It is a striking fact that about one quarter of the propeller’s work is wasted in friction, and slip. (By “slip” is meant the loss caused through the yielding of the water at the propeller, and the screw not progressing to the full extent of its pitch.) In designing the screw for a steamship, due regard must be paid to the amount of horse-power which the engines are to generate and the speed at which the vessel is to travel, but whether the inward- or outward-turning propeller is the more efficient has not yet been satisfactorily determined by experts, though the probability would seem to be with the outward-turning screws. An instance of this was recently afforded by one of the leading firms of ship-builders in this kingdom who had been commissioned to construct a vessel 300 feet long, with a speed of between 18 and 19 knots. The owner, who was a scientist, particularly stipulated that the ship’s propellers should be inward-turning, and was very positive of the advantages which would thus accrue. The builders, however, arranged the engines in such a manner that they could be driven either way with equal ease. After they had tried turning inwards, they tried outward-turning, and reversed the propellers with a decidedly satisfactory result. The same conclusion has also been arrived at by Professor W. S. Abell, who asserts that all his experience goes to prove that greater hull efficiency is obtained by outward-turning propellers. In this connection I might quote the case of the steam yacht _Niagara II._, which was built some years ago in the United States. She was about 250 feet long, with a displacement of 2,000 tons, and her deadwood aft was not cut off. Information was obtained through two six-hour trials under similar conditions, except that her screws were interchanged from side to side, so that they were inward-turning on the first trial, and outward-turning on her second. Notwithstanding that greater horse-power was used when the inward-turning propellers were employed, yet the latter did not give the ship the same amount of speed as when they were made to turn outwards. Indeed, the speed of the inward was found to average 12·8 knots, whereas the outward-turning screws gave an average of 14·12 knots. It is in the department of the propeller that fuller information is awaited with an enthusiasm that belongs to no other branch of naval architecture.