48 THE WORLD- 



all revolve around the earth in the same time, or, more philo- 

 sophically speaking, he learns that the earth turns uniform!}'' on 

 its axis performing each revolution in exactly the same interval 

 of time. The space thus obtained "on the clepsydra, for a revo- 

 lution of the heavens, we may imagine him dividing into 

 portions that will mark the subdivisions of the day. These 

 divisions would not all be equal, but decrease in length as 

 the height of the fluid column decreased. His instrument, thus 

 adjusted to measure the flight of time, we may suppose him to 

 observe the exact instant of sunset, and after an interval of a 

 day, again making the same observation. He would find upon 

 careful observation that this interval was longer than the interval 

 required for a star to revolve around the earth, by about 4 minutes, 

 if his instrument would detect so small a quantity. In other words, 

 he would find that the sun was apparently moving backward in 

 the heavens. And now, he is, perhaps, for a moment puzzled 

 which measure of time to adopt, that of the stars, or of the sun. 

 Convenience points out the latter, and consequently astronomers 

 regulate their time measurers to divide the solar dayinio 24 hours ; 

 the other is called the sidcrial day, and is about four minutes 

 shorter. 



For a long time, even after Copernitus and Galileo had estab- 

 lished the fact of a rotation of the earth on its axis, there were no 

 means of measuring intervals of time more correctly than by the 

 water-clock. It is true, that instruments made of wheels, and 

 moved by weights, were, in Galileo's time, in use, but as they 

 were without any regulators, the time was too inaccurately mea- 

 sured to be of any service. The discoveries which were being 

 made by Tycho Brahe, and Kepler, demanded some more 

 accurate method of registering the time. It is related that 

 Galileo, observing the swinging of a suspended lamp, in a 

 Church at Pisa, and noticing that the vibrations, whether long or 

 short, were performed in equal times, conceived the idea of 

 adapting such a contrivance, now called a pendulum, to measure 

 intervals of time. His apparatus was rude enough, and it was 

 necessary to employ a boy to occasionally give the pendulum a 

 slight push when it was near resting. It does not appear, at first 



