Nos. 13 and U COMPRESSED-AIR FLYING-MACHINES. 55 



time of the free flight can be determined with a reasonable degree 

 of accuracy. And the efficiency of the screw engine may be used, 

 though perhaps not quite legitimately, for reducing the enormous 

 theoretical work done by the vibrating engines to an approximately 

 correct amount. A means of making the vibrating engines lift 

 weights is a difficulty not yet surmounted. 



The revolution-counter for the free flight consists of a reel of 

 cotton on an axis parallel to the screw-shaft : an empty reel is on 

 the crank shaft secured by a set screw. The turns of cotton that 

 are wound on to reel No. 2, are counted after the flight. 



The round turn in the air-pipe is to give it sufficient spring to 

 prevent the play of the engine lifting the valve cover off the back 

 of the valve, the engine being only connected to the receiver by 

 the saddle under the forward crank shaft bearing. 



The screw shaft is lighter than the crank shaft and is backed 

 up in several ways, not shown in the drawings, to prevent the 

 engine being damaged when the machine comes to earth. 



The time of flight is taken with a sand-glass which has a loop 

 of string at each end of it. The loop at the sand end is put round 

 the right wrist and the other loop is held between the right thumb 

 and the receiver, so that the glass is turned the moment the 

 machine is let go. On the machine taking the ground the glass 

 is put horizontal and the sand that has fallen is timed at leisure. 

 This seems an obvious enough method of finding the speed, but a 

 practical way to do it was not devised previously. 



Plate v. represents one of the indicators for recording the receiver 

 and reduced pressures on the chronograph drum, they are adaptable 

 for all pressures likely to be used. The difficulty of making reliable 

 spiral springs of various strengths, was the reason for adopting 

 this form in which it is so easy to insert two. three, or more pieces 

 of the same spring steel to make the amplitude of the diagrams 

 suit the pressures and the length of the chronograph drum. The 

 piece of pipe that forms the foundation of the instrument is held 



