From arches, built of parts of circles, let us pass to the circle itself, and glance at the use of tubes of circular section as we begin to consider how resistances to motion may be minimized. The use of the bamboo not only for building, but for the carriage of water, began in the remote past. As structural material it was light and strong as we have noticed; laid upon the ground it was a ready-made water pipe of excellent form. When trees were hollowed out to convey water, when clay was modeled into tubes, the hollow cylindrical shape of the bamboo was in the mind of the Asiatic artisan, to be faithfully copied. That form has descended to all modern piping for water, steam, and gas, because the best that a pipe can take. No other shape has, proportionately to capacity, so little surface for friction inside or rust outside. A locking-bar water pipe, devised by Mephan Ferguson, of Perth, Australia, is made of two plates of equal width, curved into semi-circles which are pressed at their ends into channel bars of soft steel. As the locking-bars and joints are opposite each other, their joints can be tightly closed by a simple machine which exerts pressure in a straight line. This construction may be used not only for pipes, but for hydraulic cylinders, air receivers, mud and steam drums, tubular boilers and boiler shells where high pressures are to be withstood. A steam boiler or other vessel under severe internal strains had best be spherical if equality to resistance is particularly desired. Usually a cylindrical shape is much more convenient, and no other is given to simple steam boilers or to the tubes of water-tube or fire-tube boilers. Tubes comparatively narrow, are readily manufactured without seam, so that they may be quite safe though thin; large boilers of plates riveted together, must be built of thick metal. It was estimated by Mr. F. Reuleaux, the eminent engineer, that if such boilers could be made in one continuous piece of metal by the Mannesmann process, so successful in tube-making, an economy in weight of at least one third would be feasible. [Illustration: Hand-hole plates. Erie City water-tube boiler.]