While the discussion on heating is going on in your pages, it may not be inopportune to raise a few collateral questions on the subject of boilers and pipes, etc, on which I would be glad to hear the opinion of your correspondent, Mr Makenzie, who appears to be pretty well up on the subject, and also of others. As regards boilers, without professing to be an expert in such matters, my opinion is that the apparatus which presents the greatest amount of surface to the direct or first action of the fire, and which can be entirely enveloped in the flame of the furnace, must be the best; and it seems to me that the boiler which does this is one of the shape of a penny-piece, with a perfectly flat under surface and a slightly convex top, at the apex of which the flow-pipe would start and the returns would enter opposite each other at each side of the penny, - the water-way to be about the usual capacity of the saddle - boiler - that is, from 2 to 3 inches thick. I propose to set such a boiler above a shallow circular basin of fire-brick - the fireplace being represented by the bottom of the basin - and to cover it above with another inverted basin of fire-brick brought down to within 2 or 3 inches of the surface of the boiler, the flue going out at the flow-pipe at the crown of the boiler.

The boiler would, of course, only be slung by flanges, so as to leave a space about two inches wide round the edges for the draught: the fire being exactly under the centre of the boiler, the draught would be equal all round, and the flame would envelop the whole apparatus perfectly. In order to make the door air-tight, I propose to face it with fire-brick, and work it by a pulley and balance-weight - the door, when closed, dropping on to a bevelled seat in front of the grate, the door itself being bevelled in order to fit. The object of the bevel is to prevent ashes lying on the grate and keeping the door from shutting close. I may state that I have experimented with such doors and found them to work smoothly, and to need no banging or knocking to make them shut - practices which destroy ninety-nine furnace - doors in a hundred. Of course, the door runs in a perpendicular groove in which no obstruction could possibly settle. It may be stated that the furnace-door would be the only brick in the basin below the boiler; but the fuel is meant to lie on a circular grate of small diameter in the centre of the basin, and when the fire gets fairly ignited the red-hot cinders would be spread outwards round and up the sides of the basin, and the fresh fuel thrown in the centre, thus so far burning the smoke as it passes over the red-hot fuel towards the edges of the boiler.

I do not know that I could better explain my ideas without a plan; but your readers may guess pretty near what I mean by imagining a penny-piece placed horizontally in the air, with a saucer of the same size, and a hole in the centre of it, inverted above it, the fire being represented by a candle held under the penny, and the flame travelling over its under surface, round the edges, and over the.top and out at the hole at the crown of the saucer.

And now I will endeavour to record some experiments with a miniatare boiler of this description. I had a boiler 6 inches in diameter, and having a water-way from one-eighth to a quarter of an inch thick, and holding about a couple of wine-glassfuls of water, made of block tin, and set, complete, in the same material as it would be in firebrick, the external surface being covered with non-conducting felt. This boiler, as well as the whole of the apparatus, was made at a Midland foundry, where the experiments were carried out, and where I was kindly afforded every facility for giving the boiler a fair trial on a small scale. When the boiler was sent to me for inspection, before trial, I found written on its under surface, "The Boiler of the Future," but the best laid schemes gang oft a-gley. Well, the nominal heating-power of horticultural boilers is put at something like 50 square feet of surface of hot-water pipes to every square foot of boiler surface, though I never heard of any boiler that could do this effectively, or anything like it.

Still "the boiler of the future" ought to accomplish the very most we had, 250 feet or 3000 inches of half-inch gas-piping attached to the thin 6-inch circular boiler - about the right proportion - the piping being coiled round a drum, and the flow-piping starting from the crown of the boiler and descending the spiral coil to the bottom of the drum, and then bending off and entering the boiler by the return openings at each side - everything, in short, being arranged exactly as we heat our hothouses. A spirit-lamp was applied to heat the water, and then our difficulties began; and neither the founder nor myself could overcome or explain them at the time, though I have found a certain hypothesis on the subject since: but that is neither here nor there at present. No sooner was the lamp applied than the water began to circulate - running a considerable way round the coil by the flow, and making the pipes so hot that a touch blistered the hands. Then the pipes would as suddenly cool again in the flow-pipe, and the water would begin to flow round by the return-pipes just as far and as hotly; then circulation in that direction would cease without any apparent cause and begin at the right end again ; and so on many times, the water in the boiler all the time apparently boiling " fit to burst." Alterations were made, and the length of piping considerably diminished, etc. etc, but to no purpose.

At last the boiler burst, after hours of patient coaxing, and we had to leave the shops and run for the train as black and smutty as any Sheffield "grinder," and not a little puzzled on the subject of "hot-water circulation." Here was a boiler constructed on sound principles - on the common saddle principle, one might say - with the flow-pipe at the highest point, and the return entering at the lowest - absolutely refusing to conduct itself as in theory it ought to do. I have not had time to return to the experiment again, but meanwhile would be glad if any of your readers could explain what it was that was at fault. It was not the fault of arrangement of the pipes, or of the expansion-box, or of air in the pipes, and the outlets and inlets of the flow and return-pipes were of the same capacity. All the arrangements, indeed, were in the usual orthodox manner. The experiment, so far as it went, showed that the boiler had great and unusual power - the fault was in the circulation.

Had the experiment succeeded, it was my intention to have a boiler fixed here, and to attach to it pipes of oval instead of circular shape, that would present the same amount of heating surface as a 4-inch pipe, but which would contain considerably less water, and be sooner heated. This, I am aware, would have necessitated a more constant fire in severe weather; still I am not sure but that pipes which are soon heated and soon cooled are the best in hothouses, where it is often desirable to reduce or raise the temperature quickly. J. S. W.