R. Inglis "predicts" that a greater quantity of water would flow if the pipes were to ascend instead of descending on leaving the boiler. One of the houses I alluded to in the May number has equal to 350 feet of 4-inch pipe, and the flow runs down an incline of about 14 inches to where it enters the house; then it continues its course on a level, and on leaving drops about 13 inches, followed by a slight descent to the boiler, near which is one of Messenger's valves.

The action of this apparatus I will endeavour to describe. Thus, supposing the whole to be cold, and the valve closed: the fire is set agoing, and the water in the boiler is raised to 200o Fahr. If you then examine the boiler, which is uncased for the purpose, you will find that to the touch there is scarcely a perceptible difference in temperature between the bottom and top. The return-pipe, so far as the valve, will be hot. That part of the flow on a level with the aperture where the water leaves the boiler will be hot also, but the part which is below that level cold, as at the commencement. Open the valve. What is the result? The hot water speedily passes down the flow, and the bottom part of the boiler becomes cold by the inward rush of cold water, and continues cold, however fierce the fire may be, until the whole of the cold water in the pipes has made its circuit and become heated.

R. Inglis thinks J. H. "will no doubt be done for ever with the old system".

J. H. has tried the old system, and also what we may call the new system, and given each a fair trial. Has Mr Inglis done as much?

"What I use for a boiler is six of Cannel's flues. The space for the fire is rather large, and surrounded by fire-brick. This may appear wasteful; but on trial it was found to evaporate 6.8 lb. water per lb. of coke consumed. Not very bad, compared with steam-boilers. I name this, as some may be disposed to doubt what I stated above - viz., that there is to the touch scarcely a perceptible difference of temperature between the bottom and top of boiler when heated to 200° Fahr., and disconnected from the houses by closing the valves.

J. Hiscocks.

The Mills, Swallowclife, Salisbury.

[We think the points at issue are very important, or we would not have devoted so much space to their discussion; and we hope that all irrelevant matter will be as much as possible avoided by our correspondents. A few facts as to the practical working of the different systems are worth volumes of speculative argument. - Ed].