WATCH MAY BE USED AS A COMPASS 135 



south, and it makes one complete circuit round the points 

 of the compass in 24 hours. The hour-hand of a watch 

 also makes one complete circuit in 12 hours. Hence, if 

 the watch is held with its face in the plane of the ecliptic, 

 and the figure XII on the dial is pointed to the south, both 

 the hour-hand and the sun will be in that direction at noon. 

 Both move round in the same direction, but the angular 

 velocity of the hour-hand is twice as great as that of the 

 sun. Hence the rule. The greatest error due to the neglect 

 of the equation of time is less than 2 degrees. Of course, 

 in practice, most people would hold the face of the watch 

 horizontal, and in our latitude (that of London) no serious 

 error would thus be introduced. 



" In the southern hemisphere, or in any tropical country 

 where at noon the sun is due north, the rule will give the 

 north point instead of the south." 



