In reply to A Rigg's letter (The Engineer, vol. 55, 23 March 1883, p. 234), J I Thornycroft asserted that the principle of his guide-blade propeller was different from that of Rigg's guide-blade propeller. Riggs's propeller like an ordinary propeller acted on a stream of water that it had previously accelerated and there was no difference in velocity between the water entering and leaving the propeller.