1. F. B. Downing, A. E. Parmalee and C. J. Pedersen:U.S. Patent 2,004,160, June 11, 1935.
2. F. B. Downing and C. J. Pedersen:U.S. Patent 2,008,753, July 23, 1935; U.S. Patent 2,087,103, July 13, 1937.
3. R. G. Clarkson and C. J. Pedersen:U.S. Patent 2,054,282, Sept. 15, 1936.
4. L. Spiegler and C. J. Pedersen:U.S. Patent 2,087,098, July 13, 1937.
5. F. B. Downing and C. J. Pedersen:U.S. Patent 2,121,397, June 21, 1938.