1. E.R. Hughes, US Pat. 1 784 628 (1929).
2. T.M. Rector, US Pat. 2 018 091 (1936).
3. a Wm. Jefferies and Peirce Leslie and Co. Ltd., Br. Pats. 472 195 (1937)
4. b Wm. Jefferies and Peirce Leslie and Co. Ltd., Br. Pats. 472 196 (1937)
5. c Wm. Jefferies and Peirce Leslie and Co. Ltd., Br. Pats. 474 737 (1937).