usually impair their non-conducting qualities. A covering of hair-felt upon a steam-pipe, especially upon a horizontal pipe, will soon become so charred upon the inside, the hair assuming a pulverulent condition, in which it has been compared to snuff, that the covering hangs tangentially to the pipe, forming a channel or flue along the pipe, through which a free circulation of air and escape of heat will take place, if perchance there be any opening in the covering. It has been attempted to prevent the charring of these substances by the interposition of air-spaces and of layers of asbestos paper between the covering material and the pipe, but these attempts have met with only partial success. Asbestos paper is a fair conductor of heat, and while it somewhat retards, does not prevent this charring. Some of these fibrous organic substances, such as hair-felt, cotton-wool, etc., are, in their normal condition, excellent non-conductors of heat; and it is to be regretted that they retain their desirable qualities for only a limited time, and are too often subject to the liability of taking fire.

The mineral non-conducting materials are, as a class, less non-conducting than those composed of organic substances; but they are also less liable to deterioration, and enjoy an immunity from danger of fire. Asbestos, which has been used to some extent for covering steam

pipes, is a tolerably good conductor of heat, and there seems to be little reason for its application as a pipe-covering material. The province of asbestos is rather that of a very convenient incombustible fiber, than that of a non-conductor of heat. It is only when the fiber is in a very soft and downy condition, and full of air, that asbestos is of much value as a non-conductor. But in this respect it never approaches the nature of the structure of slag-wool, which is, at least when fresh, an excellent non-conductor of heat. The mineral wool consists of a mass of extremely fine interlocking fibers, which form multitudes. of minute air-chambers, to which is due the nonconductivity of the substance. The fiber is, however, extremely glass-like and brittle, and will not bear much handling. It doubtless merits the reputation which it has, of becoming broken and pulverized by repeated heating and cooling, and by the vibrations and jarring of steam-pipes. The powder then collects at the bottom of the paper-bags, leaving the top of the pipes comparatively unprotected. This reputation the mineral wool shares with the asbestos fiber, but is certainly more entitled to it than the latter substance. Much has been said of the corrosive action of mineral wool upon iron pipes, and Professor Egleston, in a paper read before the American Society of Civil Engineers, has demonstrated that under certain adverse condi

tions, such a corrosion does take place. But it is probable that mineral wool made from slags that are free from sulphates and sulphides, those of lime particularly, will not be liable to this objection, especially if dampness of the coverings be avoided.

The high degree of excellence of the carbonate of magnesia coverings is due to the myriads of microscopic air-cells contained in the magnesia alba, and to its entire freedom from crystalline structure. The paper coverings show a fair degree of non-conductivity, but have the drawback of being short-lived and hazardous. The mineral wool gave better results, but the covering was also much thicker than the paper cylinders. Fossil meal was the poorest of the non-conductors tried, probably because of its too great density. The covering was, however, little over five-eighths of an inch thick, and a greater thickness might have given better results. This thickness (5% inch) is that recommended by the makers, and the quantity sent by them to fill au order for covering 6 feet of 2-inch pipe, was no more than sufficient to yield a covering about five-eighths of an inch in depth.

For ease of application and removal, none of the materials tried equaled the magnesia coverings. The sections can be applied to the pipes at a rapid rate by means of the convenient metallic straps. Next come the mineral-wool cover

ings, and the paper cylinders which are secured by staples, then the hair-felt, and lastly the fossil meal. The application of the latter material requires some practice: a mason was occupied the greater part of a day in plastering the 6-foot pipe experimented with.

In point of appearance the above order also prevails, the magnesia sections being the handsomest, and the mud-like and uneven fossil-meal paste the least sightly of the coverings. The slight difference of radiation between the asbestos-paper-covered and the canvas-covered magnesia sections, was probably more due to an accident in the manufacture, than to the difference in the jacketing material. When absolute indestructibility of the external jacket is less a desideratum than beauty of exterior, the woven jackets are to be preferred. A coat of waterglass or other fire-proof paint would serve to render them uninflammable.

With reference to the economy and cost of non-conducting materials, it may be said that the material which is in the greatest degree non-conducting, incombustible and durable, will prove the most economical, even though its first cost be greater than that of an inferior article. Experiments with naked pipes show that a 2-inch pipe, carrying steam at 60-pounds pressure, will condense 180 grammnes per foot per hour. Covered with a good covering like

magnesium carbonate, the condensation will be but 38 grammes per foot per hour, a saving of 142 grammes per foot per hour, which is equivalent to 3.13 pounds of steam per day of ten hours for each foot of pipe covered. The covering of 100 feet of pipe, then, will save in a yer of 300 ten-hour days, the coal necessary to convert 93,900 pounds of water into steam. One pound. of bituminous coal is capable of making about 8.5 pounds of steam, so the saving of coal due to the 100 feet of covering would be 5%1⁄2 tons per year, which, at $4 per ton, amounts to $22. The real saving will probably amount to more than this estimate in most cases; and it may be said in round terms, that the 100 feet of covering causes each year a saving of its own first cost ($25). Inasmuch as the material pays for itself in a year, and will last indefinitely under ordinary conditions, its advantageousness is beyond question.

From all that precedes, the conclusion drawn regarding the magnesium carbonate coverings is that they are the lightest and most durable of all the mineral non-conductors, and the haudsomest and most efficient of all coverings for steam-pipes.

The appended table presents a résumé of the results of the experiments.

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