The shielding effects of struts and bars. In the previous communications, I., II., and III., the aerodynamic resistance of single bars, struts, and wires was investigated. The object of the investigation herein described was to determine the shielding effect of one bar mounted in the direct path of another bar of identical length, shape, and size.

Method of mounting the specimens. The upper bar was suspended from the fork of the balance at two points 6 inches apart in the manner described in communication II., and the identical lower bar was pivoted to the upper bar by two parallel wires 6 inches apart. Means were provided to allow the lower bar to be gradually lowered away from the upper bar. The axis of the bars were kept parallel and the ends of the two bars were kept exactly beneath one another throughout the experiments. All the bars were 24 inches long and $ inch thick.