542 SCIENCE PROGRESS. 



tration. If a ship is at rest in port and the screw is set in 

 motion, very sHght force is sufficient to keep the ship from 

 moving, for the screw only keeps churning the same portions 

 of water, and so has httle power ; but as soon as the ship is 

 under way the propeller is continually brought into con- 

 tact with fresh portions of water previously at rest, and so 

 gets a much greater grip on the water than when the ship 

 was at rest in the harbour. But for this property it would 

 be impossible to drive large ships forward with such small 

 propellers as are commonly used. And just the same 

 applies to aeroplanes. 



These simple conclusions from elementary principles 

 were verified in a striking manner by Professor Langley and 

 Mr. Maxim, and the former has embodied the results of a 

 long series of observations in his Experivicnts on Aero- 

 dynavncs, which is now regarded as a standard work of 

 reference. Mr. Maxim made similar investigations quite 

 independently at the same time, a whirling table being used 

 by both observers. The method was this : a plane was 

 made to revolve round a central axis and the force of pres- 

 sure of the air on it carefully measured so as to discover the 

 laws according- to which the resistance of the air varied 

 when the velocity or the inclination of the plane was 

 varied. 



The first conclusion that these experiments brought 

 out was that if a plane be held perfectly horizontally it falls 

 to the ground less quickly if it has a horizontal motion im- 

 parted to it (as when made to travel round and round by 

 means of this whirling table) than it would do if it were 

 simply falling vertically. In other words, the horizontal 

 velocity lengthens the time of falling. The curves obtained 

 indicate that for small speeds the effect is not very great, 

 but for speeds exceeding about thirty miles an hour the 

 lengthening of the time of fall increases very rapidly. 



The dropping plane, it will be seen, imparts a little 

 downward motion to the air as it drops, but as soon as it 

 has gone a little distance it comes in contact with fresh 

 particles of air at rest ; it is never supported for any length 

 of time by particles to which it has already imparted a per- 



