TIME MEASURING. 



481 



arrangement of wheels sets the clock going. The forms of pendulum are 



now very varied. 



But in watches the pendulum cannot 

 be used. The watch was invented by Peter 

 Hele, and his watches were called " Nurem- 

 burg eggs" from their shape. The weight 

 cannot be introduced into a watch, and so 

 the spring and fusee are used. The latter is 

 so arranged that immediately the watch is 

 wound and the spring at its greatest tension, 

 the chain is upon the smallest diameter of the 

 fusee, and the most difficult to move. But 

 as the spring is relaxed the lever arm becomes 

 longer, and the necessary compensating power 

 is retained. Watches without a "fusee" have 

 a toothed arrangement beneath the spring. 



The Pendulum. A " simple " pendu- 

 lum is impossible to make, for we cannot 

 put the connecting line between the " bob " 

 and the clock-work out of consideration, so 

 "simple pendulums" are looked upon as 

 "mathematical abstractions." The most 

 modern clocks have what is called a " dead- 

 beat" escapement, 

 and a compensat- 

 ing pendulum. 

 Clocks are liable 

 to alter by reason 

 of the state of the Fig - 



and until all our clocks are placed in vacua 

 we must be content to have them lose or gain a little. There is a magnet 

 arrangement by which the Greenwich Observatory clocks keep time by- 

 compensation, corresponding with the fall or rise of the syphon barometer 

 attached to it. The description need not be added. We may here state that 



detailed descriptions of all the instruments 

 used in the Observatory, together with full 

 information as to their use, will be found 

 in a very interesting work by Mr. Lockyer, 

 entitled "Stargazing," to which we are indebted 

 for some corrections in our summary. 



We have spoken of solar time and 

 sidereal time, and no doubt someone will 

 inquire what is meant by mean time an 

 expression so constantly applied to the 

 Greenwich clock time. Stellar time, we have 



3* 



Fig. 518. Pendulum and escapement. 



air and varying temperature 



-Balance. 



Fig. 520. Regulator. 



