A screw propeller operates on exactly the same principle as an electric fan, and ships may have one or more of these propellers, which are fastened to shafts projecting through the hull beneath the water at the stern. If the ship were tied up strongly to a pier, so that it could not move, and the propellers were turned by the engines, the result would be to set in motion a column of water away from the propeller just as an electric fan sets in motion a column of air. The resistance of the water is so great, however, that once the lines that secured the ship to the pier were thrown off, the propellers would set the ship in motion, and the propellers would progress through the water in somewhat the same way that an ordinary wood screw advances through wood when a screwdriver is properly applied. The _Majestic_ is propelled by four of these propellers, two on each side astern. Turning at 180 revolutions a minute they utilize 80,000 horse power. One might think that propellers would waste much of this power—that is, that they might turn in the water without pushing the ship forward very much, as a wood screw sometimes turns round and round without getting a grip on the wood into which the carpenter wishes to drive it. This is true to some extent when a ship is first starting, but once the ship is in motion a properly designed propeller will be 95 per cent. efficient—that is, it will go as far in 100 revolutions as if it ran without any “slip” for 95 revolutions. A propeller is measured by its diameter, just as an electric fan is measured. A propeller ten feet in diameter is one whose blades, measured from the centre of the shaft, are five feet long. Another and equally important measurement is the “pitch”—that is, the distance forward the propeller would travel in one revolution if it were running through a solid. Take a wood screw and look at it carefully. You will find that the threads run around it in a spiral. Mark a spot on one thread, and then trace the thread around the screw until it again reaches the side you marked. It will have advanced toward the point, and the direct distance between the place you first marked and the place you have arrived at would be the “pitch.” As a propeller blade travels in a path similar to the thread of a screw, its pitch is similarly measured. The four propellers on the _Majestic_ are built of manganese bronze and each carries four blades. They are 16½ feet in diameter, and their pitch is 14 feet 11½ inches. Two of these propellers turn one way and the two on the opposite side turn the other. This is to offset the tendency to swing the ship out of its line of travel, which would be very apparent if all the propellers turned one way. Ships with one propeller feel this action very plainly.