insulating properties; other things being equal, the incombustible mineral substances are to be preferred to combustible material. The following table gives the results of some actual tests of different coverings, which were conducted with great care and on a sufficiently large scale to eliminate slight errors of observation. In general the thickness of the coverings tested was I inch. Some tests were made with coverings of different thicknesses, from which it would appear that the gain in jn sulating power obtained by increasing the thickness is very slight compared with the increase in cost. If the material is a good conductor its heat-insulating power is lessened rather than diminished by increasing the thickness beyond a certain point.

PERCENTAGE OF HEAT TRANSMITTED BY VARIOUS PIPECOVERINGS, FROM TESTS MADE AT SIBLEY COLLEGE, CORNELL UNIVERSITY, AND AT MICHIGAN UNIVERSITY.* Relative Amount of Heat Transmitted.

Kind of Covering.

Naked pipe....

Two layers asbestos paper, I in. hair felt, and canvas cover..
Two layers asbestos paper, 1 in. hair felt, canvas cover, wrapped with

100.

15.2

manilla paper.....

Two layers asbestos paper, I in. hair felt..
Hair felt sectional covering, asbestos lined

15.

17.

*18.6

* These tests agree remarkably well with a series made by Prof M. E. Cooley of Michigan University, and also with some made by G. M. Brill, Syracuse, N. Y., and reported in Transactions of the American Society of Mechanical Engineers, vol. XVI.

105.5

108.7

95.0

The following table translated from Péclet's Traité de la Chaleur gives in a general way the amount of heat transmitted through coverings of various kinds and of different thicknesses; the loss from a naked pipe is taken as 100.

CHAPTER X.

DESIGN OF STEAM AND HOT-WATER SYSTEMS.

117. General Principles.-The general problem of design includes the proportioning of, first, the amount of radiating surface which will be located directly in the rooms to be heated in all systems of direct heating, and in the air-passages or flues leading to the rooms in all cases of indirect heating; second, the size of the pipes which are to convey the heated fluids to the radiating surfaces; and third, the proper size of boiler or heater.

The question of the system or method of heating which is to be adopted will usually depend upon considerations of cost or of personal preference on the part of the proprietor. The various systems of heating, whether by steam, hot water, or hot air, as commonly practised in this country, do not often come in direct competition. Hot-air heating, where the air is moved by natural draft, is adapted only to the smaller sizes of dwelling-houses, and where heat does not need to be carried any considerable distance horizontally. It is generally found that if the horizontal distance exceeds 15 or 20 feet the supply of heat becomes uncertain in amount. With steam and hotwater heating there is no such limitation as to distance; the first cost is, however, considerably greater than that of hot air, but heat can be supplied with certainty to all parts of the sys-· tem under all atmospheric conditions. Regarding the relative merits of systems of steam and hot-water heating, little can. be said. It will generally be found that the first expense of steam-heating is considerably less, and that there is considerable difference of opinion regarding the relative economy of operation of steam and hot-water heating plants. The tests which have been made have generally shown somewhat in favor of

water. The difference, however, is not great, and may be due to local conditions, but is probably due to the fact that the temperature of the discharged gases may be somewhat lower for the hot-water heater than for the steam-boiler, and also to the fact that in comparatively mild weather the fire in the hot-water heater may be regulated somewhat closer, to meet the demand for heat. The hot-water system in general requires rather better workmanship in the erection of pipe lines than steam-heating, and more care must be taken in proportioning the various pipes and fittings. The heat from hotwater radiators is somewhat less in intensity and more pleasant than that from steam-radiators, and the temperature can be regulated by simply throttling the supply-pipe of the radiators, which is not the case with steam.

Whether direct or indirect heating shall be used will depend also on circumstances. It will be found that in general the surface required for indirect heating is one third to one half greater than that for direct, and it will give off 50 per cent more heat per square foot, so that the operating expense is practically twice that of direct heating. Indirect heating assures excellent ventilation, and it is advisable to use it for certain rooms of residences because of that fact.

118. Amount of Heat and Radiating Surface required for Warming. The amount of heat required for buildings cf various constructions has been considered quite fully in Chapter III. From which it may be seen (page 59) that in ordinary building construction the amount required in heatunits, for each degree difference between inside and outside temperature, is approximately equal to the area of the glass surface plus one fourth the area of the exposed wall surface plus one fifty-fifth of the number of cubic feet of air required for ventilation.

The air required for ventilation will vary with the conditions; but in direct heating it seems necessary to allow for three changes per hour in halls, two in rooms on first floor, and one in rooms on upper floors. (See page 59.)

* See Transactions American Society Mechanical Engineers. vol. x, paper by the author. See also Report Massachusetts Experimental Station No. 8, 1870.

The amount of heat given off by one square foot of radiating surface, as determined by a great number of experiments, is given in Chapter IV, from which it is seen (pages 66 and 80) that for the ordinary radiating surface, with a temperature of 150 degrees above the surrounding air, 1.8 heat-units will be given off per square foot of surface per degree difference of temperature per hour, and when the temperature is 110 above the surrounding air about 1.7 heat-units are emitted.

The total heat emitted from radiating surfaces of different characters, corresponding to the average results of experiments is shown on the diagram, Fig. 187, in which the horizontal distances correspond to the mean difference of temperature between the air in the room and the radiator, while vertical distances, the value of which is read on the scale at the left, correspond to the total heat-units transmitted per square foot per hour.

To use the diagram assume the difference of temperature between the air of the room and the radiator, then look on vertical line until intersection with the line representing the desired condition is found, thence read results on the left. Thus, for instance, if the difference of temperature is 150 degrees the intersection of the line from this point with that representing direct ordinary radiation corresponds to 275 heatunits, and with that representing 1-inch horizontal pipe, 375 heat-units, as read on the scale at the left. The dotted lines in the diagram give the heat transmitted from various indirect surfaces for different velocities of the moving air. The results are to be found as for direct radiation, but the difference of temperature is that estimated from the mean of the surrounding air and the radiator.

Having the total heat required for warming and that which is given off from one square foot of radiating surface, it is quite evident that the surface required may be computed by the process of dividing the former by the latter.

Expressing results algebraically we can produce a formula from which the radiating surface may be calculated quickly and easily as follows:

Let R equal the total radiating surface required, the required temperature of the room, t' the temperature of the outside air, 7 the tem