S YNTHETIC Resins are of several kinds and are filled with various materials. The variety of physical properties which are obtainable from them is, therefore, great. They have not yet been developed to any extent as possible structural materials, but owing to their many valuable properties it is worth while to see what position they occupy from this point of view. The strength/weight ratios of steel, light alloys and spruce are all about the same, and any new material must roughly conform in this respect to qualify as a constructional material. The elastic modulus is the next important criterion since it determines the ratio of length to radius of gyration which must be employed in compression members in order that the full compressive strength of the material may be developed. Since the elastic modulus defines the amount of strain under load it therefore determines the deflection of a complete structure. This, however, is not the whole story, as will be seen from the fact that a spruce and steel wing (for the same conditions of loading, depth, etc.) suffer the same deflection while steel has an elastic modulus twenty times as great as spruce. This is merely because the compressive strength which has to be developed by the steel is also twenty times as great, and thus these two characteristics cancel out. The ratio of elastic modulus to stress developed fixes two important characteristics, first the deflection of the structure under load, and second the radius of gyration which must be provided in cross-sections to avoid Euler strut conditions. Steel, Duralumin and Spruce are all about equal in this respect, and have approximately similar strength/weight ratios. They are, therefore, equally suitable materials so far as the prime physical properties are concerned. There is one important point to be remembered here. Of materials of equal suitability as defined before, those of lower specific gravity will, owing to their greater bulk, require fewer artifices for securing the rigidity necessary to develop their full strength. The two criteria mentioned before are shown for various materials in Table 1. Synthetic resins may be examined with various fillers in the light of these considerations. An ordinary fabric filled insulating material has roughly the properties shown in Table 2. Since the ordinary material weighs about three times TABLE 1