Allow me to make a few more remarks on this question, and I am done with it, as I think the matter may now be safely left in the hands of Mr Hammond and his opponents. This I shall do as briefly as possible, making note of only a few extracts taken at random. These are partly side issues, it is true - still we cannot arrive at any just conclusion without considering some of these to a certain extent at least. It will not do to dogmatise on such a matter without showing our reasons for and against the several points raised. Mr Hammond says it is unnecessary to have the flow-pipe on an ascent throughout its whole length, and he is corroborated by several correspondents from apparatus in actual use. The facts, as placed before us in these papers, are worth more than any amount of theoretical rules, some of which, in actual practice, are almost worthless as evidence.

At p. 325 C. M. says, in answer to Mr Hammond: "The difference of the specific gravity of the two volumes of water is the consequent result of expansion, which must be the primary cause." Is heat not the primary cause, of which expansion is the consequent result 1 This may appear a quibble, but there is a point in it.

At p. 328 Albion proves the efficiency of the system advocated by Mr Hammond, as does also Mr Hiscocks in his first letter, together with A. II., p. 2G9, and these results are given by apparatus in actual use. In Albion's case (of which an apparatus on the same principle I once had charge of, and a better working one could not be), practically speaking, we have no flow-pipe beyond the expansion box.

At p. 375 Mr Makenzie says: "It makes no difference where the highest point is," and thereby coincides with Mr Hammond. But is there no waste of motive power in an apparatus fitted on a long slow gradient, with a gradually decreasing temperature in the body of water, and consequently an increasing density as compared with one where the rise is vertical 1 And is the friction in both cases the same?

At p. 374 he says: "By increasing the quantity (of piping) the friction increases in greater proportion than the power;" and at p. 376, "that the pressure depends on 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." I draw attention to these quotations for this reason, that we are discussing a question of hut water flowing in an apparatus fitted up on a certain principle, which is quite a different thing from a body of water of the same temperature throughout, and being raised to a certain height by pressure or gravitation, I shall trespass no further on your space, but will be content to leave the matter to those who have better opportunities of carrying it to a successful issue, by proving it unnecessary to have flow-pipes for hot water on a continuous ascent.

Robt. Stevens. Paston Northumberland.

On p. 373, owing, no doubt, to the indistinct writing, "Torricel-liana" appears for Torricelli and. On p. 375 I find that it was overlooked, in copying the original MS., to alter the words "every pump and chimney in the country proves," which should have been "every suction-pump in the country proves." Again, on p. 377, for "Deschand's Natural Philosophy," read Deschanel's Natural Philosophy; and for "1/95, that of copper-ore, 9.44," read 1/95 that of copper, or 9.44.

I have no right to object to your having expunged some irrelevant matter, the communication was far too long; but at one particular place this has led to an appearance of confusion, which I shall be obliged by your allowing me to explain. At p. 374, after the words "electric spavk," appear, "1 have no doubt they will." The complete sentence was: "This may appear to some to be curious reasoning; but all such people I refer to Mr Hammond's letter in your May number in reply to Mr Inglis, and as he proves his position, while others only assert, I have no doubt they will be satisfied." The words as printed have no meaning apart from the context, while the completed sentence explains itself. A. D. Makenzie.

2 Grove Terrace, July 28, 1879.

In your July issue your correspondent "C. M." says: "It is a well-known fact that a house situated above the level of the others is the hottest, from its having a quicker circulation." If he will call upon us we will show him an exception to this rule. We have here about 8500 feet of 4-inch pipes attached to two twin Climax boilers, which, according to the statement of the makers, Messrs Barr & Sug-den, are capable of heating 2200 feet more pipes than are attached to them, so we may safely conclude they are capable of doing their present quota of work with ease. The mains, flows, and returns, which conduct heated water to our houses, have a direct run of about 326 feet, with a gradual rise from lowest to highest point of 13 feet. The principal of the firm of Horticultural engineers who laid these mains, examined the ground before beginning the work, and declared the local conditions admirably adapted to a rapid circulation and a successful termination of his labours. The work in its various stages was superintended by no less than three practical hot-water engineers - an accumulation of engineering skill which should have placed us in possession of a favourable example of what one of your correspondents is pleased to term the "old system of heating." We do not know of a 9-feet length of flow-pipe in our houses which has not at least 1/4-inch rise in its length, and of course the returns have precisely the same descent to the mains.

Every flow-pipe leading from the mains to each of our houses is supplied with a valve so that the heat may be entirely under control.

It was implicitly believed, beyond all shadow of doubt, that the pipes situated on the highest level, taken from the boiler, would be so much the hottest, that valves were put on the mains at about 164 feet from the boilers, in order to check the expected natural inclination of the heated water to rush to the highest point, and so neglect the houses on the lower levels, - so much for theory. The actual results are directly opposed to the engineers, and, at the time, our own "rule of thumb" ideas of the natural laws governing the circulation of hot water. All the houses situated on the lowest level are the hottest, so much so as to require the greatest nicety in managing the valves, which have to be so nearly shut as to almost stop circulation altogether. The houses, situated on what we may term the middle level, are the most satisfactory, requiring little trouble in the management of valves, and are all that we could desire with somewhat slow fires; but beyond the valves in the mains mentioned above, which, by the way, are always open, we have a house 86 feet long, and a range 150 feet long, divided into three compartments. The pipes in the first of these, beginning at the end nearest the boilers, cannot be heated to the same temperature as the houses immediately below them.