for many designers think it necessary to rest the tower posts directly over the drum, thus making the diameter of the latter about forty per cent greater than the side of the square, upon the corners of which are located the axes of tower columns. Other designers make the sides of the square intersect the circle of the drum so as to divide the latter into eight equal parts, thus making the diameter of the drum about eight per cent greater than the side of the square.

The author of late years has been taking the diameter of the drum equal to the side of the square, and has obtained eight points of support by inserting four small girders in the corners of the square, at angles of forty-five degrees with its sides. As the cost of a pivot-pier varies very nearly as the square of its diameter, it follows that this method of designing the drum effects a great saving in cost of both drum and pier. Occasionally it will give a pier of very small diameter in comparison with the length of the draw-span. The remedy for this, provided the pier have the requisite stability against overturning, is not to increase the pier diameter but to anchor the draw-span to the pier in such a manner as not to interfere with the turning, but so as to offer an effective resistance to any tendency to lift the span off its support.

In the case of the Jefferson City highway bridge, the length of the draw-span is four hundred and forty feet, while the diameter of the drum is twenty-two feet-the same as the perpendicular distance between central planes of trusses. Such a ratio of span length to drum diameter is too great for safety in case of a strong lifting wind acting on one arm only, for such an uplift would have to amount to only twelve and a half pounds per square foot of floor in order to throw the span off the pier. It was therefore necessary to anchor the span to the pier by means of a long four-inch bolt passing through a wide, heavy casting which is embedded in the concrete, and projecting at the upper end between two beams and through a saddle and a heavy washer-plate. The  nut  on  the anchor-bolt  is  turned  down  so  as  nearly  but  not  quite  to touch the said washer-plate,  thus causing no obstruction to turning the