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two seconds, it must be four times as long as that which 

 swings once in one second, while to swing once in three 

 seconds it must be nine times as long, the length being 

 measured by the square of the time of vibration. 



We are thus able to " regulate " clocks by lengthening the 

 Pendulum if they be too fast, and shortening them if they be 

 too slow. The reader will probably have remarked that the 

 conditions of the atmosphere — such as heat, cold, moisture, or 

 dryness — must have an effect on the length of the Pendulum, 

 and thus alter the rating of the clock. So they do, and in 

 consequence the Compensating Pendulums have been invented, 

 some of them being made of metallic rods of different powers of 

 expansion, mostly brass and steel, while others carry a quantity 

 of mercury in a glass tube near the bottom of the Pendulum. 



Another familiar example of the Pendulum is the Metro- 

 nome, which is simply a Pendulum with a weight at the top 

 instead of the bottom, the weight being movable up or down 

 so as to decrease or hasten the pace. Generally a bell is 

 added to it, which is struck at the beginning of each bar. 



The exactness of its beats is perfect, as is known to all 

 musicians, and is calculated to take the conceit out of players 

 who are apt to disregard their time. I knew one lady, a really 

 good pianiste, before whom I placed my Metronome. Before 

 she had played many bars she broke down, exclaiming that 

 the horrid bell always said " ting " in the wrong place. 

 However, she soon acknowledged the value of the instrument, 

 and was glad to use it. 



A very good Metronome may be made by fastening a bullet 

 to the end of a piece of tape, and swinging it backwards and 

 forwards, regulating the tape according to the time required. 

 Such a Metronome is very portable, and extremely useful 

 where the conveyance of the clockwork instrument would be 

 troublesome. Moreover, its beats can be seen by a great 

 number of persons. I have often used it myself. 



Such a Metronome is used in the army, in order to regulate 

 the pace of the soldier's step, it being of the last importance 

 that the pace should always be the same. Otherwise it would 

 be impossible to calculate the time which ought to be consumed 

 in marching a certain distance, and the military calculations 



