1 86 POPULAR LECTURES AND ADDRESSES. 



Predictor (represented in Fig. 29, p. 184), now 

 actually before you: 



A long cord of which one end is held fixed 

 passes over one pulley, under another, and so on. 

 These eleven pulleys are all moved up and down 

 by cranks, and each pulley takes in or lets out 

 cord according to the direction in which it moves. 

 These cranks are all moved by trains of wheels 

 gearing into the eleven wheels fixed on this driving 

 shaft. The greatest number of teeth on any wheel 

 is 802 engaging with another of 423. All the 

 other wheels have comparatively small numbers of 

 teeth. The machine is finished now, except a 

 cast-iron sole and cast-iron back. A fly-wheel of 

 great inertia enables me to turn the machine fast, 

 without jerking the pulleys, and so to run off a 

 year's curve in about twenty-five minutes. This 

 machine is arranged for fifteen constituents in all 

 and besides that there is an arrangement for 

 analysing out the long period tides. 



The following table shows how close an 

 approximation to astronomical accuracy is given 

 by the numbers chosen for the teeth of the several 



