usual practice. Mr Makenzie now says it makes no difference where the highest point is - not a shade of a difference. Is this proving that I am "wrong fundamentally on almost every point"?In my humble opinion, it is granting what I contended for. I would also point out that if it makes "not a shade of a difference " where the highest point is, then, as sure " as one and three make four "- whether two and two does or does not - there is no necessity "in ninety-nine out of every hundred cases " for sinking the bottom of the boiler more than one foot below the floor-level on which the pipes are fixed.

And therefore those "unmitigated nuisances," deep stock-holes, need no longer exist.

I would remark, however, that it makes all the difference so far as the rapidity of the circulation is concerned, at what point of the apparatus the water attains to the highest point of elevation. If we want the most perfect circulation attainable in any form of the apparatus the water must ascend vertically from the point on which the fire acts to the highest point. I could tell Mr Makenzie how to prove the truth of this statement, but I will save space in ' The Gardener ' by giving- a quotation or two from one of Mr Makenzie's favourite authors, which will perhaps be more satisfactory proof to Mr Makenzie that I am right than anything I could advance in the same direction.

Hood, on warming buildings by hot water, page 45, says: " It has occasionally occurred that the circulation of the water in the apparatus has been reversed, the hot water passing along what should be the return-pipe, and the colder water following the course of the flow-pipe. This effect has sometimes been exceedingly puzzling; but it will be found to arise in those apparatus which have but small motive power and in which the principle has not been followed out of making the water rise to the highest point of the apparatus as soon as possible and allowing it, in its return to the boiler, to give out its heat to the various pipes, coils, or other distributing surfaces which it is intended to heat." Again, at page 169, Hood says: "In all cases where pipes are placed at various elevations above the boiler, for the purpose of warming different floors of a building, or where, from any other cause, the pipes descend by steps or gradations from a high to a lower elevation before the water returns to the boiler, it is desirable that the water should be made to ascend at once from the boiler to the highest elevation.

By this means the best possible circulation is always insured." I hope these quotations will satisfy Mr Makenzie that it makes " a shade of a difference " to the circulation at what point of the apparatus the greatest elevation occurs. Mr Makenzie says he "will show that increasing the vertical height and increasing the difference between the temperature of the two columns acts exactly in the same manner." Increased height gives increased pressure on the bottom and sides of the boiler, and all other parts of the apparatus. But pressure has no more to do with causing circulation as it takes place in the hot-water apparatus, than Mr Makenzie or I have to do with causing the magnetic needle to point northwards. If the water escaped at any point of the apparatus, then the higher the supply above the outlet the more rapid would be the efflux; but the water does not escape at any point of the apparatus, - it circulates therein; and unless there is a difference between the specific gravity of the water in the two columns - no matter what may be the vertical height of the latter above the boiler - circulation will not take place, - which is a plain proof that the only motive power at work in the hot-water apparatus is the difference of the specific gravity of the water at different points of the apparatus; and the only way by which this motive power can be increased is by increasing the difference between the temperature of the water as it leaves the boiler at the highest point, and returns thereto at the lowest point of the apparatus.