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 by pipe nipples of various kinds, with a paper or thin metal gasket between the faced surfaces of the joint. These nipples are sometimes threaded and screwed up tight by special wrenches, but what is known as the push-nipple is extensively used. These nipples are not threaded but are turned to a close fit with the holes in the loops, at the joint, which are bored out perfectly true, and they are driven tight by pressure with jacks or presses made for the purpose.

Cast-iron radiators are classified according to the kind of surface, into plain-surface and extension-surface radiators, and according to their style of construction into open and flue radiators, and of the open one, two, three and four column type, according to the formation of the loops. The different classifications will be better understood from inspection of the accompanying illustrations. The extension-surface radiators, Figures 14 and 18, as the name implies, have extensions of various kinds in the form of ridges or pins cast on to the otherwise plain surface, and are used principally for indirect radiators. The flue radiators, Figure 16, are used extensively for direct-indirect radiation, but for such purpose are provided with shields and provision of some kind for connection with the outer air. Flue radiators, such as shown in Figures 12 and 13, are sometimes spoken of as veiled-surface radiators. Most low radiators of considerable width in proportion to the height are of the flue type. The radiator that has been used for direct radiation much more widely than any other is the two-column plain-surface radiator, such as is shown in Figure 10, but the numerous other forms are being more and more extensively introduced.

Most radiators for steam heating have all the loops connected by only one steam passage through the bottom, but some are also connected at the top as well. Such radiators are adapted to steam heating from hot-water heating practice, as they are the only kind that can be used in the latter, and are hence known as the hot-water type. See Figure 20. Almost all low and wide steam radiators are made in this way, and by some authorities this construction is preferred in all kinds. Further discussion of this subject will be taken up later.

Measuring radiators.—Radiators are universally rated by the number of square feet of surface which they contain. This, at the present time, is for many reasons a very arbitrary method and