. Railroad structures and estimates . =rt Fig. 243a. and the average life given by a number of railroads is from 3 to5 years. This t>-pe of jack has been used extensively on the C. P. supports shown are 2 X 4 timbers but asbestos angleshave also been used reinforced with metal. The asbestos sheetsare used in standard sizes and the joints are simply buttedtogether. A coat of metallic paint is given all outside surfacesafter erection. ASBESTOS SMOI^ JACKS. 513 Some types of cast asbestos jacks have been quite successfuland satisfactory, and though somewhat hghter than cast ironthey cost
. Railroad structures and estimates . =rt Fig. 243a. and the average life given by a number of railroads is from 3 to5 years. This t>-pe of jack has been used extensively on the C. P. supports shown are 2 X 4 timbers but asbestos angleshave also been used reinforced with metal. The asbestos sheetsare used in standard sizes and the joints are simply buttedtogether. A coat of metallic paint is given all outside surfacesafter erection. ASBESTOS SMOI^ JACKS. 513 Some types of cast asbestos jacks have been quite successfuland satisfactory, and though somewhat hghter than cast ironthey cost, about the same, or about $125 to $150 complete inplace. (Fig. 243.) The material also for this type of jackseems to be very variable, failures have been numerous, andusually they are purchased under a guarantee; 8 to 10 yearsis given by some users as their average life. TRANSITE ASBESTOS WOODSMOKE JACK. SECTION ON END VIEW Fig. 243. 514 WOOD SMOKE JACKS. Wood. — Wood jacks were probably the first kiiid to be builtand there is a tendency at the present time to revert back tothis material, which may be accounted for by the desire of manydesigners to eliminate iron of any kind from the present-dayconstruction of engine houses, owing to the rapid deteriorationthat takes place from the smoke and sulphuric gases that areprevalent around structures of this kind. The flims\ construction of wood jacks in the past made thema fire hazard and they failed to stand up to the service overcome the fire risk the wood has been treated but thecost is said to be high. In place of treated wood fire-proofpaint has given good satisfaction. There are also wooden jacks in service that are held togetherand bound at the corners with cast-iron clamps, etc., that appearto be giving satisfactory results. A wooden jack built on the miU-t^-pe method of 243b, made of 2 X 3 timber
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