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 indirect type, the flues are much shorter and the air resistance much less than with the indirect setting. As a matter of fact, the principal air resistance with the former is due to the passage of the air through the radiator itself.

The author knows of no tests that have been made on direct-indirect radiators, but considers they would be of value if they should establish the relation between the heat given out and the air delivery per square foot of radiator for constant differences of temperature between the steam and entering air. As stated in a previous chapter, in the opinion of the author the use of the direct-indirect radiator, which has been, up to this time, and is now, very limited, will be materially increased in the immediate future, as in connection with exhaust fans they form an effective means of introducing adequate ventilation into buildings which are not very densely populated but in which there is a decided need of ventilation. This class of buildings includes especially our palatial modern office buildings and also a certain class of factories. The more extensive introduction of these radiators, as well as of the indirects, is greatly delayed by the lack of accurate information as to what the different makes on the market will do in the way of heating under various practical conditions of temperature and setting. There seems, therefore, to be a considerable field for investigation in this regard.

Circulation in radiators.—In the preceding chapter, and thus far in this one, we have discussed the action of radiators in doing the work they are intended for, and have pointed out the theoretical and practical considerations involved. Before discussing the bearing of these principles upon the design of a heating plant it is necessary to turn the attention to another consideration involved in the action of radiators, and one which is of much importance in the practical efficiency of a heating plant. This is the circulation of steam within the radiator.

Any one who has had even a slight experience with radiators well knows the troubles that arise on account of dripping air valves, water-hammer, and the pocketing of air, and other evils that are due to imperfect steam circulation. While evils of this kind are frequently attributable to imperfect circulation in the piping, yet there is a great difference in the operation of radiators in this respect.

As shown in a previous chapter, any heating system contains a