To satisfy those of your readers who have taken the trouble to follow this discussion, and whose tastes and habits of thought have not led them to investigate the matter, I may state that in a heating-apparatus I have fitted up for the very purpose of experiments, there is inserted in the flow-pipe, about 4 feet from the boiler, four glass slips 6 inches long, 1 1/2 inch broad, one on top, one on bottom, and one on each side. By watching the flow of the water, it can be seen, without the shadow of a doubt, that four minutes after the fire is put on, forward motion begins at this glass, and two minutes after it begins the whole body of the water from top to bottom, from side to side, goes forward up the hill from the boiler, and not a vestige of a return current. It is impossible, from the nature of the case, it could be otherwise. This apparatus, I may say, contains about 150 feet of 4-inch pipe, and is erected in such a way that it can be made to have a rise to far end 60 feet away from boiler; or with a continuous run down from top of boiler to return.

I may also state, that from elaborate experiments carried out by inserting a series of thermometers in the pipe, and noting the rise on the register, I find that, with a rise to far end, and 1 1/2 foot more height, the water goes round several minutes quicker than with a continuous fall, as Mr Hammond recommends. This is again exactly what one would expect - viz., that the circulation should be slower with the less height. It makes no difference where the highest point is - not a shade of difference. I may say further, that these trials have been repeated over and over again with the same result; and should any reader of ' The Gardener' have any doubts remaining, I shall be delighted to place the apparatus at his disposal, and let him satisfy himself. I can send you figures and full particulars should any one desire to see them.

Now as to the stoke-hole, which is really the important point in this discus-cussion, I have a difficulty in understanding Mr Hammond's position. What does he propose? I find in my experience, which has been considerable - having been, speaking roughly, connected with the erection of about six or seven hundred ranges of glass, large and small - the great difficulty to be, having to pass doorways and passages, and in ninety-nine out of every hundred cases where we have a deep stoke-hole this is the cause. I am sure every one concerned will be delighted to find some means of avoiding this unmitigated nuisance. "Well, what does Mr Hammond advise? My firm have several ranges of glass to heat just now with a passage running from end to end of the range about 2 feet from the back wall; the boiler is to be placed on the north side of the passage, and as the passage has to be crossed at least in three different places, we are at present in the belief that we must sink the boiler until the flow-pipe is at least a few inches below the floor level.

We are afraid of water, and may have to cut a drain, at considerable expense, to avoid being drowned out.

Now if Mr Hammond can make the apparatus work as well with the bottom of the boiler one foot below the floor level, we shall not only be delighted to have the information, but pay him handsomely for it; or, if he takes out a patent, I will guarantee that he will soon make a large fortune. I confess I am quite at a loss, after reading his three letters, what it is he proposes in this direction.

He commenced his original article in the February number by reprobating "the practice of sinking the boiler below the level of both the flow and return pipes." For my own part, I began and ended my former letter by a reference to what I consider a necessary evil - viz., a deep stoke-hole. But no one is yet any the wiser for all the information Mr Hammond has vouchsafed us.

I ask Mr Hammond, as a special favour, to go into the matter in reference to arrangements of houses as above, which, I assure him, from a pretty extensive knowledge of hothouses, is the main difficulty. I am perfectly well aware that various ingenious expedients have been recommended to overcome dips by throwing all the hot water up into a box and other means; but all who are conversant with the matter know that the success, where adopted, has not been by any means of such a nature as to lead others to follow; and if Mr H. has nothing better to recommend, he is again only bringing forward the ghost of bygone days.

If Mr Hammond's sole object is to prove that after the pipes travel, say 50 or 60 feet below the pathway with a gradual rise, they should - or rather the flow-pipe should - immediately on entering the house, rise to its highest point, then gradually slope back to the return, is the best way to lay pipes, I do not know if there is really much necessity for discussion about it. For my own part, I consider it of not the slightest importance, any further than may be desirable from accidental circumstances. If the height is the same, there is no difference between the power necessary to raise the water in a vertical pipe than in a pipe at an angle, whatever that angle may be, from the well-known law of liquids that pressure is to be counted by height only; for "the pressure exerted by a liquid in virtue of its weight (or gravity) on any portion of the liquid, or on the sides of the vessel in which it is contained, depends on the depth and density of the liquid, but is independent of the shape of the vessel and of the quantity of the liquid " - Ganot's Physics, p. 79, - italics are mine. This proves that the expenditure of power is the same in raising water by pressure in a vertical direction as at an angle.

It also further proves what I proved already, that the pressure depends on the 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.

If he objects to Ganot, let him consult any authority he pleases. Chambers, in ' Hydrostatico-Pneumatico,' p. 4, says: "When a pressure is exerted on any part of the surface of a liquid, that pressure is transmitted undiminished to all parts of the mass and in all directions;" and on p. 7: "the pressure of water increases in intensity with the depth without regard to the shape or size of the cavity or vessel containing it;" and again, p. 8: " the pressure on the horizontal bottom of a vessel is as the area of the bottom and the perpendicular height of the liquid, etc, that without regard to the shape of the vessel." It is surely unnecessary to bring forward further proof. Any one who wishes to pursue the subject further should consult the authorities named, or others - Tyndall, Lard-ner, Todmorton, Dr Golding Bird, - they all agree upon this. Having shown that the motive power in a hot-water apparatus depends directly upon the vertical height, and that it makes no difference to this motive power whether the highest point is near the boiler or at the farthest end, I have only to state, further, that the discussion on the point raised by Mr Hammond is not of yesterday.