I find that Mr Hood, in his ' Treatise,' p. 12, thus refers to it: "Some persons have imagined that if the pipes be inclined so as to allow a gradual fall of the water in its return to the boiler, additional power is gained. This at first appears very plausible, particularly with regard to some forms of the apparatus." He then proceeds, by a series of elaborate calculations, to prove this idea to be an error; go that the ghost was laid forty years ago, and the apparition Mr Hammond has conjured up is only a bogus ghost at the best, which I am sure will not trouble us much.

It only further remains for me to notice Mr Hammond's extraordinary experiment to prove that practically water is an absolute non-conductor. In any case, whatever it proves, it does not prove that water is practically a non-conductor. 1 think, if I recollect right, I have seen something about such an experiment to illustrate the doctrine of what is called the latent heat of liquefaction and the specific heat of water. Had Mr Hammond informed himself properly he would have known that the reason why his hand in his experiment will not feel any sensation of heat until the whole of the ice is melted, is not because it is an absolute non-conductor, but because of the great amount of latent heat taken up in the passage of water from the solid to the liquid state; for it is found that it takes as much heat to rnelt one pound of ice at 0° as to raise one pound of water from 0° to 79.24°; therefore the latent heat of fusion of water is fixed at 79.24° centigrade, or 142.65° Fahr. - (See Deschand's Natural Philosophy, translated by Professor Everett, part ii., pp. 426-444.) In other words, one pound of ice at 32° Fahr. requires as much heat to melt it, without raising the temperature one iota, as will suffice to raise one pound of water from 32° to 142.65° Fahr. But the conducting power of water need not be a disputed power at all: it has been fixed just as definitely as the conducting power of solids; and although a very feeble conductor compared to most of the metals, such as gold, silver, copper, etc., makes a favourable comparison with the earths.

As I said before, liquids are bad conductors, but water is one of the best. From experiments made by Despretz, he fixed the conducting powers of various substances as follows: Gold, 1000; silver, 981; copper, 897; zinc, 363; lead, 179; porcelain, 12; iron, 374; tin, 304; marble, 23; brick-earth, 11. He fixes the conducting power of water at 1/95, that of copper-ore 9.44, while copper is 8.97; therefore any one will see that although it is not a good conductor compared to some solids, it is about as good as some others, - which shows that Mr H.'s rough-and-ready way of deciding the point is worth nothing; but if he will take a rough-and-ready method, let him go to a Turkish bath at a temperature of 160°; he will find he can bear it easily, if not comfortably, - he can handle wood, cloth, etc., without inconvenience. But let him put his hand into water at 160°, and I rather think he will come to the conclusion that it is not a non-conductor, for it will part with its heat into his hand in a manner more sudden than agreeable. Or, on a frosty morning, with the temperature at zero, let him handle wood, stone, cloth, and ice in the open air, when they must, of course, be all at the same temperature.

He will find that the ice will conduct the heat from his hand sufficiently quick to make it very unpleasant, and very much quicker than wood, cloth, and many other substances. In fact, as I said already, it is exceedingly inconvenient in discussing any question to have to prove every fundamental point which ought to be known to every schoolboy. In conclusion, I have only to state that, if the explanations already given do not convince Mr Hammond that he is wrong fundamentally on almost every point, I despair of making it plainer. A. D. Makenzie.

2 Grove Terrace, Edinburgh.

I have already encroached too much on your valuable space in reply to Mr Hammond, that I cannot ask you to find room but for a few words in reply to J. S. W., who asks my opinion about the failure of the miniature hot-water apparatus described by him. While not professing to dogmatise in such matters, I am of opinion that the cause of the failure was the enormous amount of friction in such a length of small pipe. It is rather an intricate subject; but I may mention that, roughly speaking, the friction on water flowing through a half-inch pipe is relatively eight times as much as through a 4-inch pipe, - so much so that, in a very long, small pipe, friction sometimes, as in J. S. W.'s case, stops the flow altogether. When water has to circulate through a very long pipe in heating, it is necessary to use steam or other power to force the water, and various ingenious engines have been from time to time adopted for this purpose. If J. S. W. is satisfied that his "boiler of the future" is the boiler of the future, the failure of the gas-pipe need not discourage him.

Friction is apt to be forgotten, but it is a most important element, and should not be lost sight of. "Where it is not considered, the theory as well as the practice is defective. - Your obedient servant, A. D. Makenzie.

July 11, 1879.

When the discussion on this subject was first opened by Mr Hammond, I understood that his object was to show that, in ordinary cases, deep stoke-holes, and giving the "flows a continuous ascent from the top of the boiler to their farthest point of extension in the building or buildings to be heated," were unnecessary. Since various theories have been advanced as to the cause of the water circulating, it now seems to me more of a theoretical quibble than a comparison of attested facts.

In the July number, R. Inglis alludes to an "old fitter," who boasts of one pipe acting as a flow and return. In that case there would be two currents of water, travelling in opposite directions, the upper stratum going from, and the Under returning to, the boiler. This is what will occur in the flow when a continuous ascent is given them, not only at the time of " setting the fire agoing," but also while radiation of heat from the pipes continues. This I have proved by inserting a glass cylinder about 2 feet long, the same size as the flow-pipe, so that it forms a part of the same; and after mixing with the water particles of matter of about the same specific gravity as the water, the two opposite currents were plainly visible.